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Leaving Cert Chemistry: Hydrocarbons

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

HL Asked on 10 of the last 10 Higher Level papers, most recently in 2026. banker

OL Asked on 10 of the last 10 Ordinary Level papers, most recently in 2026. banker

June 2027 is the first exam on the new Chemistry course. These come from the 2016–2026 papers, so read them as a guide.

Hydrocarbons, Higher Level(7 marks)

Quick ones on Hydrocarbons.

(a)Result of shaking an alkene with bromine water?
  1. The solution turns red-brown
  2. The red-brown colour disappears
  3. A white precipitate forms
(b)General formula of the alkanes?
  1. CnH2n-2
  2. CnH2n
  3. CnH2n+2
(c)General formula of the alkenes?
  1. CnH2n
  2. CnH2n-2
  3. CnH2n+2
Show the answers

(a) The red-brown colour disappears

(b) CnH2n+2

(c) CnH2n

Your turn: Higher Level questions on Hydrocarbons.

Higher Level

Asked on 10 of the last 10 Higher Level papers, most recently in 2026. banker

Every paper, year by year

YearWhere it came up
2026Q2, Q6
2025Q6
2024Q6
2023Q2, Q6, Q10(a)
2022Q6
2021Q4(k), Q6, Q11(a)
2019Q6
2018Q6, Q10(a)
2017Q2, Q6
2016Q6

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

More Hydrocarbons questions

Hydrocarbons, 3 marks

In benzene, the carbon to carbon bonds are?

  1. Alternating long single and short double bonds around the ring
  2. All the same length as ordinary carbon to carbon double bonds
  3. All the same length, intermediate between single and double
Show the answer

All the same length, intermediate between single and double

The six p electrons are delocalised in rings above and below the plane, which also makes benzene unusually stable.

Hydrocarbons, 2 marks

Why are alkanes relatively unreactive?

  1. They are ionic compounds with very strong lattices
  2. Their C-C and C-H bonds are strong sigma bonds of low polarity
  3. They contain pi bonds that hold electrons tightly
Show the answer

Their C-C and C-H bonds are strong sigma bonds of low polarity

There is no pi bond or polar region to attract reagents, so alkanes mainly burn or react with halogens in UV light.

Hydrocarbons, 2 marks

Bond angle around each carbon atom in an alkane such as ethane?

  1. 109.5 degrees
  2. 120 degrees
  3. 180 degrees
Show the answer

109.5 degrees

Each carbon in an alkane has four single bonds arranged tetrahedrally, so the angles are 109.5 degrees.

Ordinary Level

Asked on 10 of the last 10 Ordinary Level papers, most recently in 2026. banker

Every paper, year by year

YearWhere it came up
2026Q1, Q4(j), Q6
2025Q6
2024Q1, Q4(k), Q6
2023Q1, Q6
2022Q6
2021Q1, Q6, Q8
2019Q1, Q6
2018Q1, Q6
2017Q1, Q6
2016Q1, Q4(j), Q6

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

More Hydrocarbons questions

Hydrocarbons, 2 marks

Molecular formula of butane?

  1. C4H8
  2. C4H6
  3. C4H10
Show the answer

C4H10

Alkanes are CnH2n+2. With n = 4: 2 x 4 + 2 = 10 hydrogens.

Hydrocarbons, 2 marks

Each member of a homologous series differs from the next by?

  1. A C2H2 unit
  2. A CH2 unit
  3. A CH4 unit
Show the answer

A CH2 unit

For example methane CH4, ethane C2H6, propane C3H8. Members share a general formula and similar chemical properties.

Hydrocarbons, 2 marks

Molecular formula of benzene?

  1. C6H6
  2. C6H12
  3. C6H14
Show the answer

C6H6

Benzene is a ring of six carbons, each joined to one hydrogen. C6H12 is cyclohexane and C6H14 is hexane.

Other Chemistry topics

All of Leaving Cert Chemistry