Question:
Enthalpy of combustion of carbon to $\mathrm{CO}_{2}$ is $393.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$. Calculate the heat released upon formation of $35.2 \mathrm{~g}$ of $\mathrm{CO}_{2}$ from carbon and dioxygen gas.
Solution:
Formation of CO2 from carbon and dioxygen gas can be represented as:
$\mathrm{C}_{(s)}+\mathrm{O}_{2(g)} \longrightarrow \mathrm{CO}_{2(g)}$
$\Delta_{f} H=-393.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$
$(1$ mole $=44 \mathrm{~g})$
Heat released on formation of $44 \mathrm{~g} \mathrm{CO}_{2}=-393.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$
$\therefore$ Heat released on formation of $35.2 \mathrm{~g} \mathrm{CO}_{2}$
$=\frac{-393.5 \mathrm{~kJ} \mathrm{~mol}^{-1}}{44 \mathrm{~g}} \times 35.2 \mathrm{~g}$
$=-314.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$