IB · CHEMISTRY SL

Chemistry SL

Reactivity 3: mechanisms of reaction — Reactivity 3

Name: ____________________Date: October 10, 2026

1.
Define an acid and a base according to the Brønsted-Lowry theory, in terms of proton transfer.
[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 a Brønsted-Lowry acid as a proton (H⁺) donor. 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: Defines a Brønsted-Lowry base as a proton (H⁺) acceptor. 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: In any acid-base reaction, identify which species loses a proton (the acid) and which gains one (the base) to apply Brønsted-Lowry theory correctly.
Marking points
- Defines a Brønsted-Lowry acid as a proton (H⁺) donor.
- Defines a Brønsted-Lowry base as a proton (H⁺) acceptor.
Examiner tip: In any acid-base reaction, identify which species loses a proton (the acid) and which gains one (the base) to apply Brønsted-Lowry theory correctly.
2.
Marking analysis: A learner attempts the following task: “Define an acid and a base according to the Brønsted-Lowry theory, in terms of proton transfer.” Their response addresses only this point: “Defines a Brønsted-Lowry acid as a proton (H⁺) donor.” 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 a Brønsted-Lowry acid as a proton (H⁺) donor. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Defines a Brønsted-Lowry base as a proton (H⁺) acceptor. 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 a Brønsted-Lowry acid as a proton (H⁺) donor.
- Identifies the missing requirement: Defines a Brønsted-Lowry base as a proton (H⁺) acceptor.
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.
For the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the two conjugate acid-base pairs.
[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: Identifies NH₃ and NH₄⁺ as one conjugate pair, with NH₃ the base and NH₄⁺ its conjugate acid. 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: Identifies H₂O and OH⁻ as the other conjugate pair, with H₂O the acid and OH⁻ its conjugate base. 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 a conjugate pair differs by exactly one proton (H⁺). 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: A conjugate acid-base pair always differs by exactly one H⁺ — the acid has one more proton than its conjugate base.
Marking points
- Identifies NH₃ and NH₄⁺ as one conjugate pair, with NH₃ the base and NH₄⁺ its conjugate acid.
- Identifies H₂O and OH⁻ as the other conjugate pair, with H₂O the acid and OH⁻ its conjugate base.
- States that a conjugate pair differs by exactly one proton (H⁺).
Examiner tip: A conjugate acid-base pair always differs by exactly one H⁺ — the acid has one more proton than its conjugate base.
4.
Marking analysis: A learner attempts the following task: “For the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the two conjugate acid-base pairs.” Their response addresses only this point: “Identifies NH₃ and NH₄⁺ as one conjugate pair, with NH₃ the base and NH₄⁺ its conjugate acid.” 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: Identifies NH₃ and NH₄⁺ as one conjugate pair, with NH₃ the base and NH₄⁺ its conjugate acid. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Identifies H₂O and OH⁻ as the other conjugate pair, with H₂O the acid and OH⁻ its conjugate base. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that a conjugate pair differs by exactly one proton (H⁺). 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: Identifies NH₃ and NH₄⁺ as one conjugate pair, with NH₃ the base and NH₄⁺ its conjugate acid.
- Identifies the missing requirement: Identifies H₂O and OH⁻ as the other conjugate pair, with H₂O the acid and OH⁻ its conjugate base.
- Identifies the missing requirement: States that a conjugate pair differs by exactly one proton (H⁺).
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.
Calculate the pH of a 0.0500 mol dm⁻³ solution of a strong acid, HCl, assuming complete dissociation.
[3 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 a strong monoprotic acid, [H⁺] equals the acid concentration: [H⁺] = 0.0500 mol dm⁻³. 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 pH = −log₁₀[H⁺]. 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 pH ≈ 1.30. 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 strong monoprotic acids, dissociation is assumed complete, so [H⁺] can be read directly from the given concentration with no equilibrium calculation needed.
