CALCULATIONS USING RATE EXPRESSIONSi.e., the order of reaction with respect to ethanal is 1.5. Since ethanal is the only react-
ant, the overall order is also 1.5. The order of reaction is something of a surprise, and
we could certainly not predict it from the chemical equation for the reaction.
Hydrolysis of methyl ethanoate
The equation for hydrolysis of methyl ethanoate ester by sodium hydroxide is
CH 3 COOCH 3 (l) OH(aq)CH 3 COO(aq) CH 3 OH(aq)The rate expression is found to be
rate of reactionk[CH 3 COOCH 3 (l)][OH(aq)]The order of reaction is therefore one (i.e. first order) with respect to both
methyl ethanoate and sodium hydroxide and the overall order of reaction is two (i.e.
second order).
If the sodium hydroxide is in excess (at least 10 times the concentration of ester),
theconsumptionof sodium hydroxide during the reaction will be very small and may
be neglected. This means that [OH], the hydroxide ion concentration at an instant,
may be replaced by the initial hydroxide concentration [OH] 0 .kand [OH] 0 may
now be lumped together as a new constant k. Under these conditions the rate
expression becomes
rate of reactionk[CH 3 COOCH 3 (l)]wherek[OH] 0 k. Experimentally, the reaction is now first order. We say that
the reaction is pseudo first orderand that kis the pseudo-first-order rate constant.
Here, the word ‘pseudo’ means ‘falsely’. Deliberately making one of the reactant
concentrations much higher than the other(s) is a useful strategy for forcing a reac-
tion to show first-order kinetics.
Calculations using rate expressions
Calculating the rate constant from the initial rate of
reaction
What is the rate of reaction at the start of a reaction? Immediately after mixing, the
concentrations of reactants will not have changed very much from their starting
values. We then define the initial rate of the reaction as
initial rate k[A]x 0 [B]y 0 [C]z 0...where [A] 0 , [B] 0 , and [C] 0 are the initial concentrations of reactants A, B and C.
The rate constant kat a specific temperature may now be calculated by rearrange-
ment:
k initial rate
[A]x 0 [B]y 0 [C]z 0...14.5
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