Standard Reaction Enthalpy
The enthalpy change for a chemical reaction in which all reactants and products are in their standard states at a specified temperature.
ΔH^o_rxn

Systematic way to compare energy changes of reactions.
Can be written in terms of the standard enthalpies of formation of products minus reactants (Hess's Law).

ΔH^o_rxn = Σ n_i ΔH^o_f (products) – Σ n_j ΔH^o_f (reactants)

Calculate from tables of reference data.

Example:  CaCO3(s) + heat –––> CaO(s) + CO2(g)

ΔHorxn = ΔHof (CaO, s) + ΔHof (CO2, g) – ΔHof (CaCO3, s)