Marking points
- States that for a strong monoprotic acid, [H⁺] equals the acid concentration: [H⁺] = 0.0500 mol dm⁻³.
- Uses pH = −log₁₀[H⁺].
- Obtains pH ≈ 1.30.
Examiner tip: For strong monoprotic acids, dissociation is assumed complete, so [H⁺] can be read directly from the given concentration with no equilibrium calculation needed.
6.
Marking analysis: A learner attempts the following task: “Calculate the pH of a 0.0500 mol dm⁻³ solution of a strong acid, HCl, assuming complete dissociation.” Their response addresses only this point: “States that for a strong monoprotic acid, [H⁺] equals the acid concentration: [H⁺] = 0.0500 mol dm⁻³.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 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 a strong monoprotic acid, [H⁺] equals the acid concentration: [H⁺] = 0.0500 mol dm⁻³. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Uses pH = −log₁₀[H⁺]. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: Obtains pH ≈ 1.30. 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 for a strong monoprotic acid, [H⁺] equals the acid concentration: [H⁺] = 0.0500 mol dm⁻³.
- Identifies the missing requirement: Uses pH = −log₁₀[H⁺].
- Identifies the missing requirement: Obtains pH ≈ 1.30.
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.
Define oxidation and reduction in terms of electron transfer, and use these definitions to explain why oxidation and reduction always occur together in a redox reaction.
[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: Defines oxidation as the loss of electrons. 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: Defines reduction as the gain of electrons. 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 electrons lost by the species being oxidised must be gained by another species (which is thereby reduced), so oxidation and reduction always occur simultaneously in a redox reaction. 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: Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons) — the two processes are always linked since electrons cannot simply disappear.
Marking points
- Defines oxidation as the loss of electrons.
- Defines reduction as the gain of electrons.
- States that electrons lost by the species being oxidised must be gained by another species (which is thereby reduced), so oxidation and reduction always occur simultaneously in a redox reaction.
Examiner tip: Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons) — the two processes are always linked since electrons cannot simply disappear.
8.
Marking analysis: A learner attempts the following task: “Define oxidation and reduction in terms of electron transfer, and use these definitions to explain why oxidation and reduction always occur together in a redox reaction.” Their response addresses only this point: “Defines oxidation as the loss of electrons.” 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: Defines oxidation as the loss of electrons. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Defines reduction as the gain of electrons. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that electrons lost by the species being oxidised must be gained by another species (which is thereby reduced), so oxidation and reduction always occur simultaneously in a redox reaction. 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: Defines oxidation as the loss of electrons.
- Identifies the missing requirement: Defines reduction as the gain of electrons.
- Identifies the missing requirement: States that electrons lost by the species being oxidised must be gained by another species (which is thereby reduced), so oxidation and reduction always occur simultaneously in a redox 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.
9.
Deduce the oxidation state of chromium in the dichromate ion, Cr₂O₇²⁻.
[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 oxygen has oxidation state −2, so 7 oxygens contribute −14. 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 overall ion charge is −2, so the total oxidation states must sum to −2. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Work through this mathematical step: Sets up 2x + (−14) = −2, giving x = +6 for each chromium. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Oxidation states in a neutral compound sum to zero; in an ion, they sum to the overall charge of the ion — set up and solve this equation systematically.
Marking points
- States that oxygen has oxidation state −2, so 7 oxygens contribute −14.
- States that the overall ion charge is −2, so the total oxidation states must sum to −2.
- Sets up 2x + (−14) = −2, giving x = +6 for each chromium.
Examiner tip: Oxidation states in a neutral compound sum to zero; in an ion, they sum to the overall charge of the ion — set up and solve this equation systematically.
10.
