b CONCENTRATION AND RATE OF REACTION The reaction between sodium thiosulphate and hydrochloric acid can take a noticeable time Sulphur is produced during the reaction making the solution cloudy The rate of reaction can be found by finding the time taken to reach a certain degree of cloudiness in the solution The degree of cloudiness in this case may be defined as the point at which a black cross marked below the reaction vessel can no longer be seen by looking through the solution from above In this experiment the concentration of sodium thiosulphate is made variable whilst the concentration of acid is kept constant Sodium thiosulphate may be bought as hypo which is used in photography Make up 500 mL of aqueous solution containing 20 g sodium thiosulphate 2 M hydrochloric acid is also needed Bench dilute acid is usually of this strength Using a measuring cylinder put 50 mL of thiosulphate solution into a 100 mL beaker Place the beaker on a black cross marked on a sheet of paper Add 5 mL of the acid and note the time given by the second hand of a clock Stir the acid into the solution Note the time when the cross is no longer visible through the sulphur in the solution Repeat the experiment with a smaller concentration of thiosulphate Take 40 mL of thiosulphate solution and add 10 mL of distilled water Stir and then add 5 mL of acid as before The time for the cross to become invisible should be greater than for the last experiment Repeat the experiment using 30 mL 20 mL and 10 mL of this sulphate mixed with 20 mL 30 mL and 40 mL of distilled water Plot concentration of the thiosulphate solution against time taken for the reaction Concentration values may be taken as the volume of the original thiosulphate solution used Since Itime reciprocal of time is the measure of the rate of the reaction plot thiosulphate concentrations against Itime The equation for the reaction can be written as Na2S2O3 aq  2HClaq  H2O l  SO2 g  Ss