Kinetics Study on the Reaction between Potassium
iodate and Sodium sulphite The rate of a chemical reaction may depend
on the concentration of one or more reactants. The effect of concentration of the reactant
on the rate of a chemical reaction can be studied by the reaction between potassium
iodate and sodium sulphite in presence of starch indicator at room temperature and pressure. In the first step, sulphite ions react with
potassium iodate producing iodide ions. In the second step, Iodide ions formed are
oxidized to iodine by reaction with more iodate ions. In the third step, Iodine formed in the second
step reacts immediately with sulphite ions forming iodide ions. When sulphite ions are completely consumed,
the liberated iodine reacts with starch solution, giving a blue colour. Our aim here is to study the reaction rate