IB · CHEMISTRY SL

Chemistry SL

Reactivity 1: what drives chemical reactions — Reactivity 1

Name: ____________________Date: October 10, 2026

1.
Distinguish between an exothermic and an endothermic reaction, in terms of the sign of the enthalpy change.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that an exothermic reaction releases thermal energy to the surroundings and has a negative enthalpy change (ΔH < 0). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that an endothermic reaction absorbs thermal energy from the surroundings and has a positive enthalpy change (ΔH > 0). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The sign of ΔH describes the system's energy change: negative means the system lost energy (released as heat), positive means the system gained energy (absorbed as heat).
Marking points
- States that an exothermic reaction releases thermal energy to the surroundings and has a negative enthalpy change (ΔH < 0).
- States that an endothermic reaction absorbs thermal energy from the surroundings and has a positive enthalpy change (ΔH > 0).
Examiner tip: The sign of ΔH describes the system's energy change: negative means the system lost energy (released as heat), positive means the system gained energy (absorbed as heat).
2.
Marking analysis: A learner attempts the following task: “Distinguish between an exothermic and an endothermic reaction, in terms of the sign of the enthalpy change.” Their response addresses only this point: “States that an exothermic reaction releases thermal energy to the surroundings and has a negative enthalpy change (ΔH < 0).” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: States that an exothermic reaction releases thermal energy to the surroundings and has a negative enthalpy change (ΔH < 0). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that an endothermic reaction absorbs thermal energy from the surroundings and has a positive enthalpy change (ΔH > 0). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: States that an exothermic reaction releases thermal energy to the surroundings and has a negative enthalpy change (ΔH < 0).
- Identifies the missing requirement: States that an endothermic reaction absorbs thermal energy from the surroundings and has a positive enthalpy change (ΔH > 0).
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
3.
In a calorimetry experiment, 50.0 cm³ of 1.00 mol dm⁻³ HCl is neutralised by 50.0 cm³ of 1.00 mol dm⁻³ NaOH, and the temperature rises by 6.8°C. Calculate the enthalpy change of neutralisation per mole. Assume the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.
[5 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Work through this mathematical step: Calculates the total mass of solution: 50.0 + 50.0 = 100.0 g. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Work through this mathematical step: Uses q = mcΔT = 100.0 × 4.18 × 6.8. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Develop this part of the answer: Obtains q ≈ 2842 J. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Work through this mathematical step: Calculates moles of HCl (or NaOH) reacted: 0.0500 × 1.00 = 0.0500 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
6. Work through this mathematical step: Divides to obtain ΔH = −2842/0.0500 ≈ −56.8 kJ mol⁻¹ (negative, since the reaction is exothermic). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Enthalpy of neutralisation is always reported as negative (exothermic) since temperature rises; divide the heat released by the moles of water formed (limiting reagent), not the total volume.
Marking points
- Calculates the total mass of solution: 50.0 + 50.0 = 100.0 g.
- Uses q = mcΔT = 100.0 × 4.18 × 6.8.
- Obtains q ≈ 2842 J.
- Calculates moles of HCl (or NaOH) reacted: 0.0500 × 1.00 = 0.0500 mol.
- Divides to obtain ΔH = −2842/0.0500 ≈ −56.8 kJ mol⁻¹ (negative, since the reaction is exothermic).
Examiner tip: Enthalpy of neutralisation is always reported as negative (exothermic) since temperature rises; divide the heat released by the moles of water formed (limiting reagent), not the total volume.
4.
Marking analysis: A learner attempts the following task: “In a calorimetry experiment, 50.0 cm³ of 1.00 mol dm⁻³ HCl is neutralised by 50.0 cm³ of 1.00 mol dm⁻³ NaOH, and the temperature rises by 6.8°C. Calculate the enthalpy change of neutralisation per mole. Assume the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.” Their response addresses only this point: “Calculates the total mass of solution: 50.0 + 50.0 = 100.0 g.” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[5 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: Calculates the total mass of solution: 50.0 + 50.0 = 100.0 g. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Uses q = mcΔT = 100.0 × 4.18 × 6.8. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: Obtains q ≈ 2842 J. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: Calculates moles of HCl (or NaOH) reacted: 0.0500 × 1.00 = 0.0500 mol. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
6. Requirement 5: Identifies the missing requirement: Divides to obtain ΔH = −2842/0.0500 ≈ −56.8 kJ mol⁻¹ (negative, since the reaction is exothermic). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: Calculates the total mass of solution: 50.0 + 50.0 = 100.0 g.
- Identifies the missing requirement: Uses q = mcΔT = 100.0 × 4.18 × 6.8.
