#physical_chemistry_lab
#acid_hydrolysis_ester
#physical_chemistry_experiment
An ester can be hydrolyzed under both acidic and alkaline conditions, but the mechanisms of the reactions
and hence their orders are different.
The rate of the reaction for the hydrolysis of ethyl acetate in acidic medium according to:
H
+
CH3COOC2H5 + H2O ⎯⎯→ CH3COOH + C2H5OH
May be expressed as: Rate =
dt
−d [ester]
= k´ [ester]x
[H2O]y
Since water is present in excess, its concentration remains unaffected (constant) and the rate of the
reaction is proportional to the concentration of ester only, i.e.
Rate = k [ester]x
“first order” x = 1
Notice that the hydrolysis (saponification) of ethyl acetate in alkaline solution is a second order reaction.
H
+
in this experiment is a catalyst. It is added to enhance the rate of reaction, but its concentration is not
affected. Without its presence, the reaction is extremely slow.
To follow the reaction, several aliquots of the reaction mixture are taken at different time intervals. For
each time chosen, the reaction is quenched [i.e., it is cooled to a very low temperature, so the rate is
reduced to a negligible value]. This is done by addition of ice. The extent of reaction is detected by
measuring the amount of acetic acid formed. This is done by titrating the aliquot against a base (NaOH),
using an acid / base indicator (phenolphthalein).

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