Page:Outlines of Physical Chemistry - 1899.djvu/263

 ��II. CHEMICAL DYNAMICS

Study op the Speed op Reaction

We know numerous reactions which take place sufficiently slowly to allow of the progress of the transformation being experimentally determined. Let us take as an example the decomposition of saccharose into dextrose and fructose (laevulose) by means of dilute acid. This transformation is not reversible. The acid acts by a so-called catalytic influ- ence, and its original concentration is not altered during the course of the reaction. The only variable mass which has to be considered is that of the sugar, 1 and this concentration can easily be ascertained by polarimetric analysis. As this concentration diminishes as the decomposition pro- gresses, the speed of the reaction continually decreases.

If the initial quantity of the sugar dissolved is a, then after a time t there is only a residual quantity (a— x) left unde- composed. If now the experiment be extended over a very short time dt, a further quantity of sugar dx will be trans-

formed, and consequently the speed of the reaction is j- at

the moment t. But we can also express this speed in another way : for unit weight of sugar dissolved 2 it would be k, for the quantity (#— x) it is K(a— x). We therefore have the

1 The products of decomposition need not be taken into account since the reaction is not reversible.

8 As the chemical reaction in question is uni-molecular, we may express the quantity of sugar either by units of weight (grams in solu- tion) or by the active mass (number of equivalents per litre). In the equation of pluri-molecular reactions, which we shall meet with later, only the active masses or concentrations may be used.


 * The integral calculus is here indispensable.

The constant k has a special value which requires a word of explanation. It refers to the catalytic action of the acid employed,

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