Reaction enthalpy, standard states and thermochemical equations

Reaction enthalpy ΔᵣH = Σ(enthalpies of products) − Σ(enthalpies of reactants), each multiplied by its coefficient.

Standard state means the substance taken pure and under 1 bar, at whatever temperature is specified; tabulated values are mostly for 298 K and carry the ° sign.

A thermochemical equation is a balanced equation with physical states written in and its ΔᵣH value; the coefficients stand for moles.

Multiplying an equation by a number multiplies ΔᵣH by the same number; reversing the equation reverses the sign of ΔᵣH.

Standard enthalpy of fusion, vaporisation and sublimation are enthalpy changes for melting, boiling or subliming one mole of a substance; for water ΔfusH° = 6.00 kJ mol⁻¹ at 273 K and ΔvapH° = 40.79 kJ mol⁻¹ at 373 K.

Sublimation equals fusion followed by vaporisation at the same temperature, so ΔsubH° = ΔfusH° + ΔvapH°.

Standard enthalpy of formation ΔfH° is the enthalpy change for making one mole of a compound out of its elements, each taken in its most stable form (reference state).

ΔfH° of an element in its reference state is zero; for carbon the reference state is graphite.

Using formation data: ΔᵣH° = Σ aᵢΔfH°(products) − Σ bᵢΔfH°(reactants).