Effect of concentration on rate of a reaction is given by its rate law. For a general reaction

aA + bB → Products

The rate law is,

rate = = k[A]a[B]b

Here, a and b are the orders of the reaction with respect to reactants A and B respectively. The overall order, n, is given by

n = a + b

k is the rate constant of the reaction.

Generally, on increasing the concentration of a reactant, the rate of reaction increases.

The reaction between sodium thiosulphate and hydrochloric acid occurs according to the following equation,

Na2S2O3 (aq) + 2HCl (aq) → 2 NaCl (aq) + H2O (l) + SO2 (g) + S (s)

As the reaction proceeds, the amount of precipitated sulphur increases, the solution becomes increasingly turbid and at a certain stage it becomes opaque. This stage can be fairly judged accurately by a simple method. The flask containing the reaction mixture is kept on a sheet of white paper on which a cross is marked in ink and when the reaction mixture becomes opaque, the cross is no longer visible. Rate of the reaction is inversely proportional to the time interval between the start of the reaction and the disappearance of the cross mark.

Effect of concentration of each reactant can be studied by changing the concentration of one reactant while keeping that of the other constant. The time for disappearance of cross mark (X) is noted in each case. The inverse values of the time intervals are compared.

Experiment: To study the effect of concentration on the rate of reaction between sodium thiosulphate and hydrochloric acid.

How To Perform Experiment

Take three clean and dry conical flasks and mark them 1, 2 and 3. If necessary, clean the flasks with dilute nitric acid and then wash with water thoroughly before using them. With the help of measuring cylinder, take 30 mL distilled water in the conical flask 1 and 20 mL each in flask 2 and flask 3.

Rinse the measuring cylinder with the solution of sodium thiosulphate and add 10 mL of it each in flask 1 and flask 2 and 20 mL in the flask 3. Using ball point pen, mark a cross on a sheet of plain white paper, lay it on the table and keep the flask 1 on it, so that it covers the cross-mark. Make sure that the flask is absolutely dry from outside and base before keeping it on paper. If necessary, wipe it dry with a piece of filter paper.

Wash the measuring cylinder with water and rinse it with hydrochloric acid. Now add 10 mL of hydrochloric acid to the flask 1, swirl it to mix the solutions and simultaneously start the stop watch. Keep the flask back on the white paper.

Observe the cross mark through the solution and stop the watch as soon as it disappears. Repeat the procedure, first, using flask 2 and adding 20 mL hydrochloric acid to it and then with flask 3 and adding 10 mL hydrochloric acid.