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S6 Chemistry: Energetics (Paper 4: misconception drill)
S6 Chemistry: Energetics (Paper 4: misconception drill)
Ten misconception drill items on forgetting to divide by moles, temperature differences in Celsius, reversing bond breaking and forming, a heat calculation, enthalpy per mole, doubling coefficients, taking half the mass, ethanol combustion, temperature difference conversion and the wrong unit.
Direct answers Answers in order: total heat only, yes it is fine, breaking absorbs and forming releases, 1764 J, -88 kJ per mole, -572 kJ per mole, 50 percent too small, -1260 kJ per mole, 25 K, and kJ per mole.
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: S6 Chemistry: Energetics |
| Level | S6 |
02 Questions (10 items)
- FoundationWhat goes wrong if you forget to divide the heat released by the number of moles when finding an enthalpy of combustion?
- FoundationMay the temperature rise in q equals m c delta T be taken in degrees Celsius?
- FoundationA student writes that breaking bonds releases energy and forming bonds absorbs it. What is correct?
- IntermediateHow much energy does 100 g of water absorb when it warms by 4.2 degrees Celsius with c = 4.2 J per g per K?
- IntermediateBurning 0.050 mol of a fuel releases 4.4 kJ. Find its enthalpy of combustion.
- IntermediateGiven that H2 plus half O2 giving H2O has dH -286 kJ per mole, what is dH for 2H2 plus O2 giving 2H2O?
- Challenge25 cm3 of acid is mixed with 25 cm3 of alkali. If only 25 g is taken as the mass heated instead of the true 50 g, how far off is the enthalpy of neutralisation?
- ChallengeBurning 0.92 g of ethanol, M = 46.0, raises 200 g of water by 30.0 degrees Celsius. Find the experimental enthalpy of combustion.
- ChallengeWater is heated from 20 to 45 degrees Celsius. What is delta T in kelvin?
- Error redoA student gives the unit of an enthalpy of combustion as kJ. What is the correct unit?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | You get the total heat of that experiment rather than the change per mole | The enthalpy change is defined per mole of substance, so the heat must be divided by the amount of fuel burnt. |
| 2 | Yes, a temperature difference is the same number in degrees Celsius and in kelvin | The Celsius and kelvin scales differ only by a fixed offset, so differences are identical and can be used directly. |
| 3 | Breaking bonds absorbs energy and forming bonds releases it | Energy must be put in to break a bond and is given out when a new bond forms, which is the opposite of what was written. |
| 4 | 1764 J, which is 1.76 kJ | Multiplying 100 by 4.2 by 4.2 gives 1764 J. |
| 5 | -88 kJ per mole | 4.4 divided by 0.050 gives 88, and the sign is negative because heat is released. |
| 6 | -572 kJ per mole | Doubling the whole equation doubles the enthalpy change while the sign stays the same. |
| 7 | Only half the mass is counted, so the heat found is half the true value and the enthalpy change is 50 percent too small | q is proportional to m, so halving the mass halves the heat and therefore halves the calculated value per mole. |
| 8 | -1260 kJ per mole | q is 25.2 kJ and the ethanol is 0.020 mol, so 25.2 divided by 0.020 gives 1260 with a negative sign. |
| 9 | 25 K | The difference is 45 minus 20 which is 25, and the number is unchanged in kelvin. |
| 10 | kJ per mole | An enthalpy change is the energy change per mole of substance, so the unit must carry mol to the power minus one. |
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.
Write and balance the equation first, then use q = mcDT or add formation enthalpies to find the enthalpy change.Tutor Registration →
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