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Leaving Cert Physics & Chemistry: Heats of reaction & Hess's law

How often Heats of reaction & Hess's law 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

Heats of reaction & Hess's law, Higher Level(7 marks)

Quick ones on Heats of reaction & Hess's law.

(a)Heat of combustion of a substance is defined as the…
  1. Heat given out when 1 gram burns in air
  2. Heat change when 1 mole burns completely in excess oxygen
  3. Heat change when 1 mole forms from its elements
(b)ΔH(combustion) of methane is −890 kJ mol⁻¹. Heat released when 8 g of CH4 burns? (C = 12, H = 1)
  1. 7120 kJ
  2. 890 kJ
  3. 445 kJ
(c)The heat change when 1 mole of a compound forms from its elements in their standard states is the heat of…
  1. Formation
  2. Solution
  3. Combustion
Show the answers

(a) Heat change when 1 mole burns completely in excess oxygen

(b) 445 kJ

(c) Formation

Your turn: Higher Level questions on Heats of reaction & Hess's law.

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(m), Q12(d)
2024Q7(i), Q12(c)
2023Q7(n), Q12(c)
2022Q7(m), Q7(n), Q12(b)
2021Q7(m), Q12(d)
2019Q7(k), Q12(c)
2018Q7(l), Q12(c)
2017Q7(l), Q12(d)
2016Q7(j), Q12(a)

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

More Heats of reaction & Hess's law questions

Heats of reaction & Hess's law, 3 marks

A → B, ΔH = −100 kJ. B → C, ΔH = +40 kJ. ΔH for A → C?

  1. −140 kJ
  2. +60 kJ
  3. −60 kJ
Show the answer

−60 kJ

Hess's law: the total heat change is the same by any route, so add the steps: −100 + 40 = −60 kJ. −140 ignores the sign of the second step.

Heats of reaction & Hess's law, 3 marks

Breaking chemical bonds is ___ and making bonds is ___.

  1. Exothermic; exothermic
  2. Endothermic; exothermic
  3. Exothermic; endothermic
Show the answer

Endothermic; exothermic

Energy must be put in to pull bonded atoms apart, and energy is given out when new bonds form. ΔH for a reaction is the balance of the two.

Heats of reaction & Hess's law, 3 marks

The heat of formation of an element in its standard state (e.g. O2 gas) is taken as…

  1. Zero
  2. −394 kJ mol⁻¹
  3. +1 kJ mol⁻¹
Show the answer

Zero

Forming an element from itself involves no change, so its heat of formation is zero. That is why O2 is left out when heats of formation are combined.

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(o), Q11
2024Q7(k), Q11
2023Q7(j), Q11
2022Q7(l), Q7(m), Q11
2021Q7(l), Q11
2019Q7(g), Q7(h)
2018Q7(j), Q11
2017Q11
2016Q7(l)

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

More Heats of reaction & Hess's law questions

Heats of reaction & Hess's law, 2 marks

An exothermic reaction is one that?

  1. Takes in heat from the surroundings
  2. Has no heat change
  3. Gives out heat to the surroundings
Show the answer

Gives out heat to the surroundings

Burning fuels and neutralising acids are exothermic: the surroundings get warmer. Reactions that take in heat are endothermic.

Heats of reaction & Hess's law, 2 marks

The sign of ΔH for an exothermic reaction is?

  1. Zero
  2. Negative
  3. Positive
Show the answer

Negative

The chemicals lose energy to the surroundings, so the heat change for the system is negative. Endothermic reactions have a positive ΔH.

Heats of reaction & Hess's law, 2 marks

The heat of combustion of a substance is the heat change when?

  1. 1 mole of it burns completely in oxygen
  2. 1 mole of it melts at its melting point
  3. 1 mole of it dissolves completely in water
Show the answer

1 mole of it burns completely in oxygen

It is measured in kJ/mol with a bomb calorimeter. Complete combustion of a hydrocarbon gives only carbon dioxide and water.

Other Physics & Chemistry topics

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