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S6 Chemistry: Energetics (Paper 1: basics)
S6 Chemistry: Energetics (Paper 1: basics)
Ten basic items on the sign of the enthalpy change, bond breaking and forming, the enthalpy of combustion of methane, mole ratio scaling, q equals m c delta T, temperature rise, energy per mole, equation coefficients, mass to mole conversion and the activation energy misconception.
Direct answers Answers in order: negative, bond breaking is endothermic, -890 kJ per mole, 445 kJ, 1050 J, exothermic, 210 kJ per mole, 286 kJ, about -890 kJ per mole, and an exothermic reaction still needs activation energy.
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)
- FoundationIs the enthalpy change of an exothermic reaction positive or negative?
- FoundationWhich process is endothermic, breaking bonds or forming bonds?
- FoundationOne mole of methane releases 890 kJ on complete combustion. Write its enthalpy of combustion.
- IntermediateHow many kJ are released when 0.50 mol of methane burns completely, given 890 kJ per mole?
- IntermediateHow much energy does 25.0 g of water absorb when heated from 20.0 to 30.0 degrees Celsius, with c = 4.2 J per g per K?
- IntermediateThe temperature of the mixture rises as the reaction proceeds. Is the reaction exothermic or endothermic?
- ChallengeBurning 0.010 mol of a fuel raises 100 g of water by 5.0 degrees Celsius. Find the energy released per mole in kJ.
- ChallengeFor 2H2 plus O2 giving 2H2O with dH = -572 kJ, how much energy is released per mole of hydrogen burnt?
- ChallengeBurning 1.0 g of methane, M = 16.0, releases 55.6 kJ. Find the enthalpy of combustion per mole.
- Error redoA student writes that an exothermic reaction needs no activation energy because it releases energy by itself. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Negative | In an exothermic change the system releases energy to the surroundings, so the products lie lower in energy than the reactants and the sign is negative. |
| 2 | Breaking bonds is endothermic; forming bonds is exothermic | Energy must be supplied to break a chemical bond, while energy is released when a new bond forms. |
| 3 | dHc = -890 kJ per mole | An exothermic change takes a negative sign, and since the value is already per mole of methane we write 890 with a minus sign. |
| 4 | 445 kJ | The energy released is proportional to the number of moles, and 0.50 times 890 gives 445 kJ. |
| 5 | 1050 J, which is 1.05 kJ | Using q equals m c delta T, 25.0 times 4.2 times 10.0 gives 1050 J. |
| 6 | Exothermic | The energy released raises the internal energy of the mixture, so its temperature goes up. |
| 7 | 210 kJ per mole | First 100 times 4.2 times 5.0 gives 2100 J which is 2.1 kJ, then dividing by 0.010 mol gives 210. |
| 8 | 286 kJ | The coefficient 2 stands for 2 mol of hydrogen, so the value per mole is 572 divided by 2. |
| 9 | about -890 kJ per mole | One mole of methane weighs 16.0 g, so 55.6 times 16.0 gives 889.6, about 890, with a negative sign for an exothermic change. |
| 10 | Every reaction must break bonds first and needs activation energy; being exothermic only means the overall energy change is negative | The activation energy is the barrier that starts the reaction and is unrelated to the sign of the enthalpy change, so an exothermic reaction still has to get over it. |
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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