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S6 Chemistry: Energetics (Paper 3: challenge)
S6 Chemistry: Energetics (Paper 3: challenge)
Ten challenge items on why water is weighed, excess oxygen, the insulated container, heat from hydrogen, methanol combustion, a fuel calorimetry calculation, experimental yield percentage, hydrogenation of an alkene, a three equation formation enthalpy and the effect of a catalyst.
Direct answers Answers in order: the water absorbs the heat, to ensure complete combustion, to cut heat loss, 143 kJ, -727 kJ per mole, -2520 kJ per mole, 42.4 percent, -126 kJ per mole, -75 kJ per mole, and a catalyst does not change the enthalpy change.
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)
- FoundationWhy is the mass of water measured rather than the mass of fuel in a calorimetry experiment?
- FoundationWhy is excess oxygen used when measuring an enthalpy of combustion?
- FoundationWhy use an insulated container with a lid in a calorimetry experiment?
- Intermediate1.00 g of hydrogen, M = 2.0, burns with dHc -286 kJ per mole. How many kJ are released?
- IntermediateFind the enthalpy of combustion of methanol: CH3OH plus 1.5O2 giving CO2 plus 2H2O, with dHf CH3OH -239, CO2 -394 and H2O -286.
- IntermediateBurning 0.500 g of a fuel, M = 100, raises 250 g of water by 12.0 degrees Celsius. Find its enthalpy of combustion.
- ChallengeThe standard enthalpy of combustion of ethanol is -1368 kJ per mole but the experiment gives -580. Find the experimental value as a percentage of the theoretical value.
- ChallengeUse bond energies to find the enthalpy change of C2H4 plus H2 giving C2H6, with C=C 612, C-C 348, C-H 413 and H-H 436.
- ChallengeGiven C plus O2 giving CO2 at -394, H2 plus half O2 giving H2O at -286 and CH4 plus 2O2 giving CO2 plus 2H2O at -891, find the enthalpy of formation of methane.
- Error redoA student writes that adding a catalyst makes the enthalpy change of an exothermic reaction more negative. Is this right?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Because nearly all the heat released is used to warm the water, so the water is what absorbs the energy | The m in q equals m c delta T is the mass of the substance being heated, which here is the water. |
| 2 | To ensure complete combustion, since incomplete combustion gives a heat release that is too small | Incomplete combustion produces carbon monoxide and releases noticeably less energy than complete combustion. |
| 3 | To reduce heat loss to the surroundings so the measured heat is closer to the true value | Any heat lost makes the temperature rise too small and the calculated enthalpy change too small in size. |
| 4 | 143 kJ | 1.00 g of hydrogen is 0.50 mol, and 0.50 times 286 gives 143 kJ. |
| 5 | -727 kJ per mole | For the products -394 plus 2 times -286 gives -966, and subtracting the reactant value -239 gives -727. |
| 6 | -2520 kJ per mole | q is 250 times 4.2 times 12 which is 12600 J, the amount is 0.0050 mol, and 12.6 divided by 0.0050 gives 2520 with a negative sign. |
| 7 | about 42.4 percent | 580 divided by 1368 times 100 gives 42.4, and the rest of the heat is lost to the apparatus and the air. |
| 8 | -126 kJ per mole | Breaking gives 612 plus 4 times 413 plus 436 which is 2700 while forming gives 348 plus 6 times 413 which is 2826, and the difference is -126. |
| 9 | -75 kJ per mole | Adding the first two equations gives C plus 2H2 plus 2O2 to CO2 plus 2H2O, and subtracting the third gives -394 minus 572 plus 891 which is -75. |
| 10 | No; a catalyst only lowers the activation energy and speeds the reaction up without changing the enthalpy change | A catalyst does not change the energy of the reactants or the products, so the energy gap between the two states stays the same. |
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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