Marking analysis: A learner attempts the following task: “Deduce the oxidation state of chromium in the dichromate ion, Cr₂O₇²⁻.” Their response addresses only this point: “States that oxygen has oxidation state −2, so 7 oxygens contribute −14.” 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 oxygen has oxidation state −2, so 7 oxygens contribute −14. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that the overall ion charge is −2, so the total oxidation states must sum to −2. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: Sets up 2x + (−14) = −2, giving x = +6 for each chromium. 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 oxygen has oxidation state −2, so 7 oxygens contribute −14.
- Identifies the missing requirement: States that the overall ion charge is −2, so the total oxidation states must sum to −2.
- Identifies the missing requirement: Sets up 2x + (−14) = −2, giving x = +6 for each chromium.
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.
Write the half-equations for the oxidation of Fe²⁺ to Fe³⁺ and the reduction of MnO₄⁻ to Mn²⁺ in acidic solution, then combine them into the overall balanced ionic equation.
[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. Work through this mathematical step: Writes the oxidation half-equation: Fe²⁺ → Fe³⁺ + e⁻. 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: Writes the reduction half-equation: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. 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: Multiplies the oxidation half-equation by 5 to balance electrons transferred. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Develop this part of the answer: Combines to obtain: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Always balance the number of electrons lost and gained between the two half-equations before combining them — this determines the multiplying factor needed.
Marking points
- Writes the oxidation half-equation: Fe²⁺ → Fe³⁺ + e⁻.
- Writes the reduction half-equation: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.
- Multiplies the oxidation half-equation by 5 to balance electrons transferred.
- Combines to obtain: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺.
Examiner tip: Always balance the number of electrons lost and gained between the two half-equations before combining them — this determines the multiplying factor needed.
12.
Marking analysis: A learner attempts the following task: “Write the half-equations for the oxidation of Fe²⁺ to Fe³⁺ and the reduction of MnO₄⁻ to Mn²⁺ in acidic solution, then combine them into the overall balanced ionic equation.” Their response addresses only this point: “Writes the oxidation half-equation: Fe²⁺ → Fe³⁺ + e⁻.” 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: Writes the oxidation half-equation: Fe²⁺ → Fe³⁺ + e⁻. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Writes the reduction half-equation: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: Multiplies the oxidation half-equation by 5 to balance electrons transferred. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: Combines to obtain: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺. 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: Writes the oxidation half-equation: Fe²⁺ → Fe³⁺ + e⁻.
- Identifies the missing requirement: Writes the reduction half-equation: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.
- Identifies the missing requirement: Multiplies the oxidation half-equation by 5 to balance electrons transferred.
- Identifies the missing requirement: Combines to obtain: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺.
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.
Ethene, C₂H₄, reacts with hydrogen bromide, HBr, in an addition reaction. Outline the mechanism, identifying the type of mechanism involved.
[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 this is an electrophilic addition mechanism. 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 that the electron-rich C=C double bond is attracted to and attacks the electrophilic (slightly positive) hydrogen of the polar H−Br bond. 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 this forms a carbocation intermediate and a bromide ion. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States that the bromide ion then acts as a nucleophile, bonding to the positively charged carbon to form the final product, bromoethane. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Alkenes undergo electrophilic addition due to their electron-rich double bond, which attracts electron-deficient (electrophilic) species — this is the defining reaction mechanism of the alkene functional group.
Marking points
- States that this is an electrophilic addition mechanism.
- States that the electron-rich C=C double bond is attracted to and attacks the electrophilic (slightly positive) hydrogen of the polar H−Br bond.
- States that this forms a carbocation intermediate and a bromide ion.
- States that the bromide ion then acts as a nucleophile, bonding to the positively charged carbon to form the final product, bromoethane.
Examiner tip: Alkenes undergo electrophilic addition due to their electron-rich double bond, which attracts electron-deficient (electrophilic) species — this is the defining reaction mechanism of the alkene functional group.
14.