- Identifies the missing requirement: Obtains q ≈ 2842 J.
- Identifies the missing requirement: Calculates moles of HCl (or NaOH) reacted: 0.0500 × 1.00 = 0.0500 mol.
- Identifies the missing requirement: Divides to obtain ΔH = −2842/0.0500 ≈ −56.8 kJ mol⁻¹ (negative, since the reaction is exothermic).
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
5.
Define standard enthalpy change of formation, and state the standard enthalpy of formation of any element in its standard state.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: Defines standard enthalpy of formation as the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that the standard enthalpy of formation of any element in its standard state is zero. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Elements in their standard state (e.g. O₂ gas, graphite) are defined as the reference point with zero formation enthalpy — all compound formation enthalpies are measured relative to this baseline.
Marking points
- Defines standard enthalpy of formation as the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.
- States that the standard enthalpy of formation of any element in its standard state is zero.
Examiner tip: Elements in their standard state (e.g. O₂ gas, graphite) are defined as the reference point with zero formation enthalpy — all compound formation enthalpies are measured relative to this baseline.
6.
Marking analysis: A learner attempts the following task: “Define standard enthalpy change of formation, and state the standard enthalpy of formation of any element in its standard state.” Their response addresses only this point: “Defines standard enthalpy of formation as the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: Defines standard enthalpy of formation as the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that the standard enthalpy of formation of any element in its standard state is zero. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: Defines standard enthalpy of formation as the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.
- Identifies the missing requirement: States that the standard enthalpy of formation of any element in its standard state is zero.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
7.
State Hess's law, and explain why it is a direct consequence of conservation of energy.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States Hess's law: the total enthalpy change for a reaction is the same regardless of the route taken, provided the initial and final conditions are the same. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that this follows from conservation of energy, since energy is a state function: the total energy change between two fixed states cannot depend on the path taken between them. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Hess's law allows enthalpy changes that are difficult or impossible to measure directly to be calculated indirectly via a cycle of measurable steps.
Marking points
- States Hess's law: the total enthalpy change for a reaction is the same regardless of the route taken, provided the initial and final conditions are the same.
- States that this follows from conservation of energy, since energy is a state function: the total energy change between two fixed states cannot depend on the path taken between them.
Examiner tip: Hess's law allows enthalpy changes that are difficult or impossible to measure directly to be calculated indirectly via a cycle of measurable steps.
8.
Marking analysis: A learner attempts the following task: “State Hess's law, and explain why it is a direct consequence of conservation of energy.” Their response addresses only this point: “States Hess's law: the total enthalpy change for a reaction is the same regardless of the route taken, provided the initial and final conditions are the same.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: States Hess's law: the total enthalpy change for a reaction is the same regardless of the route taken, provided the initial and final conditions are the same. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that this follows from conservation of energy, since energy is a state function: the total energy change between two fixed states cannot depend on the path taken between them. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: States Hess's law: the total enthalpy change for a reaction is the same regardless of the route taken, provided the initial and final conditions are the same.
- Identifies the missing requirement: States that this follows from conservation of energy, since energy is a state function: the total energy change between two fixed states cannot depend on the path taken between them.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
9.
Given ΔHf(CO₂) = −394 kJ mol⁻¹, ΔHf(H₂O) = −286 kJ mol⁻¹, and ΔHf(C₂H₆) = −85 kJ mol⁻¹, calculate the enthalpy change for the complete combustion of ethane: C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O.
[4 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Work through this mathematical step: Uses ΔHreaction = Σ ΔHf(products) − Σ ΔHf(reactants). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Work through this mathematical step: Calculates the products sum: 2(−394) + 3(−286) = −1646 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Work through this mathematical step: States that O₂ is an element in its standard state (ΔHf = 0), so the reactants sum is just ΔHf(C₂H₆) = −85 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Work through this mathematical step: Obtains ΔHreaction = −1646 − (−85) = −1561 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The 'products minus reactants' formula for enthalpy of formation data is a direct application of Hess's law — always remember to subtract, not add, the reactants sum.
Marking points
- Uses ΔHreaction = Σ ΔHf(products) − Σ ΔHf(reactants).
- Calculates the products sum: 2(−394) + 3(−286) = −1646 kJ mol⁻¹.
- States that O₂ is an element in its standard state (ΔHf = 0), so the reactants sum is just ΔHf(C₂H₆) = −85 kJ mol⁻¹.
- Obtains ΔHreaction = −1646 − (−85) = −1561 kJ mol⁻¹.
Examiner tip: The 'products minus reactants' formula for enthalpy of formation data is a direct application of Hess's law — always remember to subtract, not add, the reactants sum.
10.
