When magnesium ribbon is added to dilute hydrochloric acid, a vigorous reaction occurs. Describe the energy change that takes place during this reaction, and explain how the breaking and forming of chemical bonds accounts for this energy change.

Cambridge International IGCSE Chemistry (0620) — 5.1 Exothermic and endothermic reactions · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The reaction is exothermic.

Energy is required to break the bonds in the reactants.

Energy is released when new bonds are formed in the products.

More energy is released in bond forming than is absorbed in bond breaking, so a net release of energy occurs.

Examiner tips

  • Use the word "exothermic" early. Show the energy flow: break → absorb, form → release. Explain that net release = release > absorb.
  • common_mistakes
  • :
  • Saying the reaction is endothermic. Forgetting to mention that energy is absorbed to break bonds. Not stating that the net energy change is a release.

Mark scheme (4 marks)

  1. The reaction is exothermic
  2. Energy is required / absorbed to break bonds (in the reactants)
  3. Energy is released when new bonds form (in the products)
  4. More energy is released in bond forming than is absorbed in bond breaking, resulting in a net release of energy

Key terms in this question

energy

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