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S6 Chemistry Organic Chemistry (Hydrocarbons), Paper 2: combustion analysis and addition sums
S6 Chemistry Organic Chemistry (Hydrocarbons), Paper 2: combustion analysis and addition sums
Paper 2 covers the molar mass of hexane, mole ratios in propane combustion, substitution of methane, the CH2 step of a homologous series, boiling points of straight and branched chains, ethene combustion, repeat units in poly(ethene) and two back-calculation items.
Focus Combustion analysis works backwards from the moles of carbon dioxide and water, and one mole of C=C adds exactly one mole of bromine.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S6 Chemistry: Organic Chemistry (Hydrocarbons) |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationWhat is the molar mass of hexane, C6H14, and how many moles are in 4.3 g?
- FoundationPropane burns as C3H8 plus 5O2 giving 3CO2 and 4H2O. How much oxygen is needed for 0.20 mol of propane, and how much carbon dioxide and water form?
- FoundationMethane and chlorine give chloromethane and hydrogen chloride in ultraviolet light. What type of reaction is this?
- FoundationWhat is the mass difference for one CH2 unit in a homologous series, and how much heavier is butane than propane at 44.0 g per mole?
- IntermediateButane has a straight chain and a branched isomer. Which one boils higher, and why?
- IntermediateEthene burns as C2H4 plus 3O2 giving 2CO2 and 2H2O. How many moles of oxygen does 5.6 g of ethene need, and what volume is that at 24 dm3 per mole?
- IntermediateEthene polymerises to poly(ethene) with the repeat unit C2H4. What is the molar mass of a sample averaging 1,000 repeat units?
- ChallengeBurning 0.10 mol of a hydrocarbon gives 0.40 mol of carbon dioxide and 0.50 mol of water. Find its formula and the mass of 0.10 mol.
- Challenge4.2 g of an alkene adds exactly 16.0 g of bromine, whose molar mass is 160.0. Find the molar mass and the formula of the alkene.
- Error redoError repair: a student says hexene decolourises bromine water merely because bromine dissolves in it. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 86.0 g per mole, so 0.05 mol | Six carbon atoms give 72.0 and fourteen hydrogen atoms give 14.0, so the molar mass is 86.0 and 4.3 g is 0.05 mol. |
| 2 | 1.00 mol of oxygen, giving 0.60 mol of carbon dioxide and 0.80 mol of water | The coefficients run one to five to three to four, so multiply 0.20 by five, three and four. |
| 3 | Substitution | A hydrogen atom is replaced by chlorine, which makes this a substitution; addition happens at double or triple bonds and methane has none. |
| 4 | 14.0 g per mole, so butane at 58.0 is 14.0 heavier | A CH2 unit weighs 12.0 plus 2.0, which is 14.0, and 58.0 minus 44.0 gives the same 14.0. |
| 5 | The straight chain, because its longer molecules give stronger van der Waals forces | The straight chain is longer, so the molecules touch over a larger area and the van der Waals forces are stronger. |
| 6 | 0.60 mol of oxygen, which is 14.4 dm3 | 5.6 divided by 28.0 is 0.20 mol, times three gives 0.60 mol, and times 24 gives 14.4 dm3. |
| 7 | 28,000 g per mole | The repeat unit has a molar mass of 28.0 and a thousand of them give 28,000; addition polymerisation loses no atoms. |
| 8 | C4H10, butane, with a mass of 5.8 g | Carbon is 0.40 over 0.10, which is 4, hydrogen is twice 0.50 over 0.10, which is 10, and 0.10 times 58.0 is 5.8 g. |
| 9 | 0.10 mol, so 42.0 g per mole, which is propene C3H6 | 16.0 over 160.0 gives 0.10 mol of bromine, the alkene adds it one to one, so 4.2 g is 0.10 mol and the molar mass is 42.0, which fits C3H6. |
| 10 | Wrong: the carbon-carbon double bond adds bromine in an addition reaction and uses it up | Addition converts the bromine into a colourless dibromo compound, while simple dissolving only extracts bromine into the organic layer and leaves it intact. |
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.
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