Marking analysis: A learner attempts the following task: “Given ΔHf(CO₂) = −394 kJ mol⁻¹, ΔHf(H₂O) = −286 kJ mol⁻¹, and ΔHf(C₂H₆) = −85 kJ mol⁻¹, calculate the enthalpy change for the complete combustion of ethane: C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O.” Their response addresses only this point: “Uses ΔHreaction = Σ ΔHf(products) − Σ ΔHf(reactants).” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: Uses ΔHreaction = Σ ΔHf(products) − Σ ΔHf(reactants). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Calculates the products sum: 2(−394) + 3(−286) = −1646 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that O₂ is an element in its standard state (ΔHf = 0), so the reactants sum is just ΔHf(C₂H₆) = −85 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: Obtains ΔHreaction = −1646 − (−85) = −1561 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: Uses ΔHreaction = Σ ΔHf(products) − Σ ΔHf(reactants).
- Identifies the missing requirement: Calculates the products sum: 2(−394) + 3(−286) = −1646 kJ mol⁻¹.
- Identifies the missing requirement: States that O₂ is an element in its standard state (ΔHf = 0), so the reactants sum is just ΔHf(C₂H₆) = −85 kJ mol⁻¹.
- Identifies the missing requirement: Obtains ΔHreaction = −1646 − (−85) = −1561 kJ mol⁻¹.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
11.
Define bond enthalpy, and outline why using average bond enthalpies to calculate a reaction's enthalpy change gives only an approximate value.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: Defines bond enthalpy as the energy required to break one mole of a specific covalent bond in the gaseous state. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that average bond enthalpies are averaged over many different compounds/environments, so they do not reflect the exact bond strength in any specific molecule. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Bond enthalpy calculations always give approximate answers, unlike Hess's law cycles using enthalpies of formation or combustion, which give exact values from experimental data.
Marking points
- Defines bond enthalpy as the energy required to break one mole of a specific covalent bond in the gaseous state.
- States that average bond enthalpies are averaged over many different compounds/environments, so they do not reflect the exact bond strength in any specific molecule.
Examiner tip: Bond enthalpy calculations always give approximate answers, unlike Hess's law cycles using enthalpies of formation or combustion, which give exact values from experimental data.
12.
Marking analysis: A learner attempts the following task: “Define bond enthalpy, and outline why using average bond enthalpies to calculate a reaction's enthalpy change gives only an approximate value.” Their response addresses only this point: “Defines bond enthalpy as the energy required to break one mole of a specific covalent bond in the gaseous state.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: Defines bond enthalpy as the energy required to break one mole of a specific covalent bond in the gaseous state. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that average bond enthalpies are averaged over many different compounds/environments, so they do not reflect the exact bond strength in any specific molecule. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: Defines bond enthalpy as the energy required to break one mole of a specific covalent bond in the gaseous state.
- Identifies the missing requirement: States that average bond enthalpies are averaged over many different compounds/environments, so they do not reflect the exact bond strength in any specific molecule.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
13.
Calculate the enthalpy change for the reaction H₂(g) + Cl₂(g) → 2HCl(g), given bond enthalpies: H−H = 436 kJ mol⁻¹, Cl−Cl = 242 kJ mol⁻¹, H−Cl = 431 kJ mol⁻¹.
[4 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Work through this mathematical step: Uses ΔH = Σ(bonds broken) − Σ(bonds formed). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Work through this mathematical step: Calculates bonds broken: 436 + 242 = 678 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Work through this mathematical step: Calculates bonds formed: 2 × 431 = 862 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Work through this mathematical step: Obtains ΔH = 678 − 862 = −184 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Breaking bonds always requires energy input (endothermic, positive), and forming bonds always releases energy (exothermic, negative) — 'bonds broken minus bonds formed' captures this correctly.
Marking points
- Uses ΔH = Σ(bonds broken) − Σ(bonds formed).
- Calculates bonds broken: 436 + 242 = 678 kJ mol⁻¹.
- Calculates bonds formed: 2 × 431 = 862 kJ mol⁻¹.
- Obtains ΔH = 678 − 862 = −184 kJ mol⁻¹.
Examiner tip: Breaking bonds always requires energy input (endothermic, positive), and forming bonds always releases energy (exothermic, negative) — 'bonds broken minus bonds formed' captures this correctly.
14.
