Standard Enthalpy of Reactions

If enthalpy of reactants is denoted as H_reactants and total enthalpy of products as H_products, the difference between these enthalpies, is the enthalpy of the reaction

Δ_rH = H_products – H_reactants

When H_products is greater than H_reactants then ΔH is positive and heat is absorbed in the reaction, and the reaction is endothermic. For example,

H₂ (g) + I₂ (g) → 2HI (g); Δ_rH = 52.5 kJ

When H_products is less than H_reactants then H is negative and heat is evolved in the reaction, and the reaction is exothermic. For example,

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l); Δ_rH = –890.4 kJ

Enthalpy of Formation

The enthalpy change when one mole of a pure compound is formed from its elements in their most stable states is called the enthalpy of formation and is denoted by Δ_fH⁰.

For example:

C_(Graphite) + O₂ (g) → CO₂ (g) ; Δ_fH⁰ = –393.5 kJ mol^–1

Enthalpy of Combustion

Enthalpy of combustion is the enthalpy change (heat evolved) accompanying the complete combustion of 1 mole of a substance in oxygen at a given temperature and 1 bar pressure.

For example:

C₂H₅OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂O (l) ; Δ_combH⁰ = –1365.6 kJ

Enthalpy of Neutralization

Enthalpy of neutralization isthe enthalpy change (heat evolved) when one mole of hydrogen ions (H⁺) is neutralized by one mole of hydroxyl ions (OH^-) in dilute aqueous medium to form water.

For example:

H⁺ (aq) + OH^– (aq) → H₂O (l) ; Δ_neutH⁰ = –57 kJ/mol