70 GENERAL CONDITIONS OF EQUILIBRIUM, chap.
As an example of a homogeneous system in which the change can be conveniently followed by chemical investiga- tion, we may instance a solution containing sodium hydroxide and ethyl acetate which decompose according to the equation —
NaOH + CHa.COOCaHfi = CHs.COONa + CaHgOH
into sodium acetate and ethyl alcohol.
As the reaction proceeds, the quantity of sodium hydroxide in the solution decreases, and the amount present at any moment can be ascertained by titrating a portion with acid.
The amount of substance, expressed in gram-molecules per litre, which is transformed in unit of time, is termed the velocity of reaction of the system.
The velocity of reaction is frequently so great that by the methods known to us at present it is impossible to determine it. Nevertheless it can hardly be doubted that every chemical reaction requires a certain time in which to take place. In heterogeneous systems this time is considerable, for reaction can only take place at the surface of contact of the reacting substances, and consequently cannot go on suddenly. This is expressed in the old dictum : corpora non agunt nisi soluta. In a heterogeneous mixture, however, the velocity of reaction may be very high, as is shown by the explosive power of gunpowder, in which all the reacting substances are solid, and by that of flour-dust, where one of the substances is solid and the other (the oxygen of the aii*) gaseous. In all such cases the surface of contact is very great, either on account of the close incorporation or on account of the fine state of division of the reacting substances.
Chemical Equilibrium. — If ethyl acetate and water be mixed in molecular proportions and a little acid (e.g. hydro- chloric acid) added, then at the ordinary temperature a slow change takes place, the ester being converted into ethyl alcohol and acetic acid by taking up water —
CH3.COOC2H5 + HaO (+ acid) = CaHfiOH + CH3.COOH
(+ acid).
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