Enthalpy Change, ΔH of a Reaction – Reaction Enthalpy

In a chemical reaction, reactants are converted into products: reactants $\rightarrow$ products. The enthalpy change accompanying a reaction is called the reaction enthalpy, $\Delta_r H$:

Here $a_i$ and $b_i$ are the stoichiometric coefficients of the products and reactants in the balanced equation, and $H_m$ is the molar enthalpy of each substance. For the combustion of methane, $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$, this reads $\Delta_r H = [H_m(CO_2, g) + 2H_m(H_2O, l)] - [H_m(CH_4, g) + 2H_m(O_2, g)]$. Absolute molar enthalpies are not measurable, which is why the machinery below — standard states and enthalpies of formation — makes this equation practical.

$\Delta_r H$ depends on exactly how the reaction is written and the physical states of every species involved. This part sets out the bookkeeping rules that make $\Delta_r H$ values comparable and combinable:

• Standard Enthalpy of Reactions — the reference conditions (298 K, 1 bar) every tabulated value assumes.
• Enthalpy Changes during Phase Transformations — fusion, vaporisation, and sublimation enthalpies.
• Standard Enthalpy of Formation — building a reaction's $\Delta_r H$ from tabulated $\Delta_f H^\ominus$ values. …