Practical Chemistry

Experiment 14: Hess's law


To determine the enthalpy of decomposition of calcium carbonate


Calcium carbonate decomposes on heating according to the equation:

CaCO3 CaO + CO2

It is not possible to measure the enthalpy change of this reaction directly, but by judicious use of energy cycles it can be determined indirectly using Hess's law.

Calcium carbonate and calcium oxide both react with hydrochloric acid forming a solution of calcium chloride and water. In the case of calcium carbonate, carbon dioxide is also produced.


  1. Weigh out accurately about 0.05 moles of calcium carbonate
  2. Measure about 50ml of 2.0 mol dm-3 HCl into a weighed polystyrene beaker. Re-weigh after the addition.
  3. Record the temperature of the hydrochloric acid for about 1 minute to ensure stability.
  4. Add the calcium carbonate to the hydrochloric acid in the beaker stirring thoroughly and record the highest temperature attained.
  5. Repeat the procedure using calcium oxide instead of calcium carbonate.

Experimental report

You should draw out the complete energy cycle for the reactions as part of the background to the experiment.

Assessment suitable for:

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Copyright: 2003 Isis Publication