Marking analysis: A learner attempts the following task: “Calculate the enthalpy change for the reaction H₂(g) + Cl₂(g) → 2HCl(g), given bond enthalpies: H−H = 436 kJ mol⁻¹, Cl−Cl = 242 kJ mol⁻¹, H−Cl = 431 kJ mol⁻¹.” Their response addresses only this point: “Uses ΔH = Σ(bonds broken) − Σ(bonds formed).” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: Uses ΔH = Σ(bonds broken) − Σ(bonds formed). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Calculates bonds broken: 436 + 242 = 678 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: Calculates bonds formed: 2 × 431 = 862 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: Obtains ΔH = 678 − 862 = −184 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: Uses ΔH = Σ(bonds broken) − Σ(bonds formed).
- Identifies the missing requirement: Calculates bonds broken: 436 + 242 = 678 kJ mol⁻¹.
- Identifies the missing requirement: Calculates bonds formed: 2 × 431 = 862 kJ mol⁻¹.
- Identifies the missing requirement: Obtains ΔH = 678 − 862 = −184 kJ mol⁻¹.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
15.
Outline why the enthalpy of combustion of fuels is an important quantity for comparing fuels, and state the units in which it is normally expressed.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that enthalpy of combustion measures the thermal energy released when one mole of a fuel is completely burned in oxygen under standard conditions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that comparing enthalpies of combustion allows fuels to be ranked by the energy they release per mole (or per unit mass), informing practical fuel choice. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States the units as kJ mol⁻¹. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: When comparing fuels for practical use (e.g. per kg), enthalpy of combustion per mole must be converted using the fuel's molar mass, since fuels have very different molar masses.
Marking points
- States that enthalpy of combustion measures the thermal energy released when one mole of a fuel is completely burned in oxygen under standard conditions.
- States that comparing enthalpies of combustion allows fuels to be ranked by the energy they release per mole (or per unit mass), informing practical fuel choice.
- States the units as kJ mol⁻¹.
Examiner tip: When comparing fuels for practical use (e.g. per kg), enthalpy of combustion per mole must be converted using the fuel's molar mass, since fuels have very different molar masses.
16.
Marking analysis: A learner attempts the following task: “Outline why the enthalpy of combustion of fuels is an important quantity for comparing fuels, and state the units in which it is normally expressed.” Their response addresses only this point: “States that enthalpy of combustion measures the thermal energy released when one mole of a fuel is completely burned in oxygen under standard conditions.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: States that enthalpy of combustion measures the thermal energy released when one mole of a fuel is completely burned in oxygen under standard conditions. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that comparing enthalpies of combustion allows fuels to be ranked by the energy they release per mole (or per unit mass), informing practical fuel choice. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States the units as kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: States that enthalpy of combustion measures the thermal energy released when one mole of a fuel is completely burned in oxygen under standard conditions.
- Identifies the missing requirement: States that comparing enthalpies of combustion allows fuels to be ranked by the energy they release per mole (or per unit mass), informing practical fuel choice.
- Identifies the missing requirement: States the units as kJ mol⁻¹.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
17.
Sketch (in words) an enthalpy level diagram for an exothermic reaction, labelling the reactants, products, and the enthalpy change.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the reactants are drawn at a higher energy level than the products, since energy is released overall. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that an arrow points downward from reactants to products. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that ΔH is labelled as negative, representing the enthalpy difference between reactants and products. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: For an endothermic reaction, the diagram is inverted: products sit at a higher energy level than reactants, with ΔH labelled positive.
Marking points
- States that the reactants are drawn at a higher energy level than the products, since energy is released overall.
- States that an arrow points downward from reactants to products.
- States that ΔH is labelled as negative, representing the enthalpy difference between reactants and products.
Examiner tip: For an endothermic reaction, the diagram is inverted: products sit at a higher energy level than reactants, with ΔH labelled positive.
18.
Marking analysis: A learner attempts the following task: “Sketch (in words) an enthalpy level diagram for an exothermic reaction, labelling the reactants, products, and the enthalpy change.” Their response addresses only this point: “States that the reactants are drawn at a higher energy level than the products, since energy is released overall.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: States that the reactants are drawn at a higher energy level than the products, since energy is released overall. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that an arrow points downward from reactants to products. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that ΔH is labelled as negative, representing the enthalpy difference between reactants and products. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: States that the reactants are drawn at a higher energy level than the products, since energy is released overall.
- Identifies the missing requirement: States that an arrow points downward from reactants to products.
- Identifies the missing requirement: States that ΔH is labelled as negative, representing the enthalpy difference between reactants and products.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
19.
In an experiment to measure the enthalpy of combustion of ethanol, a student burns ethanol to heat water in a calorimeter. Suggest two reasons why the experimental value obtained is usually less exothermic (smaller in magnitude) than the accepted literature value.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States heat loss to the surroundings (rather than all being transferred to the water), since the setup is not perfectly insulated. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States incomplete combustion of the ethanol (e.g. producing soot/carbon monoxide instead of only carbon dioxide), which releases less energy than complete combustion. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Experimental enthalpy of combustion values obtained in a simple school laboratory setup are almost always less exothermic than literature values, due to unavoidable heat loss and incomplete combustion.
