Standard formation reactions are used to determine the standard enthalpy of formation of a compound, which is the change in enthalpy that occurs when one mole of the compound is formed from its constituent elements in their standard states. The standard enthalpy of formation of a compound is an important thermodynamic property that can be used to calculate other thermodynamic properties, such as the Gibbs free energy of formation and the entropy of formation.
The general form of a standard formation reaction is:
aA + bB + ... → cC + dD + ...
where A, B, ..., C, and D are the constituent elements of the compound, and a, b, ..., c, and d are the stoichiometric coefficients of the reaction. The standard enthalpy of formation of the compound is given by:
ΔHf° = ΣnΔHf,products° - ΣnΔHf,reactants°
where ΔHf° is the standard enthalpy of formation of the compound, ΔHf,products° are the standard enthalpies of formation of the products, and ΔHf,reactants° are the standard enthalpies of formation of the reactants.
The standard enthalpies of formation of elements in their standard states are defined to be zero. Therefore, the standard enthalpy of formation of a compound is equal to the sum of the standard enthalpies of formation of its constituent elements.