Marking analysis: A learner attempts the following task: “Ethene, C₂H₄, reacts with hydrogen bromide, HBr, in an addition reaction. Outline the mechanism, identifying the type of mechanism involved.” Their response addresses only this point: “States that this is an electrophilic addition mechanism.” 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 this is an electrophilic addition mechanism. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that the electron-rich C=C double bond is attracted to and attacks the electrophilic (slightly positive) hydrogen of the polar H−Br bond. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that this forms a carbocation intermediate and a bromide ion. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: States that the bromide ion then acts as a nucleophile, bonding to the positively charged carbon to form the final product, bromoethane. 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 this is an electrophilic addition mechanism.
- Identifies the missing requirement: States that the electron-rich C=C double bond is attracted to and attacks the electrophilic (slightly positive) hydrogen of the polar H−Br bond.
- Identifies the missing requirement: States that this forms a carbocation intermediate and a bromide ion.
- Identifies the missing requirement: States that the bromide ion then acts as a nucleophile, bonding to the positively charged carbon to form the final product, bromoethane.
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.
Methane reacts with chlorine in the presence of ultraviolet light in a substitution reaction. State the type of mechanism, and outline the initiation step.
[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. Work through this mathematical step: States that this is a free-radical substitution mechanism. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Develop this part of the answer: States that in the initiation step, ultraviolet light provides enough energy to break the Cl−Cl bond homolytically, forming two chlorine radicals (each with one unpaired electron). 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: Homolytic bond breaking (each atom keeps one electron from the shared pair) produces radicals, unlike heterolytic breaking (one atom keeps both electrons), which produces ions.
Marking points
- States that this is a free-radical substitution mechanism.
- States that in the initiation step, ultraviolet light provides enough energy to break the Cl−Cl bond homolytically, forming two chlorine radicals (each with one unpaired electron).
Examiner tip: Homolytic bond breaking (each atom keeps one electron from the shared pair) produces radicals, unlike heterolytic breaking (one atom keeps both electrons), which produces ions.
16.
Marking analysis: A learner attempts the following task: “Methane reacts with chlorine in the presence of ultraviolet light in a substitution reaction. State the type of mechanism, and outline the initiation step.” Their response addresses only this point: “States that this is a free-radical substitution mechanism.” 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 this is a free-radical substitution mechanism. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that in the initiation step, ultraviolet light provides enough energy to break the Cl−Cl bond homolytically, forming two chlorine radicals (each with one unpaired electron). 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 this is a free-radical substitution mechanism.
- Identifies the missing requirement: States that in the initiation step, ultraviolet light provides enough energy to break the Cl−Cl bond homolytically, forming two chlorine radicals (each with one unpaired electron).
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.
Explain why sulfuric acid, H₂SO₄, is described as a diprotic acid, and state the number of moles of NaOH required to fully neutralise 1 mole of H₂SO₄.
[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 diprotic acid can donate two protons (H⁺ ions) per molecule. 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 H₂SO₄ contains two ionisable hydrogen atoms, each capable of being donated to a base. 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 2 moles of NaOH are required to fully neutralise 1 mole of H₂SO₄. 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 number of acidic protons an acid can donate (monoprotic, diprotic, triprotic) directly determines the stoichiometric ratio needed for complete neutralisation.
Marking points
- States that a diprotic acid can donate two protons (H⁺ ions) per molecule.
- States that H₂SO₄ contains two ionisable hydrogen atoms, each capable of being donated to a base.
- States that 2 moles of NaOH are required to fully neutralise 1 mole of H₂SO₄.
Examiner tip: The number of acidic protons an acid can donate (monoprotic, diprotic, triprotic) directly determines the stoichiometric ratio needed for complete neutralisation.
18.
Marking analysis: A learner attempts the following task: “Explain why sulfuric acid, H₂SO₄, is described as a diprotic acid, and state the number of moles of NaOH required to fully neutralise 1 mole of H₂SO₄.” Their response addresses only this point: “States that a diprotic acid can donate two protons (H⁺ ions) per molecule.” 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 diprotic acid can donate two protons (H⁺ ions) per molecule. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that H₂SO₄ contains two ionisable hydrogen atoms, each capable of being donated to a base. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that 2 moles of NaOH are required to fully neutralise 1 mole of H₂SO₄. 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 diprotic acid can donate two protons (H⁺ ions) per molecule.
