The standard enthalpy of formation ($\Delta_f H^\circ$) is defined as the enthalpy change when exactly one mole of a compound is formed from its constituent elements in their most stable states of aggregation (reference states) . For liquid water ($H_2O$), the constituent elements in their standard states are hydrogen gas ($H_2$) and oxygen gas ($O_2$). The reaction representing the formation of one mole of $H_2O(l)$ is: $H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(l)$ This matches the second reaction provided in the question, where the enthalpy change is given as $-x_2 \text{ kJ mol}^{-1}$. Note: Reaction (i) represents the enthalpy of neutralization, not formation.