Exothermic and endothermic reactions

O Level / IGCSE Chemistry · Chemical energetics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Every reaction involves an energy change. An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings rises. An endothermic reaction takes thermal energy in, so the temperature of the surroundings falls.

The energy transferred is the enthalpy change, ΔH. It is negative for exothermic reactions and positive for endothermic ones. On a reaction pathway diagram the products sit below the reactants in an exothermic reaction and above them in an endothermic one. The hump between them is the activation energy.

Breaking bonds takes energy in. Making bonds gives energy out. The sign of ΔH depends on which amount is larger.

Rules to remember

Common mistake

Students subtract the wrong way round or forget to count every bond. Always do bonds broken minus bonds made, and multiply each bond energy by the number of those bonds in the balanced equation.

Worked example

Bond energies in kJ/mol: H–H 436, Br–Br 193, H–Br 366. What is the enthalpy change for H2 + Br2 → 2HBr?

  1. Bonds broken: one H–H and one Br–Br = 436 + 193 = 629 kJ taken in.
  2. Bonds made: two H–Br = 2 × 366 = 732 kJ given out.
  3. ΔH = broken − made = 629 − 732 = −103 kJ/mol.

Answer: ΔH = −103 kJ/mol. The sign is negative, so the reaction is exothermic.

Practice questions

Try each one, then open the answer.

1. In an exothermic reaction

  1. A
    energy is given out and the enthalpy change is negative
  2. B
    the products contain more energy than the reactants do
  3. C
    the temperature of the surroundings falls as energy is taken in
  4. D
    energy is absorbed from the surroundings and the enthalpy change is positive
Show answer

Answer: A. Exothermic reactions release energy, so the surroundings warm up and ΔH is negative (products lower in energy).

2. CH4 + 2O2 → CO2 + 2H2O. Bond energies in kJ/mol: C–H 410, O=O 496, C=O 805, O–H 460. What is the enthalpy change?

  1. A
    +818 kJ/mol
  2. B
    −818 kJ/mol
  3. C
    −1314 kJ/mol
  4. D
    −6082 kJ/mol
Show answer

Answer: B. Broken: 4 × 410 + 2 × 496 = 2632 kJ. Made: 2 × 805 + 4 × 460 = 3450 kJ. ΔH = 2632 − 3450 = −818 kJ/mol.

3. What is the activation energy of a reaction?

  1. A
    the energy released when the new bonds in the products form
  2. B
    the minimum energy that colliding particles must have in order to react
  3. C
    the total energy contained in the products of the reaction
  4. D
    the difference in energy between the reactants and the products
Show answer

Answer: B. Activation energy is the minimum energy barrier that colliding particles must reach to react; a catalyst lowers it.

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