The four enthalpy changes you must define

The enthalpy change when a reaction occurs in the molar quantities shown in the equation, with all reactants and products in their standard states at 298 K and 101 kPa.

For example, for

$2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}$

$\Delta H_\text{r}^{\ominus}$ refers to two moles of hydrogen reacting, so it is twice the value for one mole.

The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.

Example: $\text{C(graphite)} + 2\text{H}_2\text{(g)} \rightarrow \text{CH}_4\text{(g)}$.

Two consequences you can use in calculations:

• the $\Delta H_\text{f}^{\ominus}$ of an element in its standard state is zero;
• the compound must be formed from elements, not from other compounds.

The enthalpy change when one mole of a substance is completely burned in excess oxygen, under standard conditions, with all reactants and products in their standard states.

Example: $\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$.

"Completely" matters: partial combustion to carbon monoxide releases much less energy, so a value measured with a smoky flame is not $\Delta H_\text{c}^{\ominus}$.

The enthalpy change when one mole of water is formed in the reaction between an acid and a base (alkali), under standard conditions, with all species in their standard states.

Example: $\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$.

Because this ionic equation is the same for every strong acid with every strong alkali, all such neutralisations have roughly the same value, about $-57\ \text{kJ mol}^{-1}$.