- Identifies the missing requirement: States that H₂SO₄ contains two ionisable hydrogen atoms, each capable of being donated to a base.
- Identifies the missing requirement: States that 2 moles of NaOH are required to fully neutralise 1 mole of H₂SO₄.
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.
Describe a chemical test to distinguish between a solution containing Fe²⁺ ions and one containing Fe³⁺ ions, in terms of a redox reaction with acidified potassium manganate(VII).
[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 acidified potassium manganate(VII) (purple) is added dropwise to each solution. 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 with Fe²⁺, the purple colour decolourises (is reduced to pale/colourless Mn²⁺), since Fe²⁺ can be oxidised to Fe³⁺ and acts as a reducing agent. 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 with Fe³⁺, the purple colour persists (is not decolourised), since Fe³⁺ is already in its oxidised state and cannot reduce the manganate(VII) further. 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: Potassium manganate(VII) is a strong oxidising agent and a classic redox indicator: it decolourises when it oxidises a reducing agent, and stays purple in the presence of a species that cannot reduce it.
Marking points
- States that acidified potassium manganate(VII) (purple) is added dropwise to each solution.
- States that with Fe²⁺, the purple colour decolourises (is reduced to pale/colourless Mn²⁺), since Fe²⁺ can be oxidised to Fe³⁺ and acts as a reducing agent.
- States that with Fe³⁺, the purple colour persists (is not decolourised), since Fe³⁺ is already in its oxidised state and cannot reduce the manganate(VII) further.
Examiner tip: Potassium manganate(VII) is a strong oxidising agent and a classic redox indicator: it decolourises when it oxidises a reducing agent, and stays purple in the presence of a species that cannot reduce it.
20.
Marking analysis: A learner attempts the following task: “Describe a chemical test to distinguish between a solution containing Fe²⁺ ions and one containing Fe³⁺ ions, in terms of a redox reaction with acidified potassium manganate(VII).” Their response addresses only this point: “States that acidified potassium manganate(VII) (purple) is added dropwise to each solution.” 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 acidified potassium manganate(VII) (purple) is added dropwise to each solution. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that with Fe²⁺, the purple colour decolourises (is reduced to pale/colourless Mn²⁺), since Fe²⁺ can be oxidised to Fe³⁺ and acts as a reducing agent. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that with Fe³⁺, the purple colour persists (is not decolourised), since Fe³⁺ is already in its oxidised state and cannot reduce the manganate(VII) further. 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 acidified potassium manganate(VII) (purple) is added dropwise to each solution.
- Identifies the missing requirement: States that with Fe²⁺, the purple colour decolourises (is reduced to pale/colourless Mn²⁺), since Fe²⁺ can be oxidised to Fe³⁺ and acts as a reducing agent.
- Identifies the missing requirement: States that with Fe³⁺, the purple colour persists (is not decolourised), since Fe³⁺ is already in its oxidised state and cannot reduce the manganate(VII) further.
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.
Ethanoic acid has Ka = 1.8 × 10⁻⁵. Calculate the pH of a 0.0500 mol dm⁻³ solution of ethanoic acid, stating one assumption made.
[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. Develop this part of the answer: Sets up Ka ≈ [H⁺]²/[HA]₀, assuming [H⁺] is small compared to the initial acid concentration. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Work through this mathematical step: Rearranges: [H⁺] = √(Ka × [HA]₀) = √(1.8 × 10⁻⁵ × 0.0500). 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 [H⁺] ≈ 9.49 × 10⁻⁴ mol dm⁻³. 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: Uses pH = −log₁₀[H⁺] to obtain pH ≈ 3.02. 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: Unlike a strong acid, a weak acid's [H⁺] cannot be read directly from its concentration — it must be calculated from Ka, since only a small fraction of the acid actually dissociates.
