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Leaving Cert Chemistry: Fuels and heats of reaction

How often Fuels and heats of reaction 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.

Fuels and heats of reaction, Higher Level(6 marks)

Quick ones on Fuels and heats of reaction.

(a)Sign of the enthalpy change for an exothermic reaction?
  1. Positive
  2. Negative
  3. Always zero
(b)Law used to calculate a heat of reaction indirectly from other known heat changes?
  1. Boyle's law
  2. Le Chatelier's principle
  3. Hess's law
(c)Breaking the bonds in the reactants is?
  1. Endothermic
  2. Neither
  3. Exothermic
Show the answers

(a) Negative

(b) Hess's law

(c) Endothermic

Your turn: Higher Level questions on Fuels and heats of reaction.

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
2026Q6
2025Q6, Q11(b)
2024Q4(g), Q6
2023Q3, Q6
2022Q6
2021Q6
2019Q4(g), Q6
2018Q3, Q6
2017Q6
2016Q6

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

More Fuels and heats of reaction questions

Fuels and heats of reaction, 3 marks

Definition of heat of combustion?

  1. The heat change when one gram of a substance is completely burned in excess oxygen
  2. The heat needed to start one mole of a fuel burning in air
  3. The heat change when one mole of a substance is completely burned in excess oxygen
Show the answer

The heat change when one mole of a substance is completely burned in excess oxygen

It is quoted in kJ mol⁻¹ and is always negative, because combustion is exothermic.

Fuels and heats of reaction, 2 marks

Definition of bond enthalpy?

  1. Heat change when one mole of a substance burns completely in oxygen
  2. Energy needed to break one mole of a covalent bond, with all species gaseous
  3. Energy released when one mole of an ionic lattice forms from its gaseous ions
Show the answer

Energy needed to break one mole of a covalent bond, with all species gaseous

Values for bonds such as C-H are averages over many compounds, so bond-enthalpy calculations give approximate heats of reaction.

Fuels and heats of reaction, 3 marks

Heats of combustion of methanol, ethanol and propan-1-ol get more negative. Why?

  1. Each extra CH2 group adds bonds that release more energy on burning
  2. The larger alcohols are more polar, so they release more heat
  3. The larger alcohols burn more slowly, so less heat is lost
Show the answer

Each extra CH2 group adds bonds that release more energy on burning

Each step adds one CH2, so the heat of combustion becomes more negative by a roughly constant amount, about 650 kJ per mole.

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
2026Q4(b), Q11(b)
2025Q4(g)
2024Q4(d), Q6
2023Q4(g), Q4(h), Q6, Q11(c)
2022Q4(d), Q4(h), Q4(j)
2021Q3, Q4(h), Q6
2019Q4(h), Q6, Q10(a)
2018Q4(f), Q6
2017Q3, Q6
2016Q4(d), Q6

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

More Fuels and heats of reaction questions

Fuels and heats of reaction, 2 marks

Where is methane formed naturally?

  1. In the ozone layer, from sunlight acting on oxygen
  2. In limestone rocks, from the breakdown of carbonates
  3. In slurry pits and marshes, from decay without air
Show the answer

In slurry pits and marshes, from decay without air

Bacteria break down plant and animal matter in the absence of air, giving methane. It is also found in coal mines.

Fuels and heats of reaction, 2 marks

LPG (liquefied petroleum gas) is mainly a mixture of?

  1. Octane and heptane
  2. Propane and butane
  3. Methane and ethane
Show the answer

Propane and butane

Propane and butane are easily liquefied under pressure, so they can be stored in cylinders.

Fuels and heats of reaction, 3 marks

The heat of combustion of methane is -890 kJ/mol. This means?

  1. 890 kJ of heat is released when 1 mole of methane burns completely
  2. 890 kJ of heat is taken in when 1 mole of methane burns
  3. 890 kJ of heat is released when 1 gram of methane burns
Show the answer

890 kJ of heat is released when 1 mole of methane burns completely

Heat of combustion is per mole, burned completely in excess oxygen. The negative sign means heat is given out.

Other Chemistry topics

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