Enthalpy changes that are not easy to measure directly can often be found by using Hess’ Law to build an enthalpy cycle. Which enthalpy change does X represent in the enthalpy cycle shown? The cycle shows $\text{C(s)} + 2\text{H}_2\text{(g)} + 2\text{O}_2\text{(g)}$ changing to $\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)}$ with $\Delta H_f^\circ$, then to $\text{CO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$ with $\Delta H_f^\circ$, and reaction arrows labelled $\Delta H_c^\circ$ and X.
- A$-4 \times$ the enthalpy of combustion of hydrogen
- B$+4 \times$ the enthalpy of combustion of hydrogen
- C$-2 \times$ the enthalpy of formation of water
- D$+2 \times$ the enthalpy of formation of water