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 by H and
called the enthalpy:
H U + PV (definition of the enthalpy)
(2.5-1)
The enthalpy is a state function because U, P, and V are state functions.
Consider a simple system with a pressure that remains equal to a constant external
pressure. We will refer to these conditions simply as constant-pressure conditions and
assume that P(transferred) P P ext . For a process under such conditions,
dw −P ext dV −PdV (constant pressure)
(2.5-2)
This expression for dw is 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 for dw applies. If the volume changes from V 1
to V 2 at constant pressure,
w
c
dw −
c
P ext dV −
c
PdV −P(V 2 − V 1 )
(simple system, constant pressure)
(2.5-3)
The heat transferred to the system is given by
dq dU − dw dU + PdV (simple system, constant pressure)
(2.5-4)
From Eq. (2.5-1)
dH dU + PdV + VdP
(2.5-5)
At constant pressure the VdP term vanishes, so that
dq dH (simple system, constant pressure)
(2.5-6)
For a finite process,
q ∆H (simple system, constant pressure)
(2.5-7)
Although q is generally path-dependent, it is path-independent for constant-pressure
processes, for which q ∆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.
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 by H and
called the enthalpy:
H U + PV (definition of the enthalpy)
(2.5-1)
The enthalpy is a state function because U, P, and V are state functions.
Consider a simple system with a pressure that remains equal to a constant external
pressure. We will refer to these conditions simply as constant-pressure conditions and
assume that P(transferred) P P ext . For a process under such conditions,
dw −P ext dV −PdV (constant pressure)
(2.5-2)
This expression for dw is 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 for dw applies. If the volume changes from V 1
to V 2 at constant pressure,
w
c
dw −
c
P ext dV −
c
PdV −P(V 2 − V 1 )
(simple system, constant pressure)
(2.5-3)
The heat transferred to the system is given by
dq dU − dw dU + PdV (simple system, constant pressure)
(2.5-4)
From Eq. (2.5-1)
dH dU + PdV + VdP
(2.5-5)
At constant pressure the VdP term vanishes, so that
dq dH (simple system, constant pressure)
(2.5-6)
For a finite process,
q ∆H (simple system, constant pressure)
(2.5-7)
Although q is generally path-dependent, it is path-independent for constant-pressure
processes, for which q ∆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.