Page:Text-book of Electrochemistry.djvu/117

 102 VELOCITY OF REACTION. chap.

Saponification of an Ester. — Since in the reaction jnst considered the quantity of one of the reacting substances remains constant, the concentration of only one kind of molecule changes, corresponding with the given equation —

Van*t Hoflf terms such reactions monomoleciUar, Chemical reactions in which two of the reacting substances disappear in the course of the action are much more common. The best known example of such a bimoleetUar reaction is the saponification of an ester. This reaction proceeds according to the equation —

NaOH + CHsCOGCaHg = CHaCOGNa + CaHgOH.

If we start with equivalent quantities, A mols per litre, of the two substances, then after time t the same quantity of the two substances, x mols, must have disappeared, and there will remain (A — x) mols of each in a litre. Now, the quantity of sodium acetate which is formed in unit time is proportional to the concentration of the ethyl acetate and that of the sodium hydroxide ; therefore —

from which by integration can be obtained —

��A — OBn A "- xi

��= K(ti - ^o)

��where K, a constant, represents the specific velocity of the reaction.

In order to prove the validity of this equation, we cite the following results obtained by Madsen (5) in the investigation of the strengths of sugar, dextrose, and levulose as acids. The first column contains the time in minutes, the second the concentration of the base (^J^ ?i-solution being taken as unit), and the third the constant for the specific velocity of reaction.

�� �