Background

The aim of the experiment is to determine the enthalpy of decomposition of the following reaction:

Thermal decomposition of calcium carbonate

CaCO₃(s) → CaO(s) + CO₂(g)

It is not possible to find this value directly, as the reaction would involve heating the calcium carbonate strongly in order for it to decompose.

However, it is possible to determine the enthalpy change by constructing a Hess’ cycle using an intermediate stage of a solution of calcium chloride, as both calcium carbonate and calcium oxide react with hydrochloric acid forming a calcium chloride solution.

CaO(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l)

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

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Requirements

Chemicals

• Calcium carbonate powder
• Calcium oxide powder
• Hydrochloric acid, 2 mol dm^-3

Apparatus

• Polystyrene beaker and lid
• Glass beaker, 250ml
• Measuring cylinder, 50ml
• Weighing boat
• Spatula
• Stirring rod
• Electronic balance
• Thermometer or temperature probe

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Procedure

• Accurately weigh out approximately 3g of calcium oxide using a weighing boat, electronic balance and spatula.
• Measure out 100cm³ of 2M hydrochloric acid, HCl, (excess) into a weighed polystyrene beaker supported in a 250ml glass beaker.
• Record the temperature of the hydrochloric acid for one minute with stirring
• Add the calcium oxide sample to the hydrochloric acid stirring thoroughly and record the maximum temperature reached.
• Reweigh the polystyrene beaker and contents.
• Repeat twice more.
• Carry out the whole procedure again using approximately 5g of calcium carbonate instead of calcium oxide.

Take the specific heat capacity of the hydrochloric acid/calcium chloride solution to be equal to that of water (4.18 kJ kg^-1 K^-1).

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Safety

• Hydrochloric acid is corrosive
• Calcium oxide (quicklime) is caustic and will cause burns to skin. Avoid inhaling the powder and clean up spillages with plenty of water.

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