Page 44 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

(b) the pressure and (c) the concentration of the components. According to the
Le Chatelier-Braun Principle: 'If a constraint is applied to a system in
equilibrium, the system will adjust itself so as to nullify the effect of the
constraint', and the effect of the factors referred to can be considered in the
light of this statement.
(a) Temperature. The formation of ammonia from its elements is a reversible
process:

in which the forward reaction is accompanied by the evolution of heat (energy),
and is said to be an exothermic reaction; the reverse reaction absorbs heat and
is said to be endothermic. If the temperature of an equilibrium mixture of
nitrogen, hydrogen and ammonia is increased, then the reaction which absorbs
heat will be favoured, and so ammonia is decomposed.

(b) Pressure. Referring to the hydrogen iodide equilibrium system, the
stoichiometric coefficients of the molecules on each side of the equation for the
reaction are equal, and there is no change in volume when reaction occurs.
Therefore, if the pressure of the system is doubled, thus halving the total volume,
the two sides of the equation are equally affected, and so the composition of
the equilibrium mixture remains unchanged.
In the nitrogen, hydrogen, ammonia equilibrium system, there is a decrease
in volume when ammonia is produced, and hence an increase in pressure will
favour the formation of ammonia. Any gaseous equilibrium in which a change
in volume takes place will be affected by a change in pressure. For equilibrium
in the liquid phase, moderate changes in pressure have practically no effect on
the volume owing to the small compressibility of liquids, and so moderate
pressure changes do not affect the equilibrium.
(c) Concentration of reagents. If hydrogen is added to the equilibrium mixture
resulting from the thermal decomposition of hydrogen iodide, it is found that
more hydrogen iodide is present when equilibrium is restored. In accordance
with the Le Chatelier-Braun Principle, the system has reacted to remove some
of the added hydrogen.

2.2 THE LAW OF MASS ACTION
Guldberg and Waage ( 1867) clearly stated the Law of Mass Action (sometimes
termed the Law of Chemical Equilibrium) in the form: 'The velocity of a chemical
reaction is proportional to the product of the active masses of tlie reacting
substances'. 'Active mass' was interpreted as concentration and expressed in
moles per litre. By applying the law to homogeneous systems, that is to systems
in which al1 the reactants are present in one phase, for example in solution, we
can arrive at a mathematical expression for the condition of equilibrium in a
reversible reaction.
Consider first the simple reversible reaction at constant temperature:

The rate of conversion of A and B is proportional to their concentrations, or
r, = k, x [A] x [BI