Page 46 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

with oxidation-reduction systems, and with many separation problems: note
however that equilibrium constants do not give any indication of the rate of
reaction. These matters are dealt with in detail in succeeding sections of this
chapter, and in other pertinent chapters.

2.3 FACTORS AFFECTING CHEMICAL REACTIONS IN
SOLUTION
There are three main factors whose influence on chemical reactions in solution
need to be considered: (a) the nature of the solvent; (b) temperature; and
(c) the presence of catalysts.
(a) Nature of the solvent. Reactions in aqueous solution generally proceed
rapidly because they involve interaction between ions. Thus the precipitation
of silver chloride from a chloride solution by the addition of silver nitrate
solution can be formulated
Ag+ + Cl- = AgCl(so1id)
Reactions between molecules in solution, for example the formation of ethyl
acetate from acetic acid and ethanol, are generally comparatively slow. It is
therefore convenient to classify solvents as ionising solvents if they tend to
produce solutions in which the solute is ionised, and as non-ionising solvents
if they give solutions in which the solute is not ionised. Common ionising
solvents include water, acetic acid, hydrogen chloride, ammonia, amines,
bromine trifluoride and sulphur dioxide. Of these solvents, the first four are
characterised by a capability of giving rise to hydrogen ions, as for example
with water:

and with ammonia:
2NH3=NH: +NH;
These four solvents can thus be termed protogenic solvents, whilst bromine
trifluoride and sulphur dioxide which do not contain hydrogen are non-protonic
solvents. Non-ionising solvents include hydrocarbons, ethers, esters and higher
alcohols; the lower alcohols, especially methanol and ethanol, do show slight
ionising properties with appropriate solutes.
(b) Temperature. Reaction rates increase rapidly with rising temperature, and
in some analytical procedures it is necessary to heat the solution to ensure that
the required reaction takes place with sufficient rapidity.
An example of such behaviour is the titration of acidified oxalate solutions
with potassium permanganate solution. When potassium permanganate solution
is added to a solution of an oxalate containing sulphuric acid at room
temperature, reaction proceeds very slowly, and the solution sometimes acquires
a brown tinge due to the formation of manganese(1V) oxide. If, however, the
solution is heated to about 70 OC before adding any permanganate solution,
then the reaction becomes virtually instantaneous, and no manganese(1V) oxide
is produced.
(c) Catalysts. The rates of some reactions can be greatly increased by the
presence of a catalyst. This is a substance that alters the rate of a reaction