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S5 Chemistry: Enthalpy Change and Energy (Paper 2: Intermediate)
S5 Chemistry: Enthalpy Change and Energy (Paper 2: Intermediate)
Ten intermediate items: standard conditions, per-mole conversion, the direction of calorimetry error, enthalpy of solution, bond energy method and back-calculation of mass, with answers and notes.
Direct answers Answers in order: 298 K and 1 atm, -286 kJ mol-1, one mole and about -57 kJ mol-1, too small, 12.6 kJ, -44.1 kJ mol-1, -93 kJ mol-1, copper, 8.0 g, activation energy is still needed.
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)
- FoundationUnder what conditions is a standard molar enthalpy change measured?
- FoundationFor 2H2 + O2 -> 2H2O the enthalpy change is -572 kJ mol-1. What is the enthalpy change per mole of hydrogen burned?
- FoundationThe enthalpy of neutralisation refers to the formation of how many moles of water, and what is the rough value for dilute strong acid and strong alkali?
- IntermediateIf some heat escapes to the air during calorimetry, is the calculated magnitude of ΔH too large or too small?
- IntermediateIn a combustion experiment 250 cm3 of water rises by 12.0 °C. How much heat does the water absorb?
- Intermediate4.0 g of sodium hydroxide (molar mass 40 g mol-1) dissolves in 100 g of water and the temperature rises by 10.5 °C. Find the molar enthalpy change of solution.
- ChallengeGiven bond energies N=N 945, H-H 436 and N-H 391 kJ mol-1, find ΔH for N2 + 3H2 -> 2NH3.
- ChallengeWater has a specific heat capacity of 4.2 J g-1 K-1 and copper 0.385 J g-1 K-1. Equal masses absorb equal heat: which warms more?
- ChallengeThe enthalpy change of combustion of methane is -890 kJ mol-1. What mass of methane (molar mass 16 g mol-1) must burn to release 445 kJ?
- Error redoA student claims an exothermic reaction needs no activation energy and finishes instantly on contact. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 298 K and 1 atm, with each substance in its standard state | Standard conditions are 298 K and 1 atm, which makes values comparable. |
| 2 | -286 kJ mol-1 | The equation releases 572 kJ for 2 mol of hydrogen, so dividing by 2 gives -286 kJ mol-1. |
| 3 | One mole of water, about -57 kJ mol-1 | It is defined per mole of liquid water formed, and dilute strong acid with strong alkali gives about -57 kJ mol-1 because ionisation is complete. |
| 4 | Too small | The water absorbs less heat, so q underestimates the heat released and the magnitude of ΔH comes out too small. |
| 5 | 12.6 kJ | q = 250 times 4.2 times 12.0 gives 12600 J, which is 12.6 kJ. |
| 6 | -44.1 kJ mol-1 | 0.10 mol dissolves in 100 g of water, q = 100 times 4.2 times 10.5 gives 4410 J, and dividing by 0.10 gives -44.1 kJ mol-1. |
| 7 | -93 kJ mol-1 | Breaking bonds needs 945 plus 3 times 436, which is 2253, and forming bonds releases 6 times 391, which is 2346; the difference is -93 kJ mol-1. |
| 8 | Copper, because its specific heat capacity is smaller | A smaller specific heat capacity means less heat is needed per kelvin, so copper shows the larger temperature rise. |
| 9 | 8.0 g | 445 divided by 890 gives 0.50 mol, and times 16 that is 8.0 g. |
| 10 | It still needs activation energy first, so the rate is not infinitely fast | Activation energy is the initial barrier and is independent of whether the reaction is exothermic, so exothermic reactions can still be slow. |
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.
Decide first whether the question asks per mole or for the whole amount, then the units look after themselves.Tutor Registration →
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