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
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.