Marking points
- Sets up Ka ≈ [H⁺]²/[HA]₀, assuming [H⁺] is small compared to the initial acid concentration.
- Rearranges: [H⁺] = √(Ka × [HA]₀) = √(1.8 × 10⁻⁵ × 0.0500).
- Calculates [H⁺] ≈ 9.49 × 10⁻⁴ mol dm⁻³.
- Uses pH = −log₁₀[H⁺] to obtain pH ≈ 3.02.
Examiner tip: Unlike a strong acid, a weak acid's [H⁺] cannot be read directly from its concentration — it must be calculated from Ka, since only a small fraction of the acid actually dissociates.
22.
Marking analysis: A learner attempts the following task: “Ethanoic acid has Ka = 1.8 × 10⁻⁵. Calculate the pH of a 0.0500 mol dm⁻³ solution of ethanoic acid, stating one assumption made.” Their response addresses only this point: “Sets up Ka ≈ [H⁺]²/[HA]₀, assuming [H⁺] is small compared to the initial acid concentration.” 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: Sets up Ka ≈ [H⁺]²/[HA]₀, assuming [H⁺] is small compared to the initial acid concentration. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Rearranges: [H⁺] = √(Ka × [HA]₀) = √(1.8 × 10⁻⁵ × 0.0500). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: Calculates [H⁺] ≈ 9.49 × 10⁻⁴ mol dm⁻³. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: Uses pH = −log₁₀[H⁺] to obtain pH ≈ 3.02. 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: Sets up Ka ≈ [H⁺]²/[HA]₀, assuming [H⁺] is small compared to the initial acid concentration.
- Identifies the missing requirement: Rearranges: [H⁺] = √(Ka × [HA]₀) = √(1.8 × 10⁻⁵ × 0.0500).
- Identifies the missing requirement: Calculates [H⁺] ≈ 9.49 × 10⁻⁴ mol dm⁻³.
- Identifies the missing requirement: Uses pH = −log₁₀[H⁺] to obtain pH ≈ 3.02.
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.
Bromoethane reacts with aqueous sodium hydroxide in a nucleophilic substitution reaction to form ethanol. Outline the mechanism, identifying the nucleophile and stating why the carbon-bromine bond breaks heterolytically.
[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 hydroxide ion, OH⁻, acts as the nucleophile, attracted to the slightly positive (δ+) carbon atom bonded to the electronegative bromine. 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 C−Br bond is polar, since bromine is more electronegative than carbon. 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 the OH⁻ ion's lone pair forms a new bond to the carbon while the C−Br bond breaks heterolytically, with both electrons going to the bromine atom, forming a bromide ion. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States the organic product formed as ethanol, C₂H₅OH. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Nucleophilic substitution in haloalkanes always starts with the polar C-halogen bond: the more electronegative halogen leaves with both bonding electrons (heterolytic fission), which is exactly why a nucleophile (electron-pair donor) is needed to attack the electron-poor carbon.
Marking points
- States that the hydroxide ion, OH⁻, acts as the nucleophile, attracted to the slightly positive (δ+) carbon atom bonded to the electronegative bromine.
- States that the C−Br bond is polar, since bromine is more electronegative than carbon.
- States that the OH⁻ ion's lone pair forms a new bond to the carbon while the C−Br bond breaks heterolytically, with both electrons going to the bromine atom, forming a bromide ion.
- States the organic product formed as ethanol, C₂H₅OH.
Examiner tip: Nucleophilic substitution in haloalkanes always starts with the polar C-halogen bond: the more electronegative halogen leaves with both bonding electrons (heterolytic fission), which is exactly why a nucleophile (electron-pair donor) is needed to attack the electron-poor carbon.
24.
