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S5 Chemistry: Enthalpy Change and Energy (Paper 4: Misconception Drill)
S5 Chemistry: Enthalpy Change and Energy (Paper 4: Misconception Drill)
Ten items on the commonest enthalpy misconceptions: which mass to use, steam versus liquid water, breaking and forming bonds, molar conversion and units, with answers and notes.
Direct answers Answers in order: 5.0 °C, total solution mass, too small, breaking is endothermic and forming is exothermic, reactants by 300 kJ mol-1, -440 kJ mol-1, use the water formed, 25 g, endothermic with positive DH, it should be kJ mol-1.
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)
- Foundation200 g of water absorbs 4200 J of heat. By how many °C does its temperature rise?
- FoundationIn q = mcDT for a neutralisation, what mass should m be?
- FoundationIf the water formed leaves as steam when measuring an enthalpy of combustion, does the magnitude of ΔH come out too large or too small?
- IntermediateIs bond breaking endothermic or exothermic, and what about bond forming?
- IntermediateA reaction has ΔH equal to -300 kJ mol-1. Which has the higher enthalpy, and by how much?
- IntermediateBurning 0.10 mol of a fuel releases 44.0 kJ of heat. Should the molar enthalpy change be written as -44 or -440 kJ mol-1?
- ChallengeIn a neutralisation calculation, which amount should be used: acid, alkali, or water formed?
- ChallengeTaking the density of a solution as 1 g cm-3, what is the mass of 25 cm3 of it?
- ChallengeThe temperature falls after two solutions are mixed. Is the reaction exothermic or endothermic, and what is the sign of ΔH?
- Error redoA student writes the unit of a molar enthalpy change as kJ. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 5.0 °C | From q = mcDT, DT is 4200 divided by 200 times 4.2, which gives 5.0 °C. |
| 2 | The total mass of the mixed solution, including the solute | The whole solution warms up, so counting only the water underestimates the heat released. |
| 3 | Too small, because the heat of condensation is lost | Steam carries away the enthalpy of vaporisation, so the measured heat release is too low. |
| 4 | Breaking is endothermic and forming is exothermic | Energy must be supplied to pull atoms apart, whereas forming a new bond releases energy. |
| 5 | The reactants, by 300 kJ mol-1 | A negative ΔH means the products are lower in enthalpy, the difference of 300 kJ mol-1 being released as heat. |
| 6 | -440 kJ mol-1 | The molar value is per mole, so 44.0 divided by 0.10 gives 440, that is -440 kJ mol-1. |
| 7 | The moles of water formed, after identifying the limiting reagent | The enthalpy of neutralisation is defined per mole of water formed, so the limiting reagent decides the amount to use. |
| 8 | 25 g | A density of 1 g cm-3 means 1 g per cm3, so 25 cm3 has a mass of 25 g. |
| 9 | Endothermic, so ΔH is positive | A fall in temperature means heat is taken from the solution, which is endothermic and gives a positive ΔH. |
| 10 | It should be kJ mol-1; kJ is a quantity of heat, not a molar value | A molar enthalpy is quoted per mole, so without mol-1 the unit does not describe energy per mole. |
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.
Turn every wrong item today into one sentence: I thought this, but actually that.Tutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.