Modern inorganic chemistry

66 ENERGETICS

K =

Here, the reaction proceeds effectively to completion; (HC1) is

very large relative to (H 2 ) and (C1 2 ) and hence Kc (and Kp) are also

large. In these circumstances the reverse arrow is usually omitted.

The equilibrium constant at constant temperature is directly

related to the maximum energy, called the free energy AG. which is

obtainable from a reaction, the relationship being

Here G is the free energy and AG the change in free energy during

the reaction, R the gas constant and T the absolute temperature.

At 298 K, under standard conditions (G = G^)

log 10 Kp - - 0. 000733 AG^

where AG^ is the change in free energy under standard conditions.

The above equation enables us to calculate the equilibrium

constant for any value of AG or vice versa, and we readily see that

for a reaction to 'go to completion', i.e. for K to be large, AG needs

to be large and negative.

When AG = 0, the equilibrium constant K is unity. A large

positive value of AG indicates that the reaction will not 'go', being

energetically unfavourable under the specific conditions to which

AG refers.

FREE ENERGY AND ENTROPY

Free energy is related to two other energy quantities, the enthalpy

(the heat of reaction measured at constant pressure) and the entropy.

S. an energy term most simply visualised as a measure of the disorder

of the system, the relationship for a reaction taking place under

standard conditions being

where AG^ is the change in free energy, A/T^9 " the change in enthalpy,
AS^the change in entropy (all measured under standard conditions).
and T is the absolute temperature.
If overall disorder increases during a reaction, AS is positive:
where overall disorder decreases. AS is negative.