S5 Chemistry Paper: Alkanes, Alkenes, Addition Reactions And Addition Polymerisation (100 Marks)

S5 Chemistry Paper: Alkanes, Alkenes, Addition Reactions and Addition Polymerisation (100 marks)

2026-11-12

Chemistry – Organic Chemistry Basics

S5 Chemistry: 6 questions, 24 parts, 100 marks, 75 minutes. Alkanes, alkene addition, addition polymerisation, empirical formulae from percentages and bromine calculations.

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Question 1 Alkanes: General Formula, Molecular Formula and Combustion (16 marks)

Answer each part independently. Show your working and state the units (C = 12.0, H = 1.0).

#	Question	Answer and marking notes
1	The general formula of an alkane is CnH2n+2. A straight-chain alkane contains 7 carbon atoms. Give its molecular formula and the number of hydrogen atoms.	Answer: the molecular formula is C7H16; hydrogen atoms = 2 x 7 + 2 = 16. (4 marks)
2	An alkane has a relative molecular mass of 86. Find its number of carbon atoms and its molecular formula (C = 12.0, H = 1.0).	Answer: 14n + 2 = 86 gives 14n = 84, so n = 6; the molecular formula is C6H14. (4 marks)
3	1.0 mol of butane (C4H10) burns completely in excess oxygen. Find the amount of oxygen needed and the amounts of carbon dioxide and water formed.	Answer: C4H10 + 6.5 O2 -> 4 CO2 + 5 H2O; oxygen 6.5 mol, carbon dioxide 4 mol and water 5 mol. (4 marks)
4	How many structural isomers has C4H10, and how many has C5H12? Name each isomer.	Answer: C4H10 has 2 isomers (butane and 2-methylpropane); C5H12 has 3 isomers (pentane, 2-methylbutane and 2,2-dimethylpropane). (4 marks)

Question 2 Addition Reactions of Alkenes (18 marks)

Complete the parts below. Give structural or molecular formulae in your equations.

#	Question	Answer and marking notes
5	An alkene has a relative molecular mass of 70. Find its number of carbon atoms and its molecular formula.	Answer: an alkene has the formula CnH2n, so 14n = 70 and n = 5; the molecular formula is C5H10. (4 marks)
6	Ethene reacts with hydrogen bromide. Write the equation, name the product and state the reaction type.	Answer: C2H4 + HBr -> C2H5Br; the product is bromoethane; this is an addition reaction. (4 marks)
7	10.5 g of propene (C3H6, Mr = 42) reacts completely with hydrogen. Find the amount of hydrogen needed and its volume at a molar volume of 24.0 dm3/mol.	Answer: propene = 10.5 / 42 = 0.25 mol, so 0.25 mol of hydrogen is needed; volume = 0.25 x 24.0 = 6.0 dm3. (5 marks)
8	0.10 mol of an alkene decolourises bromine water completely. Find the amount and the mass of bromine (Br2, Mr = 160) used.	Answer: the alkene adds bromine in a 1 : 1 ratio, so 0.1 mol of bromine reacts; mass = 0.1 x 160 = 16.0 g. (5 marks)

Question 3 Addition Polymerisation and Polymers (18 marks)

When finding the degree of polymerisation, divide by the relative mass of the repeat unit.

#	Question	Answer and marking notes
9	A sample of poly(ethene) has a relative molecular mass of 56 000. The repeat unit is C2H4 (Mr = 28). Find the degree of polymerisation.	Answer: degree of polymerisation = 56 000 / 28 = 2000. (5 marks)
10	The repeat unit of poly(chloroethene) is C2H3Cl (Mr = 62.5), and a sample has a relative molecular mass of 125 000. Find the degree of polymerisation.	Answer: degree of polymerisation = 125 000 / 62.5 = 2000. (5 marks)
11	2.8 g of ethene is polymerised completely. If the percentage yield is 90.0 per cent, find the mass of poly(ethene) obtained.	Answer: ethene = 0.1 mol, theoretical mass = 2.8 g; actual mass = 2.8 x 0.90 = 2.52 g. (4 marks)
12	The repeat unit of poly(propene) is C3H6 (Mr = 42) and the degree of polymerisation is 1500. Find the relative molecular mass of the sample.	Answer: relative molecular mass = 42 x 1500 = 63000. (4 marks)

Question 4 Deducing Molecular Formulae from Percentage Composition (16 marks)

Give the empirical formula as the simplest whole-number ratio.

