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S6 Chemistry: Enthalpy Change and Hess Law (Paper 2: intermediate)
S6 Chemistry: Enthalpy Change and Hess Law (Paper 2: intermediate)
Ten intermediate items on the mole conversion for neutralisation, finding a formation enthalpy from a combustion enthalpy, the meaning of the sign, bond energies for hydrogen chloride, the Hess calculation for iron oxide reduction, bond energies for ammonia, percentage error, a temperature rise from a mass, the formation enthalpy of ethanol, and average bond energies as estimates.
Direct answers Answers in order: minus 56.8, minus 110.5, 890 kJ given out per mole, minus 183, minus 24.8, minus 93 kJ per mol, 4.4 percent, 26.6 K, minus 277.4 kJ per mol, and an estimate only.
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: Enthalpy Change and Hess Law |
| Level | S6 |
02 Questions (10 items)
- Foundation50.0 cm3 of 1.00 M acid and 50.0 cm3 of 1.00 M alkali are mixed, the solution weighs 100.0 g and the rise is 6.8 K. Find the enthalpy of neutralisation.
- FoundationGiven a formation enthalpy of minus 393.5 for carbon dioxide and a combustion enthalpy of minus 283.0 kJ per mol for carbon monoxide, find the formation enthalpy of carbon monoxide.
- FoundationA reaction has an enthalpy change of minus 890 kJ per mol. What does the negative sign mean?
- IntermediateUse bond energies to find the enthalpy change of hydrogen reacting with chlorine to give two moles of hydrogen chloride: H to H 436, Cl to Cl 243 and H to Cl 431 kJ per mol.
- IntermediateFind the enthalpy change when iron oxide is reduced by carbon monoxide to iron and carbon dioxide. Formation enthalpies: iron oxide minus 824.2, carbon monoxide minus 110.5 and carbon dioxide minus 393.5 kJ per mol, with iron taken as zero.
- IntermediateUse bond energies to find the enthalpy change when nitrogen and hydrogen form ammonia: N to N 945, H to H 436 and N to H 391 kJ per mol.
- ChallengeA measured enthalpy of neutralisation is 54.6 kJ per mol while the literature value is 57.1. What is the percentage error?
- ChallengeWhen 2.00 g of methane burns completely with a combustion enthalpy of minus 890 kJ per mol, all the heat raises the temperature of 1000 g of water. Find the rise, taking the molar mass as 16.0.
- ChallengeGiven a combustion enthalpy of minus 1367 for liquid ethanol, minus 393.5 for carbon dioxide and minus 285.8 kJ per mol for liquid water, find the formation enthalpy of ethanol.
- Error redoWhy is an enthalpy change calculated from average bond energies only an estimate?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Minus 56.8 kJ per mol | 100.0 times 4.18 times 6.8 gives 2842 J, and with 0.0500 mol of water formed 2.842 divided by 0.0500 gives 56.8 kJ per mol. |
| 2 | Minus 110.5 kJ per mol | Since carbon monoxide burns to carbon dioxide, the formation enthalpy is minus 393.5 plus 283.0 which is minus 110.5 kJ per mol. |
| 3 | 890 kJ of energy is given out for every mole of reaction | A negative value means the products have lower enthalpy than the reactants, so the system gives energy out to the surroundings. |
| 4 | Minus 183 kJ per mol | Breaking bonds takes in 436 plus 243 which is 679, making bonds gives out 2 times 431 which is 862, so the net value is minus 183 kJ per mol. |
| 5 | Minus 24.8 kJ per mol | The products give 3 times minus 393.5 which is minus 1180.5 and the reactants give minus 824.2 plus 3 times minus 110.5 which is minus 1155.7, so the difference is minus 24.8. |
| 6 | Minus 93 kJ per mol | Breaking bonds takes in 945 plus 3 times 436 which is 2253 and making the six N to H bonds gives out 2346, so the net value is minus 93 kJ per mol. |
| 7 | 4.4 percent | Dividing the difference of 2.5 by the literature value 57.1 and multiplying by 100 gives 4.4 percent, and the measured value is low mainly through heat loss. |
| 8 | 26.6 K | The methane is 0.125 mol which releases 111.25 kJ, and dividing by 1000 times 4.18 gives 26.6 K. |
| 9 | Minus 277.4 kJ per mol | Twice minus 393.5 plus three times minus 285.8 minus minus 1367 gives minus 277.4 kJ per mol. |
| 10 | A bond energy is an average over many molecules and the calculation ignores the physical state and intermolecular forces | The same bond has different strengths in different molecules, so an average bond energy is only approximate and the result is an estimate. |
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 q = mc(delta T) or the formation enthalpy formula first, divide by the moles only at the end, and check the sign.Tutor Registration →
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