Calcium carbonate, CaCO3(s), decomposes when heated, as shown. CaCO3(s) → CaO(s) + CO2(g) The enthalpy change of reaction, ∆Η, for the thermal decomposition of CaCO3(s) cannot be measured directly. Instead, a procedure involving two experiments is used. In each experiment, the enthalpy change of a different reaction is determined. The equation for the reaction in experiment 1 is shown. The enthalpy change for this reaction is ∆Η₁. experiment 1 CaCO3(s) + 2HCl (aq) → CaCl2(aq) + H2O(l) + CO2(g) The equation for the reaction in experiment 2 is shown. The enthalpy change for this reaction is ∆Η2. experiment 2 CaO(s) + 2HCl (aq) → CaCl2(aq) + H2O(l) Experiment 1 step 1 Weigh a 0.0500 mol sample of powdered CaCO3(s). step 2 Transfer 50.00cm³, an excess, of 2.00 moldm⁻³ hydrochloric acid, HCl(aq), into a small glass beaker. step 3 Start a timer and measure the temperature of the HCl(aq) in the beaker every 30 seconds for 2 minutes. step 4 After 3 minutes add the sample of CaCO3(s) to the HCl(aq) in the beaker. Continue measuring the temperature of the reaction mixture every 30 seconds for a further 5 minutes. Experiment 2 Repeat experiment 1 using calcium oxide, CaO(s), instead of CaCO3(s).