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Physical Chemistry 42

Enthalpy

The majority of chemical reactions, and almost all biochemical processes in υiυo, are
performed under constant pressure conditions and involve small volume changes. When a
process takes place under constant pressure, and assuming that no work other than pV
work is involved, then the relationship between the heat changes and the internal energy
of the system is given by:
dU=dq−p exdV (infinitesimal change) ∆U=q−p ex∆V (finite change)

The enthalpy, H, is defined by the expression; H=U+pV, Hence for a finite change at
constant pressure:
∆H=∆U+ p ex∆V

Thus, when the only work done by the system is pV work,
∆H=q at constant pressure

Expressed in words, the heat exchanged by a system at constant pressure is equal to the
sum of the internal energy change of that system and the work done by the system in
expanding against the constant external pressure. The enthalpy change is the heat
exchanged by the system under conditions of constant pressure.
For a reaction involving a perfect gas, in which heat is generated or taken up, ∆H is
related to ∆U by:
∆H=∆U+∆n RT

where ∆n is the change in the number of moles of gaseous components in the reaction.
Hence for the reaction , (1
−1
mole of gaseous CO 2 is created), and so ∆H=∆U+2.48 kJ mol at 298 K.

Properties of enthalpy

The internal energy, pressure and volume are all state functions (see Topic B1), and
since enthalpy is a function of these parameters, it too is a state function. As with the
internal energy, a system possesses a defined value of enthalpy for any particular system
at any specific conditions of temperature and pressure. The absolute value of enthalpy of
a system cannot be known, but changes in enthalpy can be measured. Enthalpy changes
may result from either physical processes (e.g. heat loss to a colder body) or chemical
processes (e.g. heat produced υia a chemical reaction).
An increase in the enthalpy of a system leads to an increase in its temperature (and
υice υersa), and is referred to as an endothermic process. Loss of heat from a system
lowers its temperature and is referred to as an exothermic process. The sign of ∆H
indicates whether heat is lost or gained. For an exothermic process, where heat is lost
from the system, ∆H has a negative value. Conversely, for an endothermic process in
which heat is gained by the system, ∆H is positive. This is summarized in Table 1. The