Explain why the temperature of the surroundings decreases during an endothermic reaction, using ideas about bond breaking and bond forming.

Eduqas GCSE Chemistry — C5 Energy changes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

An endothermic reaction requires energy to break the bonds in the reactants. Energy is then released when new bonds are formed in the products, but the energy needed to break the original bonds is greater than the energy released by forming the new bonds. Consequently, the reaction absorbs net energy from the surroundings, so the temperature of the surroundings falls.

Examiner tips

  • Use the term "endothermic" and mention energy absorption
  • Explain bond breaking requires energy, bond forming releases energy
  • Show the inequality that net energy is taken from surroundings
  • Link the energy absorption to a drop in temperature

Common mistakes

  • Confusing exothermic with endothermic, saying temperature rises
  • Omitting the comparison of bond breaking vs forming energies
  • Using vague phrases like "energy is used" without specifying source or effect

Mark scheme (4 marks)

  1. Energy must be supplied / taken in to break bonds in the reactants
  2. Energy is released when new bonds are formed in the products
  3. The energy needed to break bonds is greater than the energy released when bonds are formed
  4. The reaction therefore takes in / absorbs energy from the surroundings, causing the temperature of the surroundings to decrease

Key terms in this question

endothermic reaction · bond breaking · bond forming · surroundings

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