Physical Chemistry Third Edition

74 2 Work, Heat, and Energy: The First Law of Thermodynamics

2.5 Enthalpy

Many liquid systems in the laboratory are contained in vessels that are open to the

atmosphere and are thus maintained at a nearly constant pressure.^4 For convenient

analysis of constant-pressure processes we define a new variable, denoted byHand

called the enthalpy:

HU+PV (definition of the enthalpy) (2.5-1)

The enthalpy is a state function becauseU,P, andVare state functions.

Consider a simple system with a pressure that remains equal to a constant external

pressure. We will refer to these conditions simply asconstant-pressure conditionsand

assume thatP(transferred)PPext. For a process under such conditions,

dw−PextdV−PdV (constant pressure) (2.5-2)

This expression fordwis the same as that for reversible processes, Eq. (2.1-11). We

do not assert that all processes that occur at constant pressure are reversible processes,

but only that the reversible expression fordwapplies. If the volume changes fromV 1

toV 2 at constant pressure,

w

∫

c

dw−

∫

c

PextdV−

∫

c

PdV−P(V 2 −V 1 )

(simple system, constant pressure) (2.5-3)

The heat transferred to the system is given by

dqdU−dwdU+PdV (simple system, constant pressure) (2.5-4)

From Eq. (2.5-1)

dHdU+PdV+VdP (2.5-5)

At constant pressure theVdPterm vanishes, so that

dqdH (simple system, constant pressure) (2.5-6)

For a finite process,

q∆H (simple system, constant pressure) (2.5-7)

Althoughqis generally path-dependent, it is path-independent for constant-pressure

processes, for whichq∆H. Enthalpy changes of constant-pressure processes are

sometimes called “heats” of the processes.

(^4) The extreme observed sea-level barometric pressures are 1083.8 mbar (1.069 atm) and 877 mbar
(0.866 atm):Guinness Book of World Records, Guinness, 1998, p. 95.