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S6 Chemistry: Alkanes and Alkenes (Paper 4: misconception drill)
S6 Chemistry: Alkanes and Alkenes (Paper 4: misconception drill)
Ten misconception items on C4H8, the empirical formula of alkenes, the test for unsaturation, oxygen for ethene, the carbon percentage, addition against substitution, the isomers of butane, volume ratios, the molar mass of a polymer, and why bromine water fades.
Direct answers Answers in order: not an alkane, yes CH2, bromine water fading, 3 mol, 85.7 percent, addition releases no hydrogen bromide, 2, 4 dm3, 28n, and an addition reaction.
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: Alkanes and Alkenes |
| Level | S6 |
02 Questions (10 items)
- FoundationA student sees C4H8 and answers alkane. What is the correct view?
- FoundationIs the empirical formula of every member of the alkene series CH2?
- FoundationWhich test most directly shows whether a hydrocarbon is unsaturated?
- IntermediateA student says one mole of ethene needs only two moles of oxygen. What is correct?
- IntermediateA student says ethene is 50 percent carbon by mass. What is the correct value?
- IntermediateA student writes the reaction of an alkene with bromine as substitution. How are addition and substitution told apart?
- ChallengeA student says butane has four structural isomers. What is the correct number?
- ChallengeA student says 2 dm3 of ethene gives 2 dm3 of carbon dioxide. What is the correct volume?
- ChallengeA student writes the molar mass of poly(ethene) as 28 plus n. What is correct?
- Error redoA student writes that an alkene decolourises bromine water because the bromine is oxidised. What is the correct explanation?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | It is not an alkane; it has two hydrogen atoms fewer and is an alkene or a cycloalkane | An alkane with four carbon atoms is C4H10, so C4H8 with two hydrogen atoms fewer means one double bond or one ring. |
| 2 | Yes, the empirical formula of every alkene is CH2 | The general formula CnH2n fixes the carbon to hydrogen ratio at 1 to 2, so the empirical formula is always CH2. |
| 3 | Add bromine water; fading of the orange brown colour shows a carbon to carbon double bond | Burning cannot tell saturated from unsaturated because both alkanes and alkenes burn. |
| 4 | 3 mol of oxygen | 2 mol of carbon dioxide needs four oxygen atoms and 2 mol of water needs two more, giving six oxygen atoms which is 3 mol of oxygen. |
| 5 | 85.7 percent | Carbon contributes 24 out of a molar mass of 28 and 24 divided by 28 times 100 gives 85.7 percent. |
| 6 | In addition the double bond opens and one group bonds to each carbon, while in substitution a hydrogen atom is replaced and hydrogen bromide is released | Look at the products: addition releases no HBr whereas substitution does. |
| 7 | 2, namely butane and 2-methylpropane | Butane has only four carbon atoms and shortening the chain to three puts both methyl groups on the same carbon, giving just one branched isomer. |
| 8 | 4 dm3 | The mole ratio of ethene to carbon dioxide is 1 to 2 and the volume ratio is the same at the same temperature and pressure. |
| 9 | 28n, the molar mass of the repeat unit multiplied by the number of units | Polymerisation joins n units of C2H4 into a long chain, so the mass is n times 28. |
| 10 | It is an addition reaction in which bromine adds across the two carbons of the double bond so the orange brown colour disappears | The colour goes because bromine is used up to form a colourless dibromo compound, not because of an oxidation colour change. |
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 convert with the mole ratio to mass or volume.Tutor Registration →
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