Chemical Equilibrium, Law Of Mass-action : Law Of Mass-action, Effect Of Concentration

Berthollet, in addition to his proof of the reversibility of reactions, made the important discovery that the extent of reaction depends on the quantity of reacting substance present in a given volume, or its concentration. The activity of a substance, he says, is "proportional to the affinity and the quantity"; by "quantity" he meant "concentration." The activity is therefore proportional to the product: (affinity) x (concentration), which Berthollet called the [active] mass. A weak affinity could be enhanced by a large concentration, and a strong affinity weakened by high dilution.

A substance, B, may be shared between two others, A and C, to form AS and BC: A + BC <=> AB + C. If the amount of A is increased, more of B goes to A, and a new state of equilibrium is set up in which the ratio AB|BC is greater than before. Although the actual affinities of A and C for B remain unchanged, that of A appears to have increased, because the effect of A is proportional not only to its affinity, but also to its concentration; in other words, to the product of affinity and concentration, the active mass.

Thus, in reversible reactions, the extent of chemical change is proportional to the active masses of the interacting substances. This is known as the law of mass-action. If to a system of substances in equilibrium an excess of one reacting substance is added, change occurs in such a way that the concentration of that substance is diminished.

The law may be illustrated by an experiment due to J. H. Gladstone (1855). Ferric nitrate and ammonium thiocyanate react in solution to produce ferric thiocyanate, which has a blood-red colour. The reaction is reversible: Fe(NO₃)_{3 + 3NH₄CNS <=> Fe(CNS)_{3 + 3NH₄NO₃, and if an excess of Fe(NO₃)_{3 or NH₄CNS is added the intensity of the colour deepens. But if NH₄NO₃ is added, the reverse reaction is favoured by the action of mass and the colour becomes paler.

Expt. 2. - Prepare two solutions containing 3.5 gm. of crystallised ferric nitrate (Fe(NO₃)_{3,6H₂O), and 2.3 gm. of NH₄CNS, in 1 litre of water, respectively. Mix 100 c.c. of each. A dark red solution of Fe(CNS)_{3 is formed. Add 25 c.c. of this solution to 1 litre of water in each of four glass cylinders; a pale brownish-red colour is produced. Keep one jar for reference, and to the other three add: (a) 25 c.c. of the ferric nitrate solution; (b) 25 c.c. of the thiocyanate solution; (c) 25 c.c. of a saturated solution of NH₄NO₃. Observe and explain the colour change in each case.}}
The concentration of a substance is usually measured by the number of gram molecules (mols) per litre. If a gas mixture contains 3.65 gm. of HCl per litre, the concentration is 0.1. Similarly, a solution of 97 gm. of KCNS per litre has a concentration of 1. It is convenient to denote the concentration of a substance by its chemical symbol enclosed in square brackets, e.g., [KCNS] = 1 means 97 gm. of KCNS, or the amount represented by the formula, in 1 litre.

The general equation of mass-action, due to Guldberg and Waage (1864-67), can now be stated. Let the reaction:

A + B + C +... <=> A' + B' + C' +...

occur, and let it be reversible. In equilibrium:

where K is the equilibrium constant.

We shall usually write the product of the concentrations of the products of the reaction in the numerator, and the product of the concentrations of the initial substances in the denominator; the larger the value of K, therefore, the greater will have been the extent of the forward reaction when equilibrium is attained.

Thus if we consider the reaction: 3NH₄CNS + Fe(NO₃)_{3 <=> Fe(CNS)_{3 + 3NH₄NO₃, we shall have the equilibrium equation:

Addition of NH₄CNS or NH₄NO₃ will therefore displace the equilibrium to a much greater extent than addition of the equimolecular amount of Fe(NO₃)_{3 or Fe(CNS)_{3, because the cubes of the concentrations of the former substances are involved.}}}}}}}

Protein crystallography
Chemistry guide
Chemical Equilibrium, Law Of Mass-action
-L-
<<<	>>>