Marking points
- States heat loss to the surroundings (rather than all being transferred to the water), since the setup is not perfectly insulated.
- States incomplete combustion of the ethanol (e.g. producing soot/carbon monoxide instead of only carbon dioxide), which releases less energy than complete combustion.
Examiner tip: Experimental enthalpy of combustion values obtained in a simple school laboratory setup are almost always less exothermic than literature values, due to unavoidable heat loss and incomplete combustion.
20.
Marking analysis: A learner attempts the following task: “In an experiment to measure the enthalpy of combustion of ethanol, a student burns ethanol to heat water in a calorimeter. Suggest two reasons why the experimental value obtained is usually less exothermic (smaller in magnitude) than the accepted literature value.” Their response addresses only this point: “States heat loss to the surroundings (rather than all being transferred to the water), since the setup is not perfectly insulated.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: States heat loss to the surroundings (rather than all being transferred to the water), since the setup is not perfectly insulated. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States incomplete combustion of the ethanol (e.g. producing soot/carbon monoxide instead of only carbon dioxide), which releases less energy than complete combustion. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: States heat loss to the surroundings (rather than all being transferred to the water), since the setup is not perfectly insulated.
- Identifies the missing requirement: States incomplete combustion of the ethanol (e.g. producing soot/carbon monoxide instead of only carbon dioxide), which releases less energy than complete combustion.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
21.
In an experiment, 25.0 cm³ of 1.00 mol dm⁻³ copper(II) sulfate solution is added to excess zinc powder, and the temperature rises by 10.5°C. Calculate the enthalpy change per mole of copper(II) sulfate. Assume the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.
[5 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Work through this mathematical step: Calculates the mass of solution: 25.0 g. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Work through this mathematical step: Uses q = mcΔT = 25.0 × 4.18 × 10.5. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Develop this part of the answer: Obtains q ≈ 1097 J. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Work through this mathematical step: Calculates moles of CuSO₄ reacted: 0.0250 × 1.00 = 0.0250 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
6. Work through this mathematical step: Obtains ΔH = −1097/0.0250 ≈ −43.9 kJ mol⁻¹ (negative, since the displacement reaction is exothermic). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Since zinc powder is in excess, copper(II) sulfate is the limiting reactant — always divide the heat released by the moles of the limiting reactant, not the excess one.
Marking points
- Calculates the mass of solution: 25.0 g.
- Uses q = mcΔT = 25.0 × 4.18 × 10.5.
- Obtains q ≈ 1097 J.
- Calculates moles of CuSO₄ reacted: 0.0250 × 1.00 = 0.0250 mol.
- Obtains ΔH = −1097/0.0250 ≈ −43.9 kJ mol⁻¹ (negative, since the displacement reaction is exothermic).
Examiner tip: Since zinc powder is in excess, copper(II) sulfate is the limiting reactant — always divide the heat released by the moles of the limiting reactant, not the excess one.
22.
Marking analysis: A learner attempts the following task: “In an experiment, 25.0 cm³ of 1.00 mol dm⁻³ copper(II) sulfate solution is added to excess zinc powder, and the temperature rises by 10.5°C. Calculate the enthalpy change per mole of copper(II) sulfate. Assume the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.” Their response addresses only this point: “Calculates the mass of solution: 25.0 g.” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[5 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: Calculates the mass of solution: 25.0 g. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Uses q = mcΔT = 25.0 × 4.18 × 10.5. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: Obtains q ≈ 1097 J. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: Calculates moles of CuSO₄ reacted: 0.0250 × 1.00 = 0.0250 mol. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
6. Requirement 5: Identifies the missing requirement: Obtains ΔH = −1097/0.0250 ≈ −43.9 kJ mol⁻¹ (negative, since the displacement reaction is exothermic). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: Calculates the mass of solution: 25.0 g.
- Identifies the missing requirement: Uses q = mcΔT = 25.0 × 4.18 × 10.5.
- Identifies the missing requirement: Obtains q ≈ 1097 J.
- Identifies the missing requirement: Calculates moles of CuSO₄ reacted: 0.0250 × 1.00 = 0.0250 mol.
- Identifies the missing requirement: Obtains ΔH = −1097/0.0250 ≈ −43.9 kJ mol⁻¹ (negative, since the displacement reaction is exothermic).
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
23.
