中文（繁體）
中文（简体）
English
S6 Chemistry: Alkanes and Alkenes (Paper 1: basics)
S6 Chemistry: Alkanes and Alkenes (Paper 1: basics)
Ten basic items on the general formulas, the molar mass of butane, the bromine water test, the carbon percentage in ethane, oxygen needed by ethene, gas volume ratios, water from butane, addition of bromine, and the saturation of alkanes.
Direct answers Answers in order: CnH2n+2 and C3H8, 58, CnH2n and C2H4, ethene, 80.0 percent, 3 mol, 4 dm3, 90 g, 16.0 g, and alkanes are saturated so they cannot decolourise bromine water.
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)
- FoundationWhat is the general formula of alkanes, and what is the molecular formula of propane with three carbon atoms?
- FoundationWhat is the relative molecular mass of butane, C4H10?
- FoundationWhat is the general formula of alkenes, and what is the molecular formula of ethene with two carbon atoms?
- IntermediateWhich of ethene and ethane decolourises orange brown bromine water, and why?
- IntermediateWhat percentage by mass of ethane, C2H6, is carbon?
- IntermediateHow many moles of oxygen are needed to burn one mole of ethene completely?
- ChallengeAt the same temperature and pressure, what volume of carbon dioxide is formed when 2 dm3 of ethene burns completely?
- ChallengeWhat mass of water is formed when one mole of butane burns completely?
- ChallengeHow many grams of bromine are needed for 0.10 mol of ethene to react completely by addition?
- Error redoA student writes that alkanes decolourise bromine water and are therefore unsaturated. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | General formula CnH2n+2; propane is C3H8 | Substituting n equals 3 gives C3H8, since the number of hydrogen atoms is 2 times 3 plus 2, which is 8. |
| 2 | 58 | The carbon contributes 4 times 12 which is 48 and the hydrogen contributes 10 times 1 which is 10, giving 58. |
| 3 | General formula CnH2n; ethene is C2H4 | An alkene contains one carbon to carbon double bond, so the number of hydrogen atoms is twice the number of carbon atoms. |
| 4 | Ethene, because it has a carbon to carbon double bond and undergoes addition with bromine | Bromine adds across the two carbon atoms of the double bond so the orange brown colour fades, while an alkane is saturated and cannot do this. |
| 5 | 80.0 percent | Carbon contributes 24 out of a relative molecular mass of 30, and 24 divided by 30 times 100 gives 80.0 percent. |
| 6 | 3 mol | The equation C2H4 plus 3O2 gives 2CO2 plus 2H2O, so one mole of ethene needs three moles of oxygen. |
| 7 | 4 dm3 | The volume ratio of gases equals the mole ratio, and ethene to carbon dioxide is 1 to 2, so 2 dm3 gives 4 dm3. |
| 8 | 90 g | The equation C4H10 plus 6.5O2 gives 4CO2 plus 5H2O, and 5 moles of water times 18 gives 90 g. |
| 9 | 16.0 g | Ethene and bromine react in a 1 to 1 ratio and the molar mass of bromine is 160, so 0.10 times 160 gives 16.0 g. |
| 10 | Alkanes are saturated and do not decolourise bromine water; they only undergo substitution with bromine under ultraviolet light | An alkane has only single bonds, so there is no double bond for bromine to add across and the orange brown colour of bromine water stays. |
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 →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.