Standard Enthalpy of Formation - Standard Enthalpy of Reaction

Standard Enthalpy of Reaction

The standard enthalpy of formation is used in thermochemistry to find the standard enthalpy change of reaction. This is done by subtracting the sum of the standard enthalpies of formation of the reactants (each being multiplied by its respective stoichiometric coefficient, ν) from the sum of the standard enthalpies of formation of the products (each also multiplied by its respective stoichiometric coefficient), as shown in the equation below:

ΔH° = Σ(ν × ΔH_f°) (products) - Σ(ν × ΔH_f°) (reactants)

For example, for the reaction CH₄ + 2 O₂ → CO₂ + 2 H₂O:

ΔH_r° = (products) - (reactants)

If the standard enthalpy of the products is less than the standard enthalpy of the reactants, the standard enthalpy of reaction will be negative. This implies that the reaction is exothermic. The converse is also true; the standard enthalpy of reaction will be positive for an endothermic reaction.