#	Question	Answer and marking notes
13	A hydrocarbon contains 85.7 per cent carbon and 14.3 per cent hydrogen by mass. Find its empirical formula; given a relative molecular mass of 56, find its molecular formula.	Answer: carbon 85.7 / 12.0 = 7.14, hydrogen 14.3 / 1.0 = 14.3, ratio about 1 : 2, so the empirical formula is CH2; empirical mass 14.0, 56 / 14.0 = 4, so the molecular formula is C4H8. (4 marks)
14	Another hydrocarbon has a relative molecular mass of 58 and contains 82.8 per cent carbon and 17.2 per cent hydrogen by mass. Find its empirical formula and molecular formula.	Answer: carbon 82.8 / 12.0 = 6.90, hydrogen 17.2 / 1.0 = 17.2, ratio 2 : 5, empirical formula C2H5 (mass 29.0); 58 / 29.0 = 2, so the molecular formula is C4H10. (4 marks)
15	0.10 mol of a hydrocarbon burns completely to give 0.40 mol of carbon dioxide and 0.50 mol of water. Find its molecular formula.	Answer: each mole of the hydrocarbon contains 4.0 mol of carbon (4 atoms) and 10.0 mol of hydrogen (10 atoms), so the formula is C4H10. (4 marks)
16	How many structural isomers has C4H10? Name them and state which one has the higher boiling point.	Answer: there are 2 isomers, butane and 2-methylpropane; butane is a straight chain with greater contact area between molecules, so it has the higher boiling point. (4 marks)

Question 5 Reaction Types and Equations (16 marks)

State the reaction type for each part (substitution, addition, addition polymerisation or dehydrogenation).

#	Question	Answer and marking notes
17	Methane reacts with chlorine in ultraviolet light. State the reaction type and give one pair of products.	Answer: this is a substitution reaction (free-radical substitution); the products are chloromethane (CH3Cl) and hydrogen chloride (HCl). (4 marks)
18	Ethene reacts with steam with an acid catalyst. State the reaction type and name the product.	Answer: this is an addition reaction (hydration); the product is ethanol (C2H5OH). (4 marks)
19	Many ethene molecules join together with a catalyst to form poly(ethene). State the reaction type and the repeat unit.	Answer: this is addition polymerisation; the repeat unit is C2H4 (or -CH2-CH2-). (4 marks)
20	1.0 mol of propane is dehydrogenated to propene and hydrogen. Write the equation and find the total amount of gas after the reaction.	Answer: C3H8 -> C3H6 + H2; after the reaction there are 1.0 mol of propene and 1.0 mol of hydrogen, a total of 2.0 mol. (4 marks)

Question 6 Degree of Unsaturation and Combined Calculations (16 marks)

Show your working; the relative atomic mass of bromine is 80.0.

#	Question	Answer and marking notes
21	0.10 mol of a hydrocarbon (relative molecular mass 54) adds 0.20 mol of hydrogen completely. Deduce the number of unsaturated bonds and the molecular formula.	Answer: each mole of the hydrocarbon adds 2 mol of hydrogen, so it has 2 carbon-carbon double bonds or 1 carbon-carbon triple bond; the formula is C4H6 (Mr = 54). (4 marks)
22	5.6 g of butene (C4H8, Mr = 56) reacts completely with bromine. Find the mass of bromine used and the mass of the product.	Answer: butene is 0.1 mol and adds bromine in a 1 : 1 ratio, so 0.1 mol of bromine (16.0 g); the product C4H8Br2 has Mr = 216 and mass 21.6 g. (4 marks)
23	If the experiment actually gives only 19.44 g of the product, find the percentage yield.	Answer: percentage yield = 19.44 / 21.6 x 100 per cent = 90.0 per cent. (4 marks)
24	0.10 mol of butene burns completely. Find the amount of oxygen needed and the amount of carbon dioxide formed.	Answer: C4H8 + 6 O2 -> 4 CO2 + 4 H2O; oxygen = 6 x 0.10 = 0.6 mol and carbon dioxide = 4 x 0.10 = 0.4 mol. (4 marks)

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