Chapter: Chemical Equilibrium, Law Of Mass-action

Chemical Equilibrium, Law Of Mass-action, Summary

The cause of chemical change is identified with the affinities of the interacting substances. The activity of a substance, which may be measured by the velocity of reaction, was shown by Berthollet (1799) to depend not only on its affinity, but also on its concentration, i.e., the number of gram molecules in unit volume. The product of affinity and concentration is called the active mass.

The Law of Mass-Action states that the activity is proportional to the active mass, i.e., to the concentration. The product of the concentrations of the substances produced, divided by the product of the concentrations of the interacting substances, is constant when equilibrium is attained:

A + B + C + … <=> D + E + F…

, where K is the equilibrium constant.

If a system in equilibrium is subjected to an increase of pressure, a reaction will occur if possible leading to a diminution of volume, i.e. to a decrease in pressure. If a system in equilibrium is subjected to a rise in temperature a reaction will take place which is accompanied by an absorption of heat, i.e. one which tends to lower the temperature.

These results are special cases of Le Chatelier's law of reaction, which states that if a system in equilibrium is subjected to a constraint, a change occurs if possible of such a kind that the constraint is partially annulled.

The effect of adding an excess of one of the products of dissociation, either at constant volume or at constant pressure, on the degree of dissociation of a gas may be calculated by the law of mass-action.

In heterogeneous reactions the law of mass-action applies to the gaseous or dissolved components. The concentrations of pure liquids or solids in the gas phase or solution are constant, and may be included in the equilibrium constant, K.

The law of mass-action applies to the ionisation of weak electrolytes (Ostwald's dilution law), but not to strong electrolytes, when the concentrations must be replaced by the activities.

The effect of addition of a common ion on the solubility of a salt (solubility product), and the results of hydrolysis, may be considered from the point of view of the law of mass- action.

Indicators are weakly acid or basic substances, with anions or cations of different colour from the unionised molecules.

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