Iron reacts with sulfur to form iron sulfide. During the reaction, the temperature of the mixture increases. Explain, in terms of bond breaking and bond making, why this reaction is exothermic.

AQA A-Level Chemistry (7405) — 3.1.8 Thermodynamics (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Energy is required to break the Fe–Fe and S–S bonds in the reactants.
Energy is released when Fe–S bonds are formed in the product.
The energy released in forming Fe–S bonds is greater than the energy required to break the original bonds.
Therefore there is an overall release of energy to the surroundings.
This release raises the temperature of the mixture, showing the reaction is exothermic.

Examiner tips

  • Use the sequence: break bonds → form bonds → compare energies → conclude exothermic. Include the key phrase "overall release of energy to the surroundings". Mention the temperature rise as evidence of exothermicity.

Common mistakes

  • Forgetting to state that energy is required to break bonds. Not comparing the energies of bond breaking and forming. Using vague terms like "heat is produced" without linking to bond energies.

Mark scheme (5 marks)

  1. Energy is required / taken in to break bonds
  2. Energy is released when new bonds are formed
  3. The energy released forming bonds is greater than the energy required to break bonds
  4. Therefore there is an overall release of energy to the surroundings
  5. This causes the temperature of the mixture / surroundings to increase, showing the reaction is exothermic

Key terms in this question

exothermic · bond breaking · bond making

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