Given the standard enthalpies of combustion ΔHc(C, graphite) = −394 kJ mol⁻¹, ΔHc(H₂) = −286 kJ mol⁻¹ and ΔHc(C₂H₅OH) = −1367 kJ mol⁻¹, use Hess's law to calculate the standard enthalpy of formation of ethanol from the elements: 2C(graphite) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l).
[5 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Work through this mathematical step: States that for combustion data, ΔHreaction = Σ ΔHc(reactants) − Σ ΔHc(products). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Work through this mathematical step: Calculates the reactants sum: 2(−394) + 3(−286) = −1646 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Work through this mathematical step: States the products sum as ΔHc(C₂H₅OH) = −1367 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Work through this mathematical step: Substitutes ΔHf = −1646 − (−1367). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
6. Develop this part of the answer: Obtains ΔHf(ethanol) ≈ −279 kJ mol⁻¹. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The combustion-data formula (reactants minus products) is the mirror image of the formation-data formula (products minus reactants) — mixing the two up is a very common error.
Marking points
- States that for combustion data, ΔHreaction = Σ ΔHc(reactants) − Σ ΔHc(products).
- Calculates the reactants sum: 2(−394) + 3(−286) = −1646 kJ mol⁻¹.
- States the products sum as ΔHc(C₂H₅OH) = −1367 kJ mol⁻¹.
- Substitutes ΔHf = −1646 − (−1367).
- Obtains ΔHf(ethanol) ≈ −279 kJ mol⁻¹.
Examiner tip: The combustion-data formula (reactants minus products) is the mirror image of the formation-data formula (products minus reactants) — mixing the two up is a very common error.
24.
Marking analysis: A learner attempts the following task: “Given the standard enthalpies of combustion ΔHc(C, graphite) = −394 kJ mol⁻¹, ΔHc(H₂) = −286 kJ mol⁻¹ and ΔHc(C₂H₅OH) = −1367 kJ mol⁻¹, use Hess's law to calculate the standard enthalpy of formation of ethanol from the elements: 2C(graphite) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l).” Their response addresses only this point: “States that for combustion data, ΔHreaction = Σ ΔHc(reactants) − Σ ΔHc(products).” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[5 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: States that for combustion data, ΔHreaction = Σ ΔHc(reactants) − Σ ΔHc(products). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Calculates the reactants sum: 2(−394) + 3(−286) = −1646 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States the products sum as ΔHc(C₂H₅OH) = −1367 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: Substitutes ΔHf = −1646 − (−1367). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
6. Requirement 5: Identifies the missing requirement: Obtains ΔHf(ethanol) ≈ −279 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: States that for combustion data, ΔHreaction = Σ ΔHc(reactants) − Σ ΔHc(products).
- Identifies the missing requirement: Calculates the reactants sum: 2(−394) + 3(−286) = −1646 kJ mol⁻¹.
- Identifies the missing requirement: States the products sum as ΔHc(C₂H₅OH) = −1367 kJ mol⁻¹.
- Identifies the missing requirement: Substitutes ΔHf = −1646 − (−1367).
- Identifies the missing requirement: Obtains ΔHf(ethanol) ≈ −279 kJ mol⁻¹.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
25.
Define specific heat capacity, and state the equation used to calculate the heat energy change of a substance in terms of mass, specific heat capacity and temperature change.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: Defines specific heat capacity as the energy required to raise the temperature of 1 g (or 1 kg) of a substance by 1°C (or 1 K). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Work through this mathematical step: States the equation q = mcΔT. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Specific heat capacity is a property of the substance itself (e.g. water's is unusually high), which is exactly why the mass and identity of the solution matter in every calorimetry calculation.
Marking points
- Defines specific heat capacity as the energy required to raise the temperature of 1 g (or 1 kg) of a substance by 1°C (or 1 K).
- States the equation q = mcΔT.
Examiner tip: Specific heat capacity is a property of the substance itself (e.g. water's is unusually high), which is exactly why the mass and identity of the solution matter in every calorimetry calculation.
26.
Marking analysis: A learner attempts the following task: “Define specific heat capacity, and state the equation used to calculate the heat energy change of a substance in terms of mass, specific heat capacity and temperature change.” Their response addresses only this point: “Defines specific heat capacity as the energy required to raise the temperature of 1 g (or 1 kg) of a substance by 1°C (or 1 K).” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: Defines specific heat capacity as the energy required to raise the temperature of 1 g (or 1 kg) of a substance by 1°C (or 1 K). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States the equation q = mcΔT. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: Defines specific heat capacity as the energy required to raise the temperature of 1 g (or 1 kg) of a substance by 1°C (or 1 K).
- Identifies the missing requirement: States the equation q = mcΔT.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
27.
