A student dissolves a fizzing indigestion tablet in a beaker of water. The student notices the beaker feels slightly cooler to the touch after the tablet has dissolved. Explain what type of chemical reaction has taken place and describe what is happening to energy during this reaction.

OCR GCSE Chemistry A: Gateway Science (J248) — C3.3 Types of chemical reactions · Explain · 4 marks · View as Markdown

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

Indigestion tablets contain sodium hydrogencarbonate, which reacts with excess acid in the stomach. A student investigates this reaction by placing a tablet in water and monitoring the temperature change.

Model answer (4 marks)

The tablet contains sodium bicarbonate (NaHCO₃) which reacts with the acid in the stomach to give sodium chloride, water and carbon dioxide:

NaHCO₃ + H⁺ → Na⁺ + CO₂↑ + H₂O

This is an acid–base reaction, a type of double‑replacement reaction. The reaction is endothermic – energy is absorbed from the surroundings. Consequently the temperature of the water (the surroundings) falls, so the beaker feels cooler. The energy required to break the bonds in the reactants (NaHCO₃ and H⁺) is greater than the energy released when the products’ bonds are formed.

Examiner tips

  • Use the term ‘acid–base reaction’ and ‘double‑replacement’ to show understanding of the reaction type.
  • State that the reaction is endothermic and that energy is taken from the surroundings.
  • Explain that the temperature drop is due to energy absorption, linking it to the decrease in surroundings’ temperature.
  • Mention the bond energy comparison to justify why the reaction is endothermic.

Common mistakes

  • Calling the reaction exothermic or forgetting to mention it is endothermic.
  • Not identifying it as an acid–base (double‑replacement) reaction.
  • Failing to explain that the temperature drop is due to energy absorption from the surroundings.

Mark scheme (4 marks)

  1. The reaction is endothermic
  2. Energy is taken in from the surroundings
  3. The temperature of the surroundings decreases
  4. The energy needed to break bonds in the reactants is greater than the energy released when bonds in the products are formed

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