A student mixes citric acid solution with sodium hydrogencarbonate powder. Bubbles are produced and the temperature of the mixture decreases. Explain why the temperature decreases, in terms of bond breaking and bond forming, and state what type of energy change this reaction represents.

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)

Bonds in the reactants must be broken to start the reaction.
Breaking bonds requires energy, which is absorbed from the surroundings.
New bonds are then formed in the products.
The energy released when the new bonds form is less than the energy absorbed when the old bonds break.
Therefore the reaction is endothermic, so the temperature of the mixture decreases.

Examiner tips

  • Use the phrase "bonds must be broken" and "bonds are formed" to show understanding of bond breaking/formation. Show the energy flow: absorbed > released. State the reaction is endothermic to link to the temperature drop.
  • common_mistakes
  • :
  • Saying the reaction is exothermic or ignoring the energy balance. Forgetting to mention that energy is absorbed from the surroundings during bond breaking. Using vague terms like "heat is produced" instead of "heat is absorbed".

Mark scheme (5 marks)

  1. Bonds in the reactants must be broken (to start the reaction)
  2. Breaking bonds requires / absorbs energy (from the surroundings)
  3. New bonds are formed in the products
  4. The energy released when bonds form is less than the energy absorbed when bonds break
  5. The reaction is endothermic

Key terms in this question

bond breaking · bond forming

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