Formation Reactions

Standard Formation Reaction

For species $\mathrm{A}$, the standard enthalpy of formation $\Delta H_f^{\circ }(\mathrm{A},T^{\circ })$ is the enthalpy change when forming 1 mol of $\mathrm{A}$ from elements in their reference states at $(T^{\circ },p^{\circ })$.

Example (at $T^{\circ }=298\mathrm{K}$):

$$\mathrm{C}(\mathrm{graphite})+2\mathrm{H}{}_2(\mathrm{g})+\frac{1}{2}\mathrm{O}{}_2(\mathrm{g})\to \mathrm{CH}{}_4(\mathrm{g})\Rightarrow \Delta H_f^{\circ }(\mathrm{CH}{}_4)\approx -74.8\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}.$$

Hess’s law (for any reaction):

$$\Delta H_{\text{rxn}}^{\circ }=\sum _{\text{prod}}{\nu }_i\Delta H_f^{\circ }(i)-\sum _{\text{react}}{\nu }_j\Delta H_f^{\circ }(j).$$