For reactions that are done at a constant pressure (like atmospheric pressure if the beaker is open to the air), the heat change of the reaction is given a special name: Enthalpy. For the mathematics behind this, see the box at right:
Put simply: when a reaction happens, the heat is gained or lost. This heat comes from (or is removed because of) bonds breaking and forming (either chemical bonds or intermolecular bonds). This heat either gives an energy change (ΔE) or contributes to doing work (volume change).
Enthalpy is a state function, meaning all we need to know are the enthalpies of the products and reactants: