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S5 Chemistry: Enthalpy Change and Energy (Paper 3: Challenge)
S5 Chemistry: Enthalpy Change and Energy (Paper 3: Challenge)
Ten challenge items: limits of the bond energy method, the definition of enthalpy of combustion, calorimeter calibration, back-calculation of neutralisation and combustion enthalpies, reverse activation energy and catalysts, with answers.
Direct answers Answers in order: gaseous molecules, liquid water, no direct link, 240 J K-1, -52.08 kJ mol-1, -630 kJ mol-1, -802 kJ mol-1, 430 kJ mol-1, -1050 kJ mol-1, no (DH is unchanged).
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S5 Chemistry: Enthalpy Change and Energy |
| Level | S5 |
02 Questions (10 items)
- FoundationWhen average bond energies are used to find an enthalpy change, to which state of the molecules do they apply?
- FoundationIn the definition of enthalpy of combustion, what state must the water formed be in?
- FoundationIs there a direct link between the enthalpy change of a reaction and its rate?
- IntermediateA 20 W heater runs for 60 s and raises the temperature inside a calorimeter by 5.0 K. What is the heat capacity of the calorimeter?
- Intermediate50.0 cm3 of 0.50 M hydrochloric acid is mixed with 50.0 cm3 of 0.50 M sodium hydroxide solution and the temperature rises by 3.1 °C. Find the enthalpy of neutralisation.
- IntermediateBurning 0.92 g of ethanol (molar mass 46 g mol-1) raises 200 g of water by 15.0 °C. Find the enthalpy change of combustion of ethanol.
- ChallengeGiven bond energies C-H 413, O=O 498, C=O 799 and O-H 463 kJ mol-1, find ΔH for CH4 + 2O2 -> CO2 + 2H2O.
- ChallengeThe forward reaction has an activation energy of 180 kJ mol-1 and ΔH is -250 kJ mol-1. What is the activation energy of the reverse reaction?
- ChallengeIn a calorimetry experiment only 60% of the heat of combustion is absorbed by the water. The water gains 12.6 kJ while 0.020 mol of fuel burns. Find the corrected enthalpy of combustion.
- Error redoA student claims a catalyst makes a reaction release more heat so that ΔH becomes more negative. Is that right?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Gaseous molecules | Tabulated bond energies are averages for gaseous molecules; condensed phases need intermolecular effects added separately. |
| 2 | Liquid water | The definition specifies liquid water; writing steam would leave out the enthalpy of vaporisation. |
| 3 | No: DH describes the energy difference, rate describes how fast it goes | DH depends on the initial and final states, while the rate depends on activation energy and concentration. |
| 4 | 240 J K-1 | The energy supplied is 20 times 60, which is 1200 J, and dividing by 5.0 K gives 240 J K-1. |
| 5 | -52.08 kJ mol-1 | The solution is 100 g, so q = 100 times 4.2 times 3.1 gives 1302 J; 0.025 mol of water forms, so ΔH is -52.08 kJ mol-1. |
| 6 | -630 kJ mol-1 | 0.020 mol of ethanol burns and the water absorbs 12600 J, that is 12.6 kJ; dividing by 0.020 gives 630 kJ mol-1 and the sign is negative because heat is released. |
| 7 | -802 kJ mol-1 | Breaking bonds needs 4 times 413 plus 2 times 498, which is 2648; forming bonds releases 2 times 799 plus 4 times 463, which is 3450; the difference is -802 kJ mol-1. |
| 8 | 430 kJ mol-1 | The reverse activation energy equals the forward activation energy plus the heat released, so 180 plus 250 gives 430 kJ mol-1. |
| 9 | -1050 kJ mol-1 | Total heat released is 12.6 divided by 0.60, which is 21 kJ, and per mole that is -1050 kJ mol-1 with a negative sign. |
| 10 | No, a catalyst lowers the activation energy but leaves ΔH unchanged | A catalyst changes the pathway only; the enthalpy difference between the initial and final states stays the same. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Sketch the energy diagram first, then decide whether the terms add or subtract.Tutor Registration →
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