Question

For a reaction,
$CaCO_{3\left(s\right) } \to CaO_{\left(s\right)}+CO_{2\left(g\right)}$
$\Delta_{f}H^{\circ} \left(CaO\right) = - 635.1\, kj \,mol^{-1}$,
$\Delta _{f}H^{\circ }\left(CO_{2}\right)=-393.5 \,kj\, mol^{-1}$ and
$\Delta _{f}H^{\circ }\left(CaCO_{3}\right)= -1206.9 \, kj \, mol^{-1}$
Which of the following is a correct statement?

• A. $A$ large amount of heat is evolved during the decomposition of $CaCO_3$.
• B. Decomposition of $CaCO_3$ is an endothermic process and heat is provided for decomposition
• C. The amount of heat evolved cannot be calculated from the data provided
• D. $\Delta_{r}H^{\circ} = \Sigma\,\Delta_{f}H^{\circ}$ (reactants) $- \Sigma \,\Delta _{f}H^{\circ }$ (products)

Answer 1

Answer: (B)

$\Delta_{r}H = \Sigma_{f}H^{\circ}_{P} - \Sigma \,\Delta _{f}H^{\circ }_{R}$
$= \left[- 635.1 + \left(-393.5\right)\right] - \left[-1206.9\right] = + 178.4 \,kj$