Calcium carbonate, $$\(\mathrm{CaCO}_{3}(\mathrm{~s})\)$$, decomposes when heated, as shown. $$\[ \mathrm{CaCO}_{3}(\mathrm{~s}) \rightarrow \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_{2}(\mathrm{~g}) \]$$ The enthalpy change of reaction, $$\(\Delta H_{\mathrm{r}}\)$$, for the thermal decomposition of $$\(\mathrm{CaCO}_{3}\)$$ (s) cannot be measured directly. Instead, a procedure involving two experiments is used. In each experiment, the enthalpy change of a different reaction is determined. The equation for the reaction in experiment 1 is shown. The enthalpy change for this reaction is $$\(\Delta H_{1}\)$$. experiment $$\(1 \quad \mathrm{CaCO}_{3}(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CaCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{I})+\mathrm{CO}_{2}(\mathrm{~g})\)$$ The equation for the reaction in experiment 2 is shown. The enthalpy change for this reaction is $$\(\Delta H_{2}\)$$. experiment 2 $$\[ \mathrm{CaO}(\mathrm{s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CaCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \]$$ Experiment 1 step 1 Weigh a 0.0500 mol sample of powdered $$\(\mathrm{CaCO}_{3}(\mathrm{~s})\)$$. step 2 Transfer $$\(50.00 \mathrm{~cm}^{3}\)$$, an excess, of $$\(2.00 \mathrm{moldm}^{-3}\)$$ hydrochloric acid, $$\(\mathrm{HCl}(\mathrm{aq})\)$$, into a small glass beaker. step 3 Start a timer and measure the temperature of the $$\(\mathrm{HCl}(\mathrm{aq})\)$$ in the beaker every 30 seconds for $$\(2 \frac{1}{2}\)$$ minutes. step 4 After 3 minutes add the sample of $$\(\mathrm{CaCO}_{3}(\mathrm{~s})\)$$ to the $$\(\mathrm{HCl}(\mathrm{aq})\)$$ in the beaker. Continue measuring the temperature of the reaction mixture every 30 seconds for a further 5 minutes. Experiment 2 Repeat experiment 1 using calcium oxide, $$\(\mathrm{CaO}(\mathrm{s})\)$$, instead of $$\(\mathrm{CaCO}_{3}(\mathrm{~s})\)$$. Suggest why the temperature measured at 3.5 minutes is lower than the temperature measured at 4.0 minutes. .........................................................................................................................................

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