Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

144 KEACTION VELOCITY
If this important constant is known it is readily possible, as will
next be shown, to calculate in practice the extent of the transformation
in an equilibrium reaction when the reactants are no longer present
in equimolecular amounts, i.e. are not in equal molar concentrations.
The calculation of K causes no difficulty in the present example,
after what has been said about the position of the equilibrium. The
resultant mixture contained one-third of a mole each of acid and alcohol
and two-thirds of a mole each of ester and water. Therefore :
TT 3 • 3 _JL
It will now be possible to discover how and to what extent the for-
mation of the ester can be affected if acetic acid and alcohol are not
used in equimolecular proportions but, for example, in the ratio 1 : 2
moles. If the amount (in moles) of ester present at equilibrium be
x, then CE = x ; and since just as many molecules of water as of ester are
formed, C w is also equal to x. The concentration of the acid is then
1 - x, and that of the alcohol 2 - x, and, therefore :
( l s ) ( 2 s )
X . X
From this equation it follows that x = 0-85, i.e. by increasing the
concentration of the alcohol (or the acetic acid) the position of the equi-
librium can be so shifted that the yield of ester is increased to 85 per
cent. Advantage is very frequently taken of this practical possibility
of altering the position of the equilibrium.
In this connexion the following problems should be solved :
How much ester will be formed when 3 moles of alcohol react
with 1 mole of acetic acid ? How much when 30 g. of acetic acid and
50 g. of alcohol are used ? In what proportions by weight must acetic
acid and alcohol be caused to interact in order to convert 75 per cent of
the former into ester ?
In all reactions which proceed entirely in one direction the amount
of end product which could theoretically be expected can be calculated
from the stoicheiometrical ratios. But in reactions in which an equili-
brium is set up, it follows from the above considerations that we must
know the equilibrium constant, which must be deduced by analysis.
In the example chosen this evidently causes no difficulties; it is only
necessary to determine by titration the concentration of the acetic
acid in the equilibrium mixture.
The R61e of the Sulphuric Acid.—If acetic acid and alcohol alone
are heated together no noticeable reaction takes place even after a
long time. The reaction is only started by added sulphuric acid,
which may be replaced by gaseous hydrogen chloride. In all prob-
ability these mineral acids form an unstable addition compound (more
likely with the acetic acid) and this compound reacts with alcohol to