Unit 2 Advanced Higher Learning outcomes
Chemical Equilibrium
 A chemical reaction is in equilibrium when the concentration of the reactants
and products remains constant indefinitely.
 The equilibrium constant (K) characterises the composition of the reaction
mixture.
 The equilibrium constant can be measured in terms of concentrations or for
gaseous reactions, in terms of pressure.

For the general reaction:
aA + bB
cC + dD
Kc
[C]c[D]d
=
[A]a[B]b
where [A],[B],[C] and [D] are the equilibrium concentrations of A,B,C and D
respectively and a,b,c, and d are the stoichiometric coefficients of the
balanced equation.
 In a homogeneous equilibrium all the species are in the same phase.
 In a heterogeneous equilibrium the species are in more than one phase.
 The concentrations of pure solids or pure liquids are constant and are given the
value 1 in the equilibrium equation.
 Equilibrium constants are independent of the particular concentrations or
pressures of species in a given reaction.
 Equilibrium constants depend on the reaction temperature.
 Le Chatelier’s principle states that when a reaction at equilibrium is subject to
change, the composition alters in such a way as to minimise the effects of that
change.
 For endothermic reactions a rise in temperature causes an increase in K, ie the
yield of the product in increased.
 For exothermic reactions, a rise in temperature causes a decrease in K, ie, the
yield of the product in decreased.
 The effects of changes in concentration or pressure on the position of
equilibrium can be explained quantitatively in terms of a fixed equilibrium
constant.

The presence of a catalyst does not affect the equilibrium constant.
Equilibria between different phases
 When a solute is shaken in two immiscible liquids, it partitions itself between
the two liquids in a definite ratio called the partition coefficient.

The value of the partition coefficient depends on the immiscible liquids
involved, the solute and the temperature.

Solvent extraction is an application of the partition of a solid between two
liquids.

Applications of solvent extraction include the purification of water-soluble
organic acids using a suitable organic solvent.

Chromatographic separations depend on the partition equilibrium between two
phases, one stationary and the other mobile.

There are several types of chromatography, eg paper, thin-layer, gas-liquid.

In paper chromatography the stationary phase in the water held on the paper
and the mobile phase is the solvent.

In gas-liquid chromatography, the stationary phase is a liquid held on a solid
support and the mobile phase is a gas.