Distinguish between a renewable and a non-renewable energy source, giving one example of each, and state one environmental advantage of using renewable sources.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that a renewable energy source can be replenished naturally within a human timescale, e.g. biofuel, solar or wind energy. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that a non-renewable energy source exists in a finite supply and cannot be replenished on a human timescale once used, e.g. coal, oil or natural gas. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States a valid environmental advantage of renewables, e.g. they generally produce little to no direct carbon dioxide emissions during use, reducing contribution to the enhanced greenhouse effect. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The defining difference between renewable and non-renewable sources is the timescale of replenishment compared to the rate of use — not simply whether a source is 'natural' or involves combustion.
Marking points
- States that a renewable energy source can be replenished naturally within a human timescale, e.g. biofuel, solar or wind energy.
- States that a non-renewable energy source exists in a finite supply and cannot be replenished on a human timescale once used, e.g. coal, oil or natural gas.
- States a valid environmental advantage of renewables, e.g. they generally produce little to no direct carbon dioxide emissions during use, reducing contribution to the enhanced greenhouse effect.
Examiner tip: The defining difference between renewable and non-renewable sources is the timescale of replenishment compared to the rate of use — not simply whether a source is 'natural' or involves combustion.
28.
Marking analysis: A learner attempts the following task: “Distinguish between a renewable and a non-renewable energy source, giving one example of each, and state one environmental advantage of using renewable sources.” Their response addresses only this point: “States that a renewable energy source can be replenished naturally within a human timescale, e.g. biofuel, solar or wind energy.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: States that a renewable energy source can be replenished naturally within a human timescale, e.g. biofuel, solar or wind energy. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that a non-renewable energy source exists in a finite supply and cannot be replenished on a human timescale once used, e.g. coal, oil or natural gas. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States a valid environmental advantage of renewables, e.g. they generally produce little to no direct carbon dioxide emissions during use, reducing contribution to the enhanced greenhouse effect. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: States that a renewable energy source can be replenished naturally within a human timescale, e.g. biofuel, solar or wind energy.
- Identifies the missing requirement: States that a non-renewable energy source exists in a finite supply and cannot be replenished on a human timescale once used, e.g. coal, oil or natural gas.
- Identifies the missing requirement: States a valid environmental advantage of renewables, e.g. they generally produce little to no direct carbon dioxide emissions during use, reducing contribution to the enhanced greenhouse effect.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
29.
Explain how an enthalpy cycle can be used to determine the enthalpy change of a reaction that is difficult to measure directly, using the combustion of carbon to carbon monoxide (rather than fully to carbon dioxide) as an example.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the direct reaction C(s) + ½O₂(g) → CO(g) cannot be measured directly in the laboratory, since carbon burning in oxygen always also produces some carbon dioxide. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that an alternative route can be used: burn carbon completely to CO₂ (ΔHc(C), measurable), and separately burn CO completely to CO₂ (ΔHc(CO), measurable). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Work through this mathematical step: States that, by Hess's law, ΔH(C→CO) = ΔHc(C) − ΔHc(CO), since both routes start and end at the same states. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Work through this mathematical step: States that this indirect calculation gives the enthalpy change for the otherwise unmeasurable reaction. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Whenever a reaction cannot be measured directly (often because side reactions or intermediate products interfere), look for two other measurable reactions that connect the same starting and ending substances via Hess's law.
Marking points
- States that the direct reaction C(s) + ½O₂(g) → CO(g) cannot be measured directly in the laboratory, since carbon burning in oxygen always also produces some carbon dioxide.
- States that an alternative route can be used: burn carbon completely to CO₂ (ΔHc(C), measurable), and separately burn CO completely to CO₂ (ΔHc(CO), measurable).
- States that, by Hess's law, ΔH(C→CO) = ΔHc(C) − ΔHc(CO), since both routes start and end at the same states.
- States that this indirect calculation gives the enthalpy change for the otherwise unmeasurable reaction.
Examiner tip: Whenever a reaction cannot be measured directly (often because side reactions or intermediate products interfere), look for two other measurable reactions that connect the same starting and ending substances via Hess's law.
30.
Marking analysis: A learner attempts the following task: “Explain how an enthalpy cycle can be used to determine the enthalpy change of a reaction that is difficult to measure directly, using the combustion of carbon to carbon monoxide (rather than fully to carbon dioxide) as an example.” Their response addresses only this point: “States that the direct reaction C(s) + ½O₂(g) → CO(g) cannot be measured directly in the laboratory, since carbon burning in oxygen always also produces some carbon dioxide.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: States that the direct reaction C(s) + ½O₂(g) → CO(g) cannot be measured directly in the laboratory, since carbon burning in oxygen always also produces some carbon dioxide. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that an alternative route can be used: burn carbon completely to CO₂ (ΔHc(C), measurable), and separately burn CO completely to CO₂ (ΔHc(CO), measurable). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that, by Hess's law, ΔH(C→CO) = ΔHc(C) − ΔHc(CO), since both routes start and end at the same states. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: States that this indirect calculation gives the enthalpy change for the otherwise unmeasurable reaction. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: States that the direct reaction C(s) + ½O₂(g) → CO(g) cannot be measured directly in the laboratory, since carbon burning in oxygen always also produces some carbon dioxide.
