Alkenes, alcohols & carboxylic acids (Chem only) – GCSE Chemistry exam questions with mark scheme

1Grade 4–5Triple only4 marks

Ethanol is an alcohol.

(a) Give the formula of the functional group in all alcohols. [1]

(b) Ethanol can be made by fermentation of sugar solution. Give two conditions needed for fermentation. [2]

(a) –OH; allow OH / hydroxyl; do not accept OH⁻ / hydroxide(1)
(b) any two from: yeast; warm / about 30–35 °C (allow a stated temperature in the range 25–40 °C); anaerobic / no oxygen / no air — first correct condition(1)
(b) second correct condition from the list above; ignore 'aqueous / in solution' (given in the question); do not accept 'heat strongly' / high temperature(1)
(c) ethanoic acid; allow acetic acid; do not accept 'carboxylic acid' alone(1)
Answer: (a) –OH (b) any two of: yeast; warm (about 30–35 °C); anaerobic (no air) (c) ethanoic acid
Examiner tip: Fermentation needs 'warm' (about 35 °C) — high temperatures denature the enzymes in yeast.

2Grade 6–7HigherTriple only5 marks

Ethanoic acid reacts with ethanol, in the presence of an acid catalyst, to make an ester.

(a) Name the ester formed. [1]

(b) Give the other product of the reaction. [1]

(c) Ethanoic acid is a weak acid. A student adds equal lengths of magnesium ribbon to equal volumes of 1.0 mol/dm³ ethanoic acid and 1.0 mol/dm³ hydrochloric acid at the same temperature.

Explain why the ethanoic acid reacts more slowly. [3]

(a) ethyl ethanoate(1)
(b) water; allow H₂O(1)
(c) ethanoic acid is only partially ionised / dissociated (in water)(1)
(c) so there is a lower concentration of hydrogen ions / H⁺(aq); allow fewer H⁺ ions (in the same volume)(1)
(c) so fewer collisions per second / lower frequency of collisions (between H⁺ ions and magnesium); 'fewer collisions' alone does not score(1)
Answer: (a) ethyl ethanoate (b) water (c) ethanoic acid only partially ionises, so lower H⁺ concentration and less frequent collisions
Examiner tip: Same concentration of acid does not mean same concentration of H⁺ ions — that is the key idea for weak acids.

3Grade 1–3FoundationTriple only5 marks

The table shows the formulae of five compounds, A to E.

Compound	A	B	C	D	E
Formula	CH₄	C₃H₆	C₂H₅OH	C₄H₈	C₂H₆

(a) Which two compounds are alkenes? [1]

(b) Complete the general formula for the alkenes: C_nH_{__} [1]

(d) Alkenes are described as unsaturated. What does this mean? [1]

(e) Pentene burns in air with a smoky flame. Give the reason why. [1]

(a) B and D; both needed, any order(1)
(b) 2n; accept C_nH_2n(1)
(c) butene; allow but-1-ene / but-2-ene; do not accept butane or butanol(1)
(d) (molecules) contain a carbon–carbon double bond / C=C; allow have two fewer hydrogen atoms than the alkane with the same number of carbon atoms; ignore 'can react with bromine'(1)
(e) incomplete combustion (occurs); allow not enough oxygen to burn completely / produces carbon (particles) or soot; ignore 'it is an alkene'(1)
Answer: (a) B and D (b) C_nH_2n (c) butene (d) contain a C=C double bond (e) incomplete combustion
Examiner tip: In (c) check the ending: '-ene' for alkenes, '-ane' for alkanes and '-anol' for alcohols. Swapping them is the most common way to drop this mark.

4Grade 4–5Triple only5 marks

The diagram shows the displayed formula of propene.

(a) Propene is bubbled through bromine water. Give the colour change seen. [1]

(b) Propene reacts with chlorine, Cl₂.

(i) Name the type of reaction. [1]

(ii) Draw the displayed formula of the product. [2]

(a) orange to colourless; allow yellow / brown for orange; do not accept 'clear' for colourless; both colours needed(1)
(b)(i) addition; ignore 'halogenation'; do not accept substitution(1)
(b)(ii) chain of three carbon atoms with all single bonds and six H atoms (all bonds shown)(1)
(b)(ii) one Cl atom on each of two neighbouring carbon atoms that were joined by the double bond (CH₂Cl–CHCl–CH₃ displayed); max 1 if C=C still shown(1)
(c) (nickel) catalyst; allow named catalyst e.g. nickel / platinum / palladium; ignore temperature values(1)
Answer: (a) orange → colourless (b)(i) addition (ii) CH₂Cl–CHCl–CH₃ drawn fully displayed: one Cl on each of the two carbons that had the double bond, all single bonds (c) a (nickel) catalyst
Examiner tip: In addition reactions the double bond becomes a single bond: if your drawing still has C=C, or has the two Cl atoms on the same carbon, you lose the structure mark.

