Page 43 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CHAPTER 2
FUNDAMENTAL THEORETICAL

PRINCIPLES OF REACTIONS IN
SOLUTION

Many of the reactions of qualitative and quantitative chemical analysis take
place in solution; the solvent is most commonly water but other liquids may
also be used. It is, therefore, necessary to have a general knowledge of the
conditions which exist in solutions, and also of the factors which influence
chemical reactions.

2.1 CHEMICAL EOUlLlBRlUM
If a mixture of hydrogen and iodine vapour is heated to a temperature of about
450 OC in a closed vessel, the two elements combine and hydrogen iodide is
formed. It is found, however, that no matter how long the duration of the
experiment, some hydrogen and iodine remain uncombined. If pure hydrogen
iodideis heated in a closed vessel to a temperature of about 450 OC, the substance
decomposes to form hydrogen and iodine, but again, no matter how prolonged
the heating, some hydrogen iodide remains unchanged. This is an example of
a reversible reaction in the gaseous phase.

H2(g) + I2(g) = 2HI(g)
An example of a reversible reaction in the liquid phase is afforded by the
esterification reaction between ethanol and acetic (ethanoic) acid forming ethyl
acetate and water. Since, however, ethyl acetate undergoes conversion to acetic
acid and ethanol when heated with water, the esterification reaction never
proceeds to completion.
C2H50H + CH3COOH = CH3COOC2H5 + H20
It is found that after the elapse of a sufficient time interval, al1 reversible
reactions reach a state of chemical equilibrium. In this state the composition
of the equilibrium mixture remains constant, provided that the temperature
(and for some gaseous reactions, the pressure also) remains constant.
Furthermore, provided that the conditions (temperature and pressure) are
maintained constant, the same state of equilibrium may be obtained from either
direction of a given reversible reaction. In the equilibrium state, the two opposing
reactions are taking place at the same rate so that the system is in a state of
dynamic equilibrium.
It must be emphasised that the composition of a given equilibrium system
can be altered by changing the conditions under which the system is maintained
and it is necessary to consider the effect of changes in (a) the temperature,