School / IB / CHEMISTRY SL / Reactivity 1: what drives chemical reactions Question practice
Reactivity 1: what drives chemical reactions About this practice Measuring enthalpy change, energy cycles, and energy from fuels.
Chemistry SL Reactivity 1: what drives chemical reactions
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1 Distinguish between an exothermic and an endothermic reaction, in terms of the sign of the enthalpy change. Easy 2 marks No calculator + 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. Marking analysis Easy 2 marks No calculator + 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. Medium 5 marks Calculator + 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. Marking analysis Medium 5 marks Calculator + 5 Define standard enthalpy change of formation, and state the standard enthalpy of formation of any element in its standard state. Easy 2 marks No calculator + 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. Marking analysis Easy 2 marks No calculator + 7 State Hess's law, and explain why it is a direct consequence of conservation of energy. Easy 2 marks No calculator + 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. Marking analysis Easy 2 marks No calculator + 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. Medium 4 marks Calculator + 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. Marking analysis Medium 4 marks Calculator + 11 Define bond enthalpy, and outline why using average bond enthalpies to calculate a reaction's enthalpy change gives only an approximate value. Easy 2 marks No calculator + 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. Marking analysis Easy 2 marks No calculator + 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. Medium 4 marks Calculator + 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. Marking analysis Medium 4 marks Calculator + 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. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 17 Sketch (in words) an enthalpy level diagram for an exothermic reaction, labelling the reactants, products, and the enthalpy change. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 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. Easy 2 marks No calculator + 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. Marking analysis Easy 2 marks No calculator + 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. Medium 5 marks Calculator + 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. Marking analysis Medium 5 marks Calculator + 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). Medium 5 marks Calculator + 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. Marking analysis Medium 5 marks Calculator + 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. Easy 2 marks No calculator + 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. Marking analysis Easy 2 marks No calculator + 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. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 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. Medium 4 marks No calculator + 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. Marking analysis Medium 4 marks No calculator + 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. Medium 4 marks Calculator + 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. Marking analysis Medium 4 marks Calculator + Self-assessed Not marked yet
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