To find the standard enthalpy change of a chemical reaction (ΔHrxn), one typically uses a Hess's law approach, where the ΔHrxn is calculated based on known standard enthalpies of formation of the reactants and products involved in the reaction.
Hess's law states that the overall enthalpy change for a reaction is the same, regardless of whether the reaction occurs in one step or a series of steps. This means that the enthalpy change for the overall reaction can be determined by algebraically summing up the enthalpy changes of the individual steps.
Hess's law is related to the standard enthalpies of formation (ΔHf) of the reactants and products, which are the enthalpy changes when one mole of a compound is formed from its elements in their standard states at a specified temperature and pressure. By using these known values, along with the stoichiometry of the reaction, one can calculate the ΔHrxn for the overall chemical equation.