Marking analysis: A learner attempts the following task: “Bromoethane reacts with aqueous sodium hydroxide in a nucleophilic substitution reaction to form ethanol. Outline the mechanism, identifying the nucleophile and stating why the carbon-bromine bond breaks heterolytically.” Their response addresses only this point: “States that the hydroxide ion, OH⁻, acts as the nucleophile, attracted to the slightly positive (δ+) carbon atom bonded to the electronegative bromine.” 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 hydroxide ion, OH⁻, acts as the nucleophile, attracted to the slightly positive (δ+) carbon atom bonded to the electronegative bromine. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that the C−Br bond is polar, since bromine is more electronegative than carbon. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that the OH⁻ ion's lone pair forms a new bond to the carbon while the C−Br bond breaks heterolytically, with both electrons going to the bromine atom, forming a bromide ion. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
5. Requirement 4: Identifies the missing requirement: States the organic product formed as ethanol, C₂H₅OH. 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 hydroxide ion, OH⁻, acts as the nucleophile, attracted to the slightly positive (δ+) carbon atom bonded to the electronegative bromine.
- Identifies the missing requirement: States that the C−Br bond is polar, since bromine is more electronegative than carbon.
- Identifies the missing requirement: States that the OH⁻ ion's lone pair forms a new bond to the carbon while the C−Br bond breaks heterolytically, with both electrons going to the bromine atom, forming a bromide ion.
- Identifies the missing requirement: States the organic product formed as ethanol, C₂H₅OH.
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.
State what is measured by a standard electrode potential, and state the reference value assigned to the standard hydrogen electrode.
[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 a standard electrode potential measures the tendency of a half-cell to undergo reduction, relative to a standard reference, 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 hydrogen electrode is assigned a standard electrode potential of exactly 0.00 V by convention. 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: Only the relative values of electrode potentials matter for predicting whether a redox reaction is feasible — the hydrogen electrode's 0.00 V is simply an agreed reference point, not a claim about its actual reactivity.
Marking points
- States that a standard electrode potential measures the tendency of a half-cell to undergo reduction, relative to a standard reference, under standard conditions.
- States that the standard hydrogen electrode is assigned a standard electrode potential of exactly 0.00 V by convention.
Examiner tip: Only the relative values of electrode potentials matter for predicting whether a redox reaction is feasible — the hydrogen electrode's 0.00 V is simply an agreed reference point, not a claim about its actual reactivity.
26.
Marking analysis: A learner attempts the following task: “State what is measured by a standard electrode potential, and state the reference value assigned to the standard hydrogen electrode.” Their response addresses only this point: “States that a standard electrode potential measures the tendency of a half-cell to undergo reduction, relative to a standard reference, 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: States that a standard electrode potential measures the tendency of a half-cell to undergo reduction, relative to a standard reference, 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 hydrogen electrode is assigned a standard electrode potential of exactly 0.00 V by convention. 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 a standard electrode potential measures the tendency of a half-cell to undergo reduction, relative to a standard reference, under standard conditions.
- Identifies the missing requirement: States that the standard hydrogen electrode is assigned a standard electrode potential of exactly 0.00 V by convention.
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.
State what is meant by a buffer solution, and outline how a mixture of a weak acid and its conjugate base can resist changes in pH when a small amount of acid is added.
[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 buffer solution resists changes in pH when small amounts of acid or base are added. 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 mixture contains a significant reservoir of both the weak acid and its conjugate base. 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 added H⁺ ions react with (are consumed by) the conjugate base, so the free [H⁺] and hence the pH change only slightly. 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: A buffer needs comparable amounts of both a weak acid and its conjugate base, so that it can neutralise small additions of either H⁺ or OH⁻ in either direction.
Marking points
- States that a buffer solution resists changes in pH when small amounts of acid or base are added.
- States that the mixture contains a significant reservoir of both the weak acid and its conjugate base.
