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S6 Chemistry Organic Chemistry, Paper 1: alkanes, alkenes, isomers and addition
S6 Chemistry Organic Chemistry, Paper 1: alkanes, alkenes, isomers and addition
Ten graded items on alkane and alkene formulae, molar masses, combustion stoichiometry, the bromine water test, structural isomer counts and addition of hydrogen, each with an answer.
Focus Alkanes are CnH2n+2 and alkenes CnH2n, so find moles before using equation coefficients, and remember that bromine adds across C=C.
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)
- FoundationAlkanes follow CnH2n+2. How many hydrogen atoms are in hexane, C6H14?
- FoundationWhat is the molar mass of methane, CH4, and how many moles are in 8.0 g?
- FoundationMethane burns as CH4 + 2O2 giving CO2 and 2H2O. How many moles of oxygen are needed to burn 0.50 mol of methane completely?
- FoundationBromine water is added to hexane and to hexene. Which one decolourises it, and why?
- IntermediateHow many structural isomers exist for C4H10 and for C5H12?
- IntermediateAlkenes follow CnH2n. How many hydrogen atoms fewer does C5H10 have than the alkane C5H12?
- IntermediateEthene adds hydrogen to give ethane. How many moles are in 2.8 g of ethene, and what mass of hydrogen is needed?
- ChallengeBurning 0.20 mol of an alkene gives 0.60 mol of carbon dioxide. Find the carbon number, the molecular formula and the mass of 0.20 mol.
- ChallengeEthene burns as C2H4 + 3O2 giving 2CO2 and 2H2O. How many moles of oxygen react with 5.6 g of ethene?
- Error redoError repair: a student writes that alkenes decolourise bromine water simply because bromine dissolves in them. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Fourteen | Twice six plus two is fourteen, which matches the alkane formula. |
| 2 | 16.0 g per mole, so 0.50 mol | 12.0 plus four is 16.0, and 8.0 divided by 16.0 is 0.50. |
| 3 | 1.0 mol | Each mole of methane needs two moles of oxygen, so 0.50 times 2 is 1.0. |
| 4 | Hexene, because its carbon-carbon double bond adds bromine | Hexane has only single bonds and does not react, while the C=C in hexene adds bromine and removes the orange colour. |
| 5 | Two for C4H10 and three for C5H12 | C4H10 gives butane and 2-methylpropane, while C5H12 adds 2,2-dimethylpropane for three in total. |
| 6 | Two fewer | Five times two is ten, and twelve minus ten is two, one hydrogen molecule fewer. |
| 7 | 0.10 mol, needing 0.20 g of hydrogen | The molar mass is 28.0, so 2.8 g is 0.10 mol, and 0.10 times 2.0 gives 0.20 g of hydrogen. |
| 8 | Three carbons, so propene C3H6, with a mass of 8.4 g | 0.60 over 0.20 gives three carbons per molecule, C3H6 has a molar mass of 42.0, and 0.20 times 42.0 is 8.4 g. |
| 9 | 0.60 mol | 5.6 divided by 28.0 is 0.20 mol of ethene, and three moles of oxygen per mole gives 0.60 mol. |
| 10 | Wrong: bromine is consumed by addition across the C=C bond, forming a colourless dibromo compound | Dissolving is only physical mixing and the colour would remain, whereas decolourising means bromine has been consumed by addition to form a colourless dibromo compound. |
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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