Using standard enthalpies of formation
Enthalpies of reaction (ΔrH°) can be calculated from standard enthalpies of formation. These can be found in data tables. '
ΔrH° = ΣΔfH°(products) - ΣΔfH°(reactants)
Σ means sum of
Enthalpy changes depend on amount.
ΔfH° are quoted per mol of substance.
Therefore ΔfH° for each reactant and product compound must be multiplied by its coefficient in the balanced equation for the reaction before they are combined as shown above.
ΔrH° = ΣΔfH°(products) - ΣΔfH°(reactants)
Σ means sum of
Enthalpy changes depend on amount.
ΔfH° are quoted per mol of substance.
Therefore ΔfH° for each reactant and product compound must be multiplied by its coefficient in the balanced equation for the reaction before they are combined as shown above.
Note that as the cycle below shows, this calculation corresponds to using an alternative path for the reaction in which reactants decompose to their elements which in turn react to form products.
HCO2H(l) + CH3OH(l)  2C(s) + 3H2(g) + 3/2O2(g) |  HCO2CH3(l) + H2O(l) |