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Leaving Cert Physics & Chemistry: Hydrocarbons

How often Hydrocarbons comes up on the Physics & Chemistry papers, every year it was asked, and questions to try.

HL Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

OL Asked on 9 of the last 9 Ordinary Level papers, most recently in 2025. banker

Hydrocarbons, Higher Level(6 marks)

Quick ones on Hydrocarbons.

(a)What is a hydrocarbon?
  1. Any compound that contains carbon
  2. A compound of carbon and hydrogen only
  3. A compound of carbon, hydrogen and oxygen
(b)Ethene is bubbled through bromine water. What colour change is seen?
  1. Purple to colourless
  2. Colourless to red-brown
  3. Red-brown to colourless
(c)General formula of the alkanes?
  1. CnH2n+2
  2. CnH2n-2
  3. CnH2n
Show the answers

(a) A compound of carbon and hydrogen only

(b) Red-brown to colourless

(c) CnH2n+2

Your turn: Higher Level questions on Hydrocarbons.

Higher Level

Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

Every paper, year by year

YearWhere it came up
2025Q7(q), Q7(r), Q11
2024Q7(o), Q7(p), Q7(r), Q11
2023Q7(o), Q7(p), Q7(r), Q11
2022Q11
2021Q7(o), Q7(p), Q7(r), Q11
2019Q7(l), Q7(m), Q7(n), Q11
2018Q7(m), Q7(o), Q11
2017Q7(m), Q7(o), Q11
2016Q7(m), Q11

Links open the State Examinations Commission’s paper for that year.

More Hydrocarbons questions

Hydrocarbons, 3 marks

How many moles of O2 are needed to burn 1 mole of ethene completely?

  1. 2
  2. 4
  3. 3
Show the answer

3

C2H4 + 3O2 → 2CO2 + 2H2O. The products contain 4 + 2 = 6 O atoms, which is 3 O2 molecules.

Hydrocarbons, 2 marks

General formula of the alkenes?

  1. CnH2n-2
  2. CnH2n
  3. CnH2n+2
Show the answer

CnH2n

Each alkene has one C=C double bond, so it has two fewer H atoms than the alkane with the same carbons: ethene C2H4, propene C3H6.

Hydrocarbons, 2 marks

Aromatic compounds are compounds that contain…

  1. A benzene ring
  2. Only single carbon–carbon bonds
  3. An –OH group
Show the answer

A benzene ring

Aromatic compounds, such as benzene and methylbenzene, contain the benzene ring. Compounds without it, such as alkanes and alkenes, are aliphatic.

Ordinary Level

Asked on 9 of the last 9 Ordinary Level papers, most recently in 2025. banker

Every paper, year by year

YearWhere it came up
2025Q7(k), Q11
2024Q7(o), Q7(p), Q11
2023Q7(p), Q11
2022Q11
2021Q7(p), Q7(r), Q11
2019Q7(o), Q11
2018Q7(o), Q11
2017Q7(k), Q11
2016Q7(n), Q11

Links open the State Examinations Commission’s paper for that year.

More Hydrocarbons questions

Hydrocarbons, 2 marks

Molecular formula of propane?

  1. C3H6
  2. C3H4
  3. C3H8
Show the answer

C3H8

Alkanes follow CnH2n+2. For n = 3: 2 × 3 + 2 = 8. C3H6 is propene, an alkene (CnH2n).

Hydrocarbons, 2 marks

Which of these compounds is aromatic?

  1. Ethene
  2. Benzene
  3. Ethane
Show the answer

Benzene

Aromatic compounds contain a benzene ring, a flat ring of 6 carbons with delocalised electrons. Benzene usually reacts by substitution, not addition.

Hydrocarbons, 2 marks

Bromine water is shaken with an unsaturated compound such as ethene. Its colour changes from?

  1. Red-brown to colourless
  2. Colourless to red-brown
  3. Blue to red
Show the answer

Red-brown to colourless

Bromine adds across the C=C double bond, so its colour disappears. Acidified potassium permanganate is another test: purple to colourless.

Other Physics & Chemistry topics

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