Page:Outlines of Physical Chemistry - 1899.djvu/266

 248 OUTLINES OF PHYSICAL CHEMISTRY

(a — x) of sugar remaining at the time t n is proportional to the difference between the rotations a n and a.

The result of this experiment is to give the value 0*00205 for k, if we use decimal logarithms (instead of natural logarithms as indicated in equation 1).

2. Catalytic action op Dilute Acids on Methyl Acetate. — The experiment is carried out at 25°C. in a small flask which contains for 20 cubic centimetres of semi-normal acid (hydrochloric, sulphuric, &c.) 1 cubic centimetre of methyl acetate. The first acidimetric titration, for which 1 cubic centimetre of mixture is used, is made immediately after the reaction is started. After continuously increasing intervals, small quantities are drawn off and titrated ; but the last 5 cubic centimetres must be left in the digester for at least two days, and these are used for the final determination. Denoting by a ot the first titre (the number of cubic centimetres of ^-normal baryta- water equivalent to 1 cubic centimetre of the acid mixture), by a,a mi a n the successive titres correspond- ing to the intervals of time t, t mi t n, and by a the final

titre after the interval t, we have the equation

K = 1 r .log^L».

t n A — d n

This formula is only a modification of equation 1, and is easily understood. For the operation described and for hydrochloric acid the value 0*0018 has been found for k (using decimal logarithms).

The Equating of Problems relating to the Speed op Reactions. 1 — The examples cited have been chosen from amongst the simplest cases, and now we must treat the question from a more general point of view. We must distinguish between reactions which go on to comple-

1 The procedure given here is the classic one. For a simpler method of equation see Th. MilUer, Bulletin de la SocUU chimique de Paris, [3], 19, p. 337. N.B. — It is necessary to rigorously apply the definition of active mass.

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