Page 199 - Physical chemistry understanding our chemical world

166 REACTION SPONTANEITY AND THE DIRECTION OF THERMODYNAMIC CHANGE

the amount of chalk on the top line must also fall. These changes must occur in tandem
if K is to remain constant. In other words, decreasing the amount of CO 2 in a chicken’s
blood means less chalk is available for shell production. Conversely, the same reason-
ing suggests that increasing the concentration of carbonate – by adding carbonated
water to the chicken’s drink – will increases the bottom line of Equation (4.61), and
the chalk term on the top increases to maintain a constant value of K.

Le Chatelier’s principle

Arguments of this type illustrate Le Chatelier’s principle, which was formulated in
1888. It says:

Any system in stable chemical equilibrium, subjected to the influence
Le Chatelier’s prin-
of an external cause which tends to change either its temperature or
ciple is named after
Henri Louis le Chatelier its condensation (pressure, concentration, number of molecules in unit
(1850–1937). He also volume), either as a whole or in some of its parts, can only undergo such
spelt his first name the internal modifications as would, if produced alone, bring about a change
English way, as ‘Henry’. of temperature or of condensation of opposite sign to that resulting from
the external cause.

The principle represents a kind of ‘chemical inertia’, seeking to minimize the changes
of the system. It has been summarized as, ‘if a constraint is applied to a system in
equilibrium, then the change that occurs is such that it tends to annul the constraint’.
It is most readily seen in practice when:

(1) The pressure in a closed system is increased (at fixed temperature) and
shifts the equilibrium in the direction that decreases the system’s volume,
i.e. to decrease the change in pressure.
(2) The temperature in a closed system is altered (at fixed pressure), and the
equilibrium shifts in such a direction that the system absorbs heat from its
surroundings to minimize the change in energy.

4.6 The effect of temperature on
thermodynamic variables

Why does egg white denature when cooked but
remain liquid at room temperature?

Effects of temperature on G : the Gibbs–Helmholtz equation
O
Boiling an egg causes the transparent and gelatinous albumen (‘egg white’) to modify
chemically, causing it to become a white, opaque solid. Like all chemical reactions,