98
PART I
THERMODYNAMICS AND KINETICS
The expression for the reaction Gibbs energy of a mixture becomes:
(db5.14)
Assuming behavior of an ideal solution, this can be written in terms of the concentrations
because the activity coefficients all become 1 and the change in the Gibbs energy can be
expressed in terms of an equilibrium constant, K eq , according to:
(db5.15)
(db5.16)
( )
( )
ln
Δ
Δ
G
G
R T
a a
a a
rec
rec
c d
a b
=
° +
C D
A B
RESPONSE OF THE EQUILIBRIUM CONSTANT TO
CONDITION CHANGES
Equilibrium conditions for any given reaction will shift in response to
changes to a system, such as a change in pressure, temperature, or concentrations of the various reactants and products. The equilibrium conditions are not altered by the presence of an enzyme; this only changes
the rate of a process. The response of a reaction due to such a change
was originally described by the French chemist Henri Le Chatelier, who
proposed what is now known as Le Chatelier’s principle:
Le Chatelier’s principle states that when a system at
equilibrium is subjected to a disturbance, the system
will respond such that the effect of the disturbance is
minimized.
Consider a chamber containing gas molecules with the
pressure controlled by a piston (Figure 5.3). The gas molecules can be present in equilibrium as both monomers and
dimers through a reaction A ↔ A 2 . If the piston is pushed
into the chamber the gas will be compressed, as the response
of the system will be to minimize the increase in pressure. In
this case, the pressure increase can be reduced by decreasing
the number of particles in the gas phase; that is, shifting
the equilibrium towards a larger number of dimers.
(a)
(b)
Figure 5.3 When
gas molecules are
compressed they will
respond to minimize
the effect, as shown
in this case by
forming dimers, thus
decreasing the
number of particles
in the gas phase.
K
c c
c c
eq
c d
a b
=
C D
A B
( )
( )
ln
( )
Δ
Δ
Δ
G
G
R T
c c
c c
G
rec
rec
c d
a b
=
° +
=
°
C D
A B
r rec
eq
RT K
ln
+
9781405124362_4_005.qxd 4/30/08 19:06 Page 98
PART I
THERMODYNAMICS AND KINETICS
The expression for the reaction Gibbs energy of a mixture becomes:
(db5.14)
Assuming behavior of an ideal solution, this can be written in terms of the concentrations
because the activity coefficients all become 1 and the change in the Gibbs energy can be
expressed in terms of an equilibrium constant, K eq , according to:
(db5.15)
(db5.16)
( )
( )
ln
Δ
Δ
G
G
R T
a a
a a
rec
rec
c d
a b
=
° +
C D
A B
RESPONSE OF THE EQUILIBRIUM CONSTANT TO
CONDITION CHANGES
Equilibrium conditions for any given reaction will shift in response to
changes to a system, such as a change in pressure, temperature, or concentrations of the various reactants and products. The equilibrium conditions are not altered by the presence of an enzyme; this only changes
the rate of a process. The response of a reaction due to such a change
was originally described by the French chemist Henri Le Chatelier, who
proposed what is now known as Le Chatelier’s principle:
Le Chatelier’s principle states that when a system at
equilibrium is subjected to a disturbance, the system
will respond such that the effect of the disturbance is
minimized.
Consider a chamber containing gas molecules with the
pressure controlled by a piston (Figure 5.3). The gas molecules can be present in equilibrium as both monomers and
dimers through a reaction A ↔ A 2 . If the piston is pushed
into the chamber the gas will be compressed, as the response
of the system will be to minimize the increase in pressure. In
this case, the pressure increase can be reduced by decreasing
the number of particles in the gas phase; that is, shifting
the equilibrium towards a larger number of dimers.
(a)
(b)
Figure 5.3 When
gas molecules are
compressed they will
respond to minimize
the effect, as shown
in this case by
forming dimers, thus
decreasing the
number of particles
in the gas phase.
K
c c
c c
eq
c d
a b
=
C D
A B
( )
( )
ln
( )
Δ
Δ
Δ
G
G
R T
c c
c c
G
rec
rec
c d
a b
=
° +
=
°
C D
A B
r rec
eq
RT K
ln
+
9781405124362_4_005.qxd 4/30/08 19:06 Page 98