- Identifies the missing requirement: States that an alternative route can be used: burn carbon completely to CO₂ (ΔHc(C), measurable), and separately burn CO completely to CO₂ (ΔHc(CO), measurable).
- Identifies the missing requirement: States that, by Hess's law, ΔH(C→CO) = ΔHc(C) − ΔHc(CO), since both routes start and end at the same states.
- Identifies the missing requirement: States that this indirect calculation gives the enthalpy change for the otherwise unmeasurable reaction.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
31.
A student dissolves 3.50 g of KCl(s) in 100.0 g of water in an uninsulated cup. The temperature falls from 22.4 °C to 19.1 °C. Use c = 4.18 J g⁻¹ K⁻¹ and M(KCl) = 74.55 g mol⁻¹, and initially neglect the cup's heat capacity. (a) Calculate the heat change of the water. (b) Determine the molar enthalpy change of solution of KCl. (c) State the sign of ΔH and explain it. (d) Predict how heat entering from the surroundings affects the calculated magnitude.
[4 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Work through this mathematical step: Calculates qwater = mcΔT = 100.0(4.18)(−3.3) = −1.38 kJ. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Work through this mathematical step: Uses n(KCl) = 3.50/74.55 mol and qreaction = +1.38 kJ to obtain ΔHsol ≈ +29.4 kJ mol⁻¹. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Develop this part of the answer: States that ΔH is positive because the dissolving process absorbs heat from the water/endothermic surroundings. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Work through this mathematical step: Predicts that heat entering from the surroundings makes the observed temperature fall too small, so the calculated positive magnitude is underestimated. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Keep the signs of the water and reaction heat changes opposite, and state which mass your heat-capacity model uses.
Marking points
- Calculates qwater = mcΔT = 100.0(4.18)(−3.3) = −1.38 kJ.
- Uses n(KCl) = 3.50/74.55 mol and qreaction = +1.38 kJ to obtain ΔHsol ≈ +29.4 kJ mol⁻¹.
- States that ΔH is positive because the dissolving process absorbs heat from the water/endothermic surroundings.
- Predicts that heat entering from the surroundings makes the observed temperature fall too small, so the calculated positive magnitude is underestimated.
Examiner tip: Keep the signs of the water and reaction heat changes opposite, and state which mass your heat-capacity model uses.
32.
Marking analysis: A learner attempts the following task: “A student dissolves 3.50 g of KCl(s) in 100.0 g of water in an uninsulated cup. The temperature falls from 22.4 °C to 19.1 °C. Use c = 4.18 J g⁻¹ K⁻¹ and M(KCl) = 74.55 g mol⁻¹, and initially neglect the cup's heat capacity. (a) Calculate the heat change of the water. (b) Determine the molar enthalpy change of solution of KCl. (c) State the sign of ΔH and explain it. (d) Predict how heat entering from the surroundings affects the calculated magnitude.” Their response addresses only this point: “Calculates qwater = mcΔT = 100.0(4.18)(−3.3) = −1.38 kJ.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
2. Requirement 1: Recognises credit for the stated point: Calculates qwater = mcΔT = 100.0(4.18)(−3.3) = −1.38 kJ. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Uses n(KCl) = 3.50/74.55 mol and qreaction = +1.38 kJ to obtain ΔHsol ≈ +29.4 kJ mol⁻¹. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that ΔH is positive because the dissolving process absorbs heat from the water/endothermic surroundings. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: Predicts that heat entering from the surroundings makes the observed temperature fall too small, so the calculated positive magnitude is underestimated. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.
Marking points
- Recognises credit for the stated point: Calculates qwater = mcΔT = 100.0(4.18)(−3.3) = −1.38 kJ.
- Identifies the missing requirement: Uses n(KCl) = 3.50/74.55 mol and qreaction = +1.38 kJ to obtain ΔHsol ≈ +29.4 kJ mol⁻¹.
- Identifies the missing requirement: States that ΔH is positive because the dissolving process absorbs heat from the water/endothermic surroundings.
- Identifies the missing requirement: Predicts that heat entering from the surroundings makes the observed temperature fall too small, so the calculated positive magnitude is underestimated.
Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.