Calcium carbonate, $$\(\mathrm{CaCO}_{3}(\mathrm{~s})\)$$, decomposes when heated, as shown. $$\[ \mathrm{CaCO}_{3}(\mathrm{~s}) \rightarrow \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_{2}(\mathrm{~g}) \]$$ The enthalpy change of reaction, $$\(\Delta H_{\mathrm{r}}\)$$, for the thermal decomposition of $$\(\mathrm{CaCO}_{3}\)$$ (s) cannot be measured directly. Instead, a procedure involving two experiments is used. In each experiment, the enthalpy change of a different reaction is determined. The equation for the reaction in experiment 1 is shown. The enthalpy change for this reaction is $$\(\Delta H_{1}\)$$. experiment $$\(1 \quad \mathrm{CaCO}_{3}(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CaCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{I})+\mathrm{CO}_{2}(\mathrm{~g})\)$$ The equation for the reaction in experiment 2 is shown. The enthalpy change for this reaction is $$\(\Delta H_{2}\)$$. experiment 2 $$\[ \mathrm{CaO}(\mathrm{s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CaCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \]$$ Experiment 1 step 1 Weigh a 0.0500 mol sample of powdered $$\(\mathrm{CaCO}_{3}(\mathrm{~s})\)$$. step 2 Transfer $$\(50.00 \mathrm{~cm}^{3}\)$$, an excess, of $$\(2.00 \mathrm{moldm}^{-3}\)$$ hydrochloric acid, $$\(\mathrm{HCl}(\mathrm{aq})\)$$, into a small glass beaker. step 3 Start a timer and measure the temperature of the $$\(\mathrm{HCl}(\mathrm{aq})\)$$ in the beaker every 30 seconds for $$\(2 \frac{1}{2}\)$$ minutes. step 4 After 3 minutes add the sample of $$\(\mathrm{CaCO}_{3}(\mathrm{~s})\)$$ to the $$\(\mathrm{HCl}(\mathrm{aq})\)$$ in the beaker. Continue measuring the temperature of the reaction mixture every 30 seconds for a further 5 minutes. Experiment 2 Repeat experiment 1 using calcium oxide, $$\(\mathrm{CaO}(\mathrm{s})\)$$, instead of $$\(\mathrm{CaCO}_{3}(\mathrm{~s})\)$$. A student carries out experiment 1 and obtains the results given in Table 1.1. (i) Plot a graph on the grid in Fig. 1.1 to show the relationship between temperature and time. Use a cross ( $$\(x\)$$ ) to plot each data point. The points and line of best fit for the data before 3 minutes have been drawn for you. Draw a line of best fit for the data after 3 minutes that will enable you to determine the theoretical temperature increase at 3.0 minutes. (ii) Use your graph to determine the theoretical temperature increase at 3.0 minutes. theoretical temperature increase at 3.0 minutes $$\(=\)$$ .......................... $$\({ }^{\circ} \mathrm{C}\)$$

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