A chemical action of this sort in which one of the
substances present, although essential for the speed of the
reaction, does not suffer any change, is termed catalysis or
a catalytic reaction. The decomposition of cane sugar into
, invert sugar is a similar process.
The catalysis of ethyl acetate does not proceed, as might be expected from the chemical equation, so that the whole quantity of ester is changed into alcohol and acetic acid, but the reaction approaches a limit, the so-called limit of reaction. In this case the limit is reached when two-thirds of the ethyl acetate has decomposed. On the other hand, if equimolecular quantities of alcohol and acetic acid be mixed, and a little hydrochloric acid added, ethyl acetate and water are gradu- ally formed according to the equation —
CaHgOH + CH3.COOH (+ acid) = CHs.COOCaHg + H2O
(+ add),
that is to say, a reaction opposed to the above catalysis takes place. This reaction also approaches a limit which is the same as that already mentioned, and is reached when a third of the alcohol and acetic acid have formed ester.
In order to express that a measurable equilibrium is established, the reaction is written in the following way, according to the suggestion of van't Hoff —
CHa.COOCaHfi + HaO (+ acid) ^ C2H5OH + CHs.COOH
(+ acid).
The double arrow sign (^), used in place of the usual sign of equality ( = ), denotes that the reaction may proceed in one direction or the other, depending on the concentrations of the reacting substances, and that finally an equilibrium will be established when the two opposite reactions take place at the same speed.
Besides these " incomplete " resictions — Le, those in which the extent of reaction is limited — there are other reactions in which practically the whole of the substance originally present is transformed; the inversion of cane sugar is an
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