S5 Chemistry Practice Paper: Organic Chemistry Basics - Alkanes And Alkenes (12 Questions, 100 Marks, 75 Minutes)

S5 Chemistry Practice Paper: Organic Chemistry Basics – Alkanes and Alkenes (12 Questions, 100 Marks, 75 Minutes)

2026-11-09

S5 Chemistry: Alkanes, Alkenes and Functional Groups

Grade 11 chemistry paper on alkanes and alkenes: naming, isomerism, formulae, combustion, addition, polymerisation, cracking and bioethanol. 12 questions with full answers.

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Instructions: This paper has three sections, A, B and C. Answer all questions. Total marks: 100. Time allowed: 75 minutes.

Show the formula used, the values substituted and the units. Give numerical answers to three significant figures.

In explanations, name the law used, for example the law of alkane or conservation of energy.

Spend about 20 minutes on Section A, 30 minutes on Section B and the last 25 minutes on Section C.

Section A: Formulae and Isomerism (Questions 1-4)

Four questions, 28 marks in total. Suggested time: 21 minutes.

#	Question	Answer and marking notes
1	Butane has the molecular formula C4H10. (a) Calculate the molar mass of butane. (b) Calculate the percentage by mass of carbon in butane, giving your answer to three significant figures.	(a) Molar mass = 4 x 12.011 + 10 x 1.008 = 58.1 g mol-1 (3 marks); (b) percentage by mass of C = 4 x 12.011 / 58.124 x 100% = 82.7% (1 mark for working, 2 marks for answer).
2	(a) State the general formula of the alkane homologous series. (b) An alkane contains 8 carbon atoms. Write its molecular formula and the number of hydrogen atoms. (c) Give two features common to all members of the alkane homologous series.	(a) CnH2n+2 (2 marks); (b) C8H18, 18 hydrogen atoms (2 marks); (c) any two: members differ by one CH2 unit, similar chemical properties, physical properties vary regularly with chain length (2 marks).
3	A hydrocarbon contains 85.6% carbon and 14.4% hydrogen by mass. Its molar mass is 56.1 g mol-1. (a) Find its empirical formula. (b) Find its molecular formula. (c) State a homologous series it may belong to and give one example.	(a) C:H = 85.6/12.011 : 14.4/1.008 = 7.13 : 14.29, i.e. 1:2, so empirical formula is CH2 (3 marks); (b) empirical formula mass 14.03, 56.1 / 14.03 is about 4, so molecular formula is C4H8 (3 marks); (c) alkene or cycloalkane series, e.g. but-1-ene (2 marks).
4	(a) Name all the structural isomers with molecular formula C4H10. (b) How many structural isomers does C5H12 have? Name any two of them.	(a) butane and 2-methylpropane (1 mark each); (b) 3 isomers (2 marks): pentane, 2-methylbutane, 2,2-dimethylpropane (any two, 2 marks each).

Section B: Combustion, Addition and Polymerisation (Questions 5-8)

Four questions, 32 marks in total. Suggested time: 24 minutes.

