THEORY:
The higher the concentration of a reactant or reactants, the greater is the rate of the reaction. This effect of concentration can be well investigated by following the reaction of sodium thiosulphate of different concentrations with the hydrochloric acid of the same concentration. Thiosulphate ion reacts with acid as follows with the formation of colloidal sulphur.
S₂O3 — + 2H+ ———> H₂O + SO₂ + S
APPARATUS :
1. Conical flask 250 ml
2. Measuring cylinder 100 ml and 5 or 10 ml
3. Stop watch
CHEMICALS:
1. 0.2 M sodium thiosulphate
2. 2 M hydrochloric acid.
PROCEDURE:
i) Pour 50 ml of 0.2 M sodium thiosulphate into a conical flask from a measuring cylinder. Also keep ready 5 ml of 2M HCL in another small measuring cylinder.
ii) Place the conical flask over a white filter paper on which a cross mark has been written.
iii) Add the 5 ml of the acid into the flask, start a stopwatch immediately, swirl the flask once and keep it still on the filter paper.
iv) Look vertically down through the solution and stop the stop watch when the cross mark just disappears. Note the time in seconds. Rinse the flask immediately.
v) Repeat the experiment four times more by taking 40, 30, 20 and 10 ml of 0.2 M sodium thiosulphate, diluting it up to 50 ml with water each time.
vi) Plot (i) a graph of molar concentration of the thiosulphate vs time (by taking concentration along the x-axis and time on the y-axis).
(ii) a graph of molar concentration of the thiosulphate vs reciprocal of time. (by taking concn along the x-axis and 1/T on the y-axis).
Write the conclusion you can draw from the graph.
RESULTS:
| No. of obs. | Vol of 0.2 M thiosul-hate(ml) | Vol. of water(ml) | Molarity of solutions | Time for the cross to disappear (T) | Reciprocal of time 1/T |
| 1 | 50 | 0 | |||
| 2 | 40 | 10 | |||
| 3 | 30 | 20 | |||
| 4 | 20 | 30 | |||
| 5 | 10 | 40 |