A student adds a small amount of anhydrous calcium chloride to a test tube of water. The student notices that the outside of the test tube becomes noticeably warm. Explain why the temperature of the surroundings increases during this process, referring to bond breaking and bond making.

OCR GCSE Chemistry A: Gateway Science (J248) — C3.2 Energetics · Explain · 4 marks · View as Markdown

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

Anhydrous calcium chloride is a white solid that dissolves readily in water. It is sometimes used as a drying agent and in self-heating food packaging.

Model answer (4 marks)

The dissolution of CaCl₂ requires breaking the ionic lattice bonds between Ca²⁺ and Cl⁻ ions, which consumes energy. Simultaneously, new bonds are formed as the ions interact with water molecules (hydrogen bonding and ion–dipole interactions), releasing more energy than was absorbed. Because the net energy change is negative, the excess energy is released to the surroundings, warming the test tube.

Examiner tips

  • Use the term ‘exothermic’ to show understanding of heat transfer
  • Mention both bond breaking (lattice) and bond making (hydration)
  • Show the energy flow: absorbed > released

Common mistakes

  • Confusing the direction of heat flow – saying the solution heats up but not the surroundings
  • Forgetting to mention the ionic lattice breaking
  • Using vague terms like ‘heat’ without linking to bond energy

Mark scheme (4 marks)

  1. Energy is required to break bonds in the reactants
  2. Energy is released when new bonds are formed in the products
  3. More energy is released in bond making than is taken in during bond breaking
  4. Therefore energy is transferred to the surroundings, so this is an exothermic reaction

Key terms in this question

bond breaking · bond making · surroundings

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