BioPHYSICAL chemistry

98 PARTI 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,Keq, according to:

(db5.15)

(db5.16)

()()lnΔΔGGRT

aa

recrec aa

cd

=°+ ab

CD

AB

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 con-

centrations of the various reactants and products. The equilibrium con-

ditions 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 mole-

cules 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 5.3When
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

cc

eq cc

cd

= ab

CD

AB

()()ln()ΔΔGGRT Δ

cc

cc

rec rec G

cd

=°+ ab=°

CD

AB

rrec+RTKln eq