#	Question	Answer and marking notes
5	5.80 g of butane burns completely in excess oxygen. (a) Write the balanced equation. (b) Calculate the number of moles of oxygen required. (c) Calculate the mass of carbon dioxide formed. (d) Calculate the mass of water formed.	(a) C4H10 + 13/2 O2 -> 4CO2 + 5H2O, or 2C4H10 + 13O2 -> 8CO2 + 10H2O (2 marks); (b) n = 5.80 / 58.12 = 0.0998 mol, oxygen = 6.5 x 0.0998 = 0.649 mol (2 marks); (c) carbon dioxide = 4 x 0.0998 x 44.01 = 17.6 g (2 marks); (d) water = 5 x 0.0998 x 18.02 = 8.99 g (2 marks).
6	At room temperature and pressure, 2.24 dm3 of ethene reacts with bromine water. (a) State the type of reaction and write the equation. (b) Calculate the mass of bromine decolourised (Br = 79.9). (c) Describe the colour change observed.	(a) Addition, C2H4 + Br2 -> C2H4Br2 (3 marks); (b) ethene = 2.24 / 22.4 = 0.100 mol, bromine = 0.100 x 159.8 = 16.0 g (3 marks); (c) the orange-brown bromine water is decolourised (2 marks).
7	Propene undergoes an addition reaction with hydrogen bromide. (a) Give the structure and name of the major product. (b) Explain, in terms of carbocation stability, why this product is major. (c) Name the minor product.	(a) 2-bromopropane, CH3-CHBr-CH3 (3 marks); (b) a secondary carbocation is more stable than a primary one, so bromine adds to the carbon with fewer hydrogen atoms (3 marks); (c) 1-bromopropane (2 marks).
8	Ethene undergoes addition polymerisation. (a) Draw the repeat unit. (b) Calculate the molar mass of a poly(ethene) molecule containing 1000 repeat units. (c) State one difference between addition polymerisation and condensation polymerisation.	(a) -CH2-CH2- (2 marks); (b) 1000 x 28.054 = 28054, i.e. about 2.81 x 10^4 g mol-1 (3 marks); (c) addition polymerisation releases no small molecule while condensation polymerisation releases water or another small molecule (3 marks).

Section C: Chemical Tests, Cracking and Applications (Questions 9-12)

Four questions, 40 marks in total. Suggested time: 30 minutes.

#	Question	Answer and marking notes
9	Dodecane, C12H26, is cracked to give pentane and one alkene. (a) Give the molecular formula and name of the alkene. (b) Use molar masses to show that mass is conserved. (c) Explain the industrial purpose of cracking.	(a) C7H14, hept-1-ene (3 marks); (b) molar masses: dodecane 170.3, pentane 72.2, heptene 98.2; 72.2 + 98.2 = 170.4, matching 170.3, so mass is conserved (4 marks); (c) to convert long-chain alkanes into short-chain alkanes (petrol) and alkenes (chemical feedstock) that are in greater demand (3 marks).
10	Use chemical tests to distinguish hexane from hex-1-ene. (a) Describe two tests and the observation for each. (b) Write the equation for one of the tests. (c) Explain why these two tests cannot distinguish hexane from heptane.	(a) Bromine water: the alkene decolourises it while the alkane does not; acidified potassium permanganate: the alkene turns it colourless or gives a brown precipitate while the alkane does not (6 marks); (b) C6H12 + Br2 -> C6H12Br2 (2 marks); (c) both are alkanes and give no reaction with these reagents (2 marks).
11	2.24 dm3 of ethene burns completely at room temperature and pressure. (a) Write the balanced equation. (b) Calculate the volume of oxygen required. (c) Calculate the volume of carbon dioxide formed. (d) If oxygen supply is limited, name one possible product.	(a) C2H4 + 3O2 -> 2CO2 + 2H2O (2 marks); (b) volume of oxygen = 3 x 2.24 = 6.72 dm3 (3 marks); (c) volume of carbon dioxide = 2 x 2.24 = 4.48 dm3 (3 marks); (d) carbon monoxide, or carbon (soot) (2 marks).
12	Ethanol can be made by fermentation. (a) Write the equation for fermentation of glucose. (b) Calculate the mass of ethanol obtainable from 90.0 g of glucose at 100% yield. (c) State one environmental advantage and one limitation of using ethanol as a fuel.	(a) C6H12O6 -> 2C2H5OH + 2CO2 (3 marks); (b) glucose = 90.0 / 180.16 = 0.4996 mol, ethanol = 0.9991 mol, mass = 0.9991 x 46.07 = 46.0 g (5 marks); (c) advantage: renewable biomass with a shorter carbon cycle; limitation: lower heat of combustion, large land area needed (2 marks).

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