Page 45 - The engineering of chemical reactions

Approximate Reactions 29

APPROXIMATE REACTIONS

Consider the hydrolysis or saponification of an ester (ethyl acetate) into an alcohol (ethanol)
and an acid (acetic acid),

CHjCOOC2H5 + H20 2 CH3COOH + C2HjOH
This reaction and its reverse take place readily in basic aqueous solution. We write this
reaction as
A+BzC+D
Next consider the addition of water to an olefin to form an alcohol,

RCH = CH2 + HZ0 2 RCH2CH20H
which can be written as

In many situations we carry out these reactions in dilute aqueous solutions, where
there is a large excess of water. The concentration of pure liquid water is 55 moles/liter,
and the concentration of water in liquid aqueous solutions is nearly constant even when
the above solutes are added up to fairly high concentrations. The rates of these forward
reactions are

r = ~[CH~COOC~HS][H~O]
and
r = k[RCH = CH2][H20]
respectively. However, since the change of the concentration of water [H;?O] is usually
immeasurably small whenever water is a solvent, we may simplify these reactions as
CH3COOC2HS + CH$OOH + C2H50H, r = ~[CH~COOC~HS]
and
RCH = CH2 -+ RCH2CH20H, r = k[RCH = CHz]
These reactions do not satisfy total mass conservation because the mole of water is omitted
as a reactant. We have also redefined a new rate coefficient as k = k[HzO] by grouping the
nearly constant [Hz01 with k. After grouping the concentration of the solvent [Hz01 into
the rate coefficient, we say that we have a pseudo-first-order rate expression.
It is fairly common to write reactions in this fashion omitting Hz0 from the chemical
equation and the rate; so these reactions become of the type
A+C+D, r = kCA
and

A + C, r = kCA
respectively. Thus, besides isomerization, there are in fact a number of reactions that we
write approximately as A + B or A -+ products; so our use of these simple rate expressions
is in fact appropriate for a large number of reaction systems.