The rate of reaction depends upon the temperature. Arrhenius equation gives the relationship between the two:
k = A.e–Ea/RT
where k is the rate constant of the reaction at temperature T. A and Ea are constants for a given reaction. A is known as Arrhenius factor and Ea as activation energy. Activation energy is the minimum energy which the reactant molecules must possess over and above of their average energy in order to undergo chemical change. On raising the temperature, more reactant molecules possess sufficient energy required for reaction. Hence, the rate of the 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 temperature can be studied by starting the reaction with the same concentrations of both the reactants at different temperatures. The inverse values of the time intervals for the cross mark to disappear are compared.
How To Perform Experiment
Take three clean and dry conical flasks and mark them 1, 2 and 3. If necessary, before using, clean them with dilute nitric acid and wash with water thoroughly. Using a clean measuring cylinder take 30 mL distilled water in each flask. Rinse the measuring cylinder with sodium thiosulphate solution and add 10 mL of it in each of these three conical flasks.
Using a ball-point pen, make a cross mark on a plain white sheet of paper and lay it on table. Wash the measuring cylinder with water and rinse with hydrochloric acid. Take another conical flask (other than the three marked flasks) and take 10 mL hydrochloric acid in it using the measuring cylinder.
Heat both the flasks (flask No. 1 containing Na2S2O3 and the other containing HCl) to a temperature of about 40°C separately on two tripod stands with wire gauze on them. Place the flask 1 on the sheet of white paper covering the cross mark. Quickly pour the hot HCl solution into the flask 1. Mix the solutions by swirling and start the stop watch simultaneously.
Keep it back on the white sheet of paper. Measure the temperature of the mixture solution and record it. Observe the cross mark through the solution and stop the watch as soon as it disappears. Record the time interval and its inverse.
Reset the zero of the stop watch for next reading. Repeat this procedure two more times. First time use the flask 2, and heat the solutions to about 50°C. Second time use flask 3 and heat the solutions to about 60°C.