The standard heat of reaction can be calculated by using the following equation. ΔHo = ∑nΔHo f (products) − ∑nΔHo f (reactants) The symbol Σ is the Greek letter sigma and means "the sum of".

ΔH° rxn = [ (c) (ΔH f ° C) + (d) ( ΔH f ° D) ] – [ (a) (ΔH f ° A)+ (b) (ΔH f ° B) ] Where: c = # mols of product C ΔH f ° C = enthalpy of C d = #mols of product D ΔH f ° D = enthalpy of D

To calculate the Standard Formation Enthalpy, we employ the following formula: ΔHf° = ∑ (n * ΔHf° (products)) - ∑ (m * ΔHf° (reactants)) ΔH f ° = ∑(n ∗ ΔH f °(products)) − ∑(m ∗ ΔH f °(reactants))

ΔH for a reaction is equal to the sum of the heats of formation of the product compounds minus the sum of the heats of formation of the reactant compounds: ΔH = Σ ΔH f products - Σ ΔH f reactants

ΔH o reaction = ΣΔH o f (p) − ΣΔH o f (r), where p = products and r = reactants. EXAMPLE: The ΔH o reaction for the oxidation of ammonia 4NH₃ (g) + 5O₂ (g) → 4NO (g) + 6H₂O (g) is -905.2 kJ.