When gases are produced in a chemical reaction, work must be done in pushing against the pressure of the atmosphere to make room for them. Conversely, if gases disappear or condense to form liquids or solids during a reaction, work is done on the reacting substances by the external world.

If we make a small correction for any work that gases in a reaction might do in pushing against the atmosphere when they are generated, then we obtain a modified energy, the enthalpy, H. Enthalpy is defined formally by H = E + PV, in which PV is the product of pressure and volume.

When a reaction takes place at a constant surrounding pressure, which is true for reactions conducted in the open at the surface of the Earth (e.g., at a laboratory bench), then the heat of reaction is a measure of the change in enthalpy, DH = H(products) - H(reactants). The difference between energy and enthalpy is only a few percent, and enthalpy should be thought of as a "corrected" energy, with allowance made for atmospheric pressure.