Chemistry - Thermochemistry Question with Solution | TestHub

Question Explanation:

Calculate the enthalpy change (heat released) when infinitely dilute solutions of ${\mathrm{CaCl}}_2$ and ${\mathrm{Na}}_2{\mathrm{CO}}_3$ are mixed.

Concept:

Enthalpy of Precipitation.

Solution:

The net reaction is the precipitation of ${\mathrm{CaCO}}_3(\mathrm{s}):{\mathrm{Ca}}^{2+}(\mathrm{aq})+{\mathrm{CO}}_3{}^{2-}(\mathrm{aq})\to {\mathrm{CaCO}}_3(\mathrm{s})$. The enthalpy change is calculated using the enthalpies of formation of the ions and the solid precipitate.

$$\begin{array}{rl}& {\mathrm{Ca}}^{2+}+{\mathrm{CO}}_3^{2-}\to {\mathrm{CaCO}}_3(\mathrm{s})\\ & \Delta \mathrm{H}=\Delta {\mathrm{H}}_{\mathrm{f}}^{\circ }{\mathrm{CaCO}}_3(\mathrm{s})-\Delta {\mathrm{H}}_{\mathrm{f}}^{\circ }{\mathrm{Ca}}^{2+}-\Delta {\mathrm{H}}^{\circ }{\mathrm{CO}}_3^{2-}\\ & =-288+129+161\\ & =+2\mathrm{Kcal}/\mathrm{mole}\end{array}$$