AS Level Chemistry chapter 14: Hydrocarbons MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
44 exam-style questions on Hydrocarbons, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. Why are long-chain alkanes from crude oil cracked?
A
to produce shorter-chain alkanes for fuels and alkenes for making polymers
B
to make larger alkanes that are needed for lubricating oils
C
to remove sulfur impurities before the fuel is burned
D
to convert alkenes into alkanes, which are more useful
Show answer
Answer: A. Demand for petrol-sized alkanes is greater than the supply from fractional distillation, and the alkenes formed are valuable feedstock for polymers and other chemicals. Cracking breaks long chains, so it makes smaller molecules, not larger ones.
2. Which alkane gives only one monochlorinated product when it reacts with chlorine in ultraviolet light?
A
pentane
B
2-methylbutane
C
2,3-dimethylbutane
D
2,2-dimethylpropane
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Answer: D. In 2,2-dimethylpropane, C(CH3)4, all 12 hydrogen atoms are on equivalent CH3 groups, so any substitution gives (CH3)3CCH2Cl. 2,3-dimethylbutane looks symmetrical but has two kinds of H (on CH3 and on CH), giving two products.
3. What mass of carbon dioxide is formed when 5.80 g of butane, C4H10, burns completely?
A
4.40 g
B
8.80 g
C
17.6 g
D
35.2 g
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Answer: C. n(C4H10) = 5.80 ÷ 58.0 = 0.100 mol; each molecule gives 4CO2, so 0.400 mol × 44.0 = 17.6 g. Taking the 8 from 2C4H10 + 13O2 → 8CO2 + 10H2O as moles of CO2 per mole of butane gives 35.2 g.
4. What is observed when ethene is bubbled through aqueous bromine?
A
the orange colour is decolourised
B
a white precipitate forms
C
the solution turns purple
D
the solution turns green
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Answer: A. Bromine adds across the C=C bond to give colourless 1,2-dibromoethane (and some 2-bromoethanol), so the orange colour disappears. Alkanes give no change unless in UV light.
5. Why is the incomplete combustion of alkane fuels in a car engine dangerous?
A
it produces carbon monoxide, which is toxic
B
it produces sulfur dioxide
C
it produces nitrogen dioxide
D
it produces ozone
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Answer: A. With limited oxygen, carbon is only partly oxidised to CO (and soot), and CO binds to haemoglobin. Nitrogen oxides come from N2 and O2 at high temperature, not from incomplete combustion.
6. What is the role of ultraviolet light in the reaction of methane with bromine?
A
it heats the mixture so that the molecules collide more often
B
it breaks C–H bonds to form H+ ions
C
it provides the energy for homolytic fission of Br–Br bonds, forming bromine free radicals
D
it acts as a catalyst that is regenerated
Show answer
Answer: C. UV light supplies the energy to split Br2 evenly into two Br• radicals (initiation). The Br–Br bond is much weaker than the C–H bond, so it breaks first.
7. Which reagents and conditions are used to make ethanol from ethene industrially?
A
aqueous sodium hydroxide, heat under reflux
B
cold dilute acidified potassium manganate(VII)
C
hydrogen with a nickel catalyst
D
steam with a phosphoric acid catalyst, high temperature and pressure
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Answer: D. Steam adds across the C=C bond using an H3PO4 catalyst at about 300 °C and 6–7 MPa. Cold dilute KMnO4 would give a diol, and hydrogen would give ethane.
8. Which reagents and conditions convert propene into propane?
A
hydrogen with a nickel catalyst and heat
B
hydrogen in ultraviolet light
C
steam with a phosphoric acid catalyst
D
aqueous sodium hydroxide and heat
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Answer: A. Hydrogenation adds H2 across the C=C bond and needs a Ni (or Pt) catalyst and heat. Steam with H3PO4 would give an alcohol.
9. Which reagents and conditions convert ethanol into ethene?
A
NaOH in ethanol, heat
B
H2 with Ni catalyst, heat
C
acidified K2Cr2O7, reflux
D
concentrated H2SO4 (or heated Al2O3), heat
Show answer
Answer: D. Dehydration of an alcohol removes water: C2H5OH → C2H4 + H2O, using a concentrated acid or hot aluminium oxide. Ethanolic NaOH is for eliminating HX from a halogenoalkane.
10. An alkene with the formula C5H10 is heated with hot, concentrated, acidified potassium manganate(VII). The only organic products are propanone and ethanoic acid. What is the alkene?
A
pent-2-ene
B
3-methylbut-1-ene
C
2-methylbut-1-ene
D
2-methylbut-2-ene
Show answer
Answer: D. Propanone comes from a (CH3)2C= end and ethanoic acid from a =CHCH3 end, so the alkene is (CH3)2C=CHCH3. Pent-2-ene would give ethanoic and propanoic acids; the but-1-enes have a =CH2 end, which gives CO2.
44, each with the answer and a short explanation, across 2 topics: Alkanes, Alkenes. Every chapter test draws ten of them, so you can take it many times.
Is the AS Level Chemistry chapter 14 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
