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Leaving Cert Chemistry: Isomerism

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

HL Asked on 6 of the last 10 Higher Level papers, most recently in 2026. most years

OL Asked on 2 of the last 10 Ordinary Level papers, most recently in 2026.

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

Isomerism, Higher Level(8 marks)

Quick ones on Isomerism.

(a)Type of structural isomerism shown by but-1-ene and but-2-ene?
  1. Functional group isomerism
  2. Positional isomerism
  3. Chain isomerism
(b)Which isomer of C4H10 is branched?
  1. 2-methylbutane
  2. Butane (n-butane)
  3. 2-methylpropane
(c)Number of structural isomers with the molecular formula C4H10?
  1. 2
  2. 4
  3. 3
Show the answers

(a) Positional isomerism

(b) 2-methylpropane

(c) 2

Your turn: Higher Level questions on Isomerism.

Higher Level

Asked on 6 of the last 10 Higher Level papers, most recently in 2026. most years

Every paper, year by year

YearWhere it came up
2026Q6
2025Q4(g)
2024Q6, Q11(b)
2023Not asked
2022Not asked
2021Not asked
2019Q6
2018Q8
2017Q8
2016Not asked

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

More Isomerism questions

Isomerism, 2 marks

Definition of structural isomers?

  1. Compounds with the same structural formula but different molecular masses
  2. Compounds with the same empirical formula only, but different numbers of atoms
  3. Compounds with the same molecular formula but different structural formulas
Show the answer

Compounds with the same molecular formula but different structural formulas

Because the atoms are joined differently, isomers have different physical properties such as boiling point.

Isomerism, 3 marks

Why can alkenes such as but-2-ene have cis and trans forms?

  1. They contain a benzene ring that cannot twist
  2. There is no free rotation about the C=C double bond
  3. They contain a triple bond that holds the groups in place
Show the answer

There is no free rotation about the C=C double bond

The pi bond would have to break for the groups to swap sides. Each carbon of the C=C also needs two different groups attached.

Isomerism, 2 marks

In a trans isomer, the two matching groups are?

  1. On opposite sides of the double bond
  2. On the same side of the double bond
  3. On the same carbon atom
Show the answer

On opposite sides of the double bond

Trans means across. In trans-but-2-ene the two CH3 groups are diagonally opposite across the C=C.

Ordinary Level

Asked on 2 of the last 10 Ordinary Level papers, most recently in 2026.

Every paper, year by year

YearWhere it came up
2026Q6
2025Not asked
2024Q6
2023Not asked
2022Not asked
2021Not asked
2019Not asked
2018Not asked
2017Not asked
2016Not asked

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

More Isomerism questions

Isomerism, 2 marks

Butane and methylpropane both have the formula C4H10. They are?

  1. The same compound
  2. Polymers of each other
  3. Structural isomers
Show the answer

Structural isomers

They have the same molecular formula but different structures. Methylpropane has a branched chain.

Isomerism, 2 marks

Which of these is an isomer of pentane, C5H12?

  1. Hexane, C6H14
  2. 2,2-dimethylpropane
  3. Butane, CH3CH2CH2CH3
Show the answer

2,2-dimethylpropane

2,2-dimethylpropane is also C5H12, with a branched structure. Butane is C4H10 and hexane is C6H14.

Other Chemistry topics

All of Leaving Cert Chemistry