5Grade 6–7HigherTriple only8 marks

Ethanol, C₂H₅OH, can be made industrially from ethene, C₂H₄:

$$\text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_5\text{OH}$$

(a) Give two conditions used for this reaction. [2]

(b) Calculate the maximum mass of ethanol, in tonnes, that can be made from 4.2 tonnes of ethene.

Relative atomic masses ($A_r$): H = 1, C = 12, O = 16 [3]

$$\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$$

Calculate the percentage atom economy for making ethanol by fermentation. Give your answer to 3 significant figures. [2]

(d) Explain why the atom economy for making ethanol from ethene is 100%. [1]

(a) any two from: (water as) steam; (phosphoric acid) catalyst; high temperature (allow 250–300 °C); high pressure (allow 60–70 atm) — first condition(1)
(a) second condition from the list; ignore 'heat' without 'high'; do not accept yeast / 30 °C (fermentation conditions)(1)
(b) $M_r$ ethene = 28 and $M_r$ ethanol = 46(1)
(b) 4.2 ÷ 28 (= 0.15) or 4.2 × 10⁶ ÷ 28 = 1.5 × 10⁵ mol; allow ratio 46/28 seen(1)
(b) 6.9 (tonnes); correct answer with no working scores 3(1)
(c) (2 × 46) ÷ 180 × 100; allow 92/180 × 100(1)
(c) 51.1 (%); correct answer with no working scores 2; 51 or 51.11 scores 1 (not 3 s.f.)(1)
(d) ethanol is the only product / there is no waste (by-)product; allow all atoms of reactants end up in the desired product(1)
Answer: (a) steam + catalyst (high temperature / pressure) (b) 6.9 tonnes (c) 51.1% (d) only one product
Examiner tip: In (b) the reaction is 1:1, so mass of ethanol = 4.2 × 46/28; dividing the wrong way round (28/46) gives 2.6 t and loses two marks.

6Grade 1–3FoundationTriple only5 marks

A student added a small piece of sodium to 10 cm³ of four different alcohols. Each piece of sodium was the same size.

The table shows the results.

Alcohol	Number of carbon atoms	Time for the sodium to disappear (s)
methanol	1	45
ethanol	2	70
propanol	3	130
butanol	4	210

(a) Bubbles of gas were seen in each alcohol. Name the gas. [1]

(b) Describe a test for this gas and give the result. [1]

(d) Methanol mixes with water. Universal indicator is added. What is the pH of the solution? [1]

(e) Propanol reacts with an oxidising agent. Name the type of compound formed. [1]

(a) hydrogen; allow H₂(1)
(b) (lighted / burning splint) squeaky pop; both test and result needed; do not accept glowing splint(1)
(c) the more carbon atoms (in the alcohol) the longer the time / the slower the reaction; allow the reverse argument; allow comparison of first and last results; ignore 'it increases' unless qualified(1)
(d) 7; allow neutral (solution); do not accept a range including other values(1)
(e) carboxylic acid; allow propanoic acid; do not accept 'acid' alone(1)
Answer: (a) hydrogen (b) lighted splint gives a squeaky pop (c) the more carbon atoms, the longer the time (slower reaction) (d) pH 7 (e) a carboxylic acid (propanoic acid)
Examiner tip: In (c), a pattern must link both variables: say what happens to the time as the number of carbon atoms increases, not just 'the time goes up'.

7Grade 4–5Triple only4 marks

Alcohols are used as fuels.

(a) Balance the equation for the complete combustion of methanol. [1]

$$\_\_\,\text{CH}_3\text{OH} + \_\_\,\text{O}_2 \rightarrow \_\_\,\text{CO}_2 + \_\_\,\text{H}_2\text{O}$$

(b) Write a balanced equation for the complete combustion of butanol, C₄H₉OH. [2]

(a) 2, 3, 2, 4; allow multiples; allow 1, 1½, 1, 2(1)
(b) C₄H₉OH + O₂ → CO₂ + H₂O (correct formulae)(1)
(b) C₄H₉OH + 6O₂ → 4CO₂ + 5H₂O (balancing); allow multiples; allow C₄H₁₀O; second mark dependent on first(1)
(c) (as a) solvent; allow (ethanol) in alcoholic drinks; allow (in) hand sanitiser / perfume; ignore 'drinking' alone(1)
Answer: (a) 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O (b) C₄H₉OH + 6O₂ → 4CO₂ + 5H₂O (c) solvent (or alcoholic drinks)
Examiner tip: When balancing oxygen, remember the alcohol molecule already contains one O atom: count it before deciding how many O₂ are needed.

8Grade 6–7Triple only6 marks

A student found the energy released when one mole of each of four alcohols burns. The graph shows the results.

(a) Use the graph to predict the energy released when one mole of pentanol (5 carbon atoms) burns. [1]

(b) Describe the pattern shown by the graph. Use numbers from the graph in your answer. [2]

A camping stove needs a fuel that releases as much energy as possible per gram.

Which alcohol, methanol or butanol, releases more energy per gram? Show your working.