- States that added H⁺ ions react with (are consumed by) the conjugate base, so the free [H⁺] and hence the pH change only slightly.
Examiner tip: A buffer needs comparable amounts of both a weak acid and its conjugate base, so that it can neutralise small additions of either H⁺ or OH⁻ in either direction.
28.
Marking analysis: A learner attempts the following task: “State what is meant by a buffer solution, and outline how a mixture of a weak acid and its conjugate base can resist changes in pH when a small amount of acid is added.” Their response addresses only this point: “States that a buffer solution resists changes in pH when small amounts of acid or base are added.” 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 buffer solution resists changes in pH when small amounts of acid or base are added. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: States that the mixture contains a significant reservoir of both the weak acid and its conjugate base. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: States that added H⁺ ions react with (are consumed by) the conjugate base, so the free [H⁺] and hence the pH change only slightly. 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 buffer solution resists changes in pH when small amounts of acid or base are added.
- Identifies the missing requirement: States that the mixture contains a significant reservoir of both the weak acid and its conjugate base.
- Identifies the missing requirement: States that added H⁺ ions react with (are consumed by) the conjugate base, so the free [H⁺] and hence the pH change only slightly.
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.
State what is meant by an amphiprotic species, and explain, using equations, how the hydrogencarbonate ion, HCO₃⁻, can act as both a Brønsted-Lowry acid and a base.
[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 an amphiprotic species can act as either an acid (proton donor) or a base (proton acceptor), depending on what it reacts with. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Work through this mathematical step: Writes an equation showing HCO₃⁻ acting as an acid, donating a proton: HCO₃⁻ → CO₃²⁻ + H⁺. 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: Writes an equation showing HCO₃⁻ acting as a base, accepting a proton: HCO₃⁻ + H⁺ → H₂CO₃. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: An amphiprotic species must have both a proton it can donate and a lone pair (or negative charge) capable of accepting one — HCO₃⁻ satisfies both, which is exactly why it plays a central role in blood's natural buffering system.
Marking points
- States that an amphiprotic species can act as either an acid (proton donor) or a base (proton acceptor), depending on what it reacts with.
- Writes an equation showing HCO₃⁻ acting as an acid, donating a proton: HCO₃⁻ → CO₃²⁻ + H⁺.
- Writes an equation showing HCO₃⁻ acting as a base, accepting a proton: HCO₃⁻ + H⁺ → H₂CO₃.
Examiner tip: An amphiprotic species must have both a proton it can donate and a lone pair (or negative charge) capable of accepting one — HCO₃⁻ satisfies both, which is exactly why it plays a central role in blood's natural buffering system.
30.
Marking analysis: A learner attempts the following task: “State what is meant by an amphiprotic species, and explain, using equations, how the hydrogencarbonate ion, HCO₃⁻, can act as both a Brønsted-Lowry acid and a base.” Their response addresses only this point: “States that an amphiprotic species can act as either an acid (proton donor) or a base (proton acceptor), depending on what it reacts with.” 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 an amphiprotic species can act as either an acid (proton donor) or a base (proton acceptor), depending on what it reacts with. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
3. Requirement 2: Identifies the missing requirement: Writes an equation showing HCO₃⁻ acting as an acid, donating a proton: HCO₃⁻ → CO₃²⁻ + H⁺. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
4. Requirement 3: Identifies the missing requirement: Writes an equation showing HCO₃⁻ acting as a base, accepting a proton: HCO₃⁻ + H⁺ → H₂CO₃. 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 an amphiprotic species can act as either an acid (proton donor) or a base (proton acceptor), depending on what it reacts with.
- Identifies the missing requirement: Writes an equation showing HCO₃⁻ acting as an acid, donating a proton: HCO₃⁻ → CO₃²⁻ + H⁺.
- Identifies the missing requirement: Writes an equation showing HCO₃⁻ acting as a base, accepting a proton: HCO₃⁻ + H⁺ → H₂CO₃.
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.