Relative atomic masses ($A_r$): H = 1, C = 12, O = 16 [3]

(a) any value from 3250 to 3400 (kJ/mol); allow line extended correctly on graph(1)
(b) energy released increases as the number of carbon atoms increases; allow positive correlation / directly related(1)
(b) by about 650 kJ/mol for each extra carbon atom; allow values from 600 to 700; allow 'increases by the same amount each time' / linear with a quoted pair of values from the graph(1)
(c) $M_r$ methanol = 32 and $M_r$ butanol = 74(1)
(c) 726 ÷ 32 = 22.7 (kJ/g) and 2676 ÷ 74 = 36.2 (kJ/g); allow 22.6875 / 36.16 or rounded to 2 s.f. (23 and 36)(1)
(c) butanol (releases more energy per gram); dependent on correct comparison of calculated values; allow ecf from incorrect M_r(1)
Answer: (a) about 3320 kJ/mol (3250–3400) (b) increases by about 650 kJ/mol per extra carbon atom (c) methanol 22.7 kJ/g, butanol 36.2 kJ/g, so butanol
Examiner tip: 'Use numbers from the graph' means the second mark needs a quoted value, such as the increase per carbon atom; a description with no numbers is capped at 1.

9Grade 8–9HigherTriple only6 marks

A student measured the pH of two acids, to the nearest whole number. Both acids had a concentration of 0.10 mol/dm³.

Acid	pH
hydrochloric acid	1
ethanoic acid	3

(a) The student added sodium carbonate to the ethanoic acid.

(i) Give one observation. [1]

(ii) Name the salt formed. [1]

(b) Explain, in terms of ionisation, why ethanoic acid has a higher pH than hydrochloric acid of the same concentration. [2]

(c) How many times greater is the concentration of hydrogen ions in the hydrochloric acid than in the ethanoic acid? Explain your answer. [2]

(a)(i) fizzing / bubbles / effervescence; allow solid disappears / dissolves; ignore 'gas produced' or 'carbon dioxide produced' (not an observation)(1)
(a)(ii) sodium ethanoate; do not accept sodium ethanoic(1)
(b) ethanoic acid is only partially ionised (in water) whereas hydrochloric acid is completely / fully ionised; allow dissociated for ionised(1)
(b) so the concentration of hydrogen ions / H⁺(aq) is lower (in ethanoic acid); 'fewer H⁺ ions' alone does not score unless linked to the same volume / concentration(1)
(c) difference of 2 pH units and each decrease of 1 pH unit increases the H⁺ concentration by a factor of 10(1)
(c) 100 (times); allow 10²; 100 with no explanation scores 1; 2 or 20 times scores 0(1)
Answer: (a)(i) fizzing (ii) sodium ethanoate (b) ethanoic acid only partially ionises, so lower H⁺ concentration (c) pH differs by 2, each unit is a factor of 10, so 10 × 10 = 100 times
Examiner tip: Each 1 unit drop in pH means 10 times more H⁺ ions, so a difference of 2 pH units is 10 × 10 = 100 times, not 2 or 20 times.

10Grade 6–7Triple only6 marks

A technician has three bottles of colourless liquid. The labels have come off.

One bottle contains pentene, one contains ethanol and one contains a solution of propanoic acid in water.

Describe how you could use chemical tests to identify which liquid is in each bottle.

You should give the result of each test for each liquid. [6]

add bromine water (to a sample of each liquid and shake)(1)
pentene decolourises bromine water / orange to colourless(1)
ethanol and propanoic acid: bromine water stays orange / no change(1)
add sodium carbonate (or any metal carbonate / hydrogencarbonate, e.g. marble chips)(1)
propanoic acid fizzes / bubbles; the gas turns limewater milky / cloudy (carbon dioxide)(1)
ethanol (and pentene): no fizzing with sodium carbonate(1)
alternative: add universal indicator (or pH probe) — propanoic acid pH 3–6 / orange or red (acidic); ethanol pH 7 / green (neutral)(1)
alternative: add water — pentene does not mix / forms a separate layer; ethanol mixes completely(1)
a logical order, e.g. use bromine water to find pentene, then a carbonate or indicator to tell ethanol from propanoic acid(1)
ignore tests with sodium metal unless results correctly linked (both ethanol and aqueous propanoic acid give hydrogen; pentene does not)(1)

Level	Marks	What it looks like
Level 3	5–6	A clear, logically ordered method using at least two suitable tests that between them distinguish all three liquids, with correct results given for each liquid in each test.
Level 2	3–4	Suitable tests are described that would distinguish at least two of the liquids, with mostly correct results; the method may be incomplete or lack some 'no change' results.
Level 1	1–2	At least one relevant test is given, with a correct result for one liquid, or relevant results with the test unclear.
No relevant content	0	Nothing worthy of credit.

Answer: Bromine water: only pentene turns it from orange to colourless. Sodium carbonate: only propanoic acid fizzes (CO₂ turns limewater milky); ethanol gives no fizzing. (Universal indicator: propanoic acid acidic, ethanol pH 7.)
Examiner tip: Top-level answers give a result for every liquid in each test, including the 'no change' results; describing only the positive result for one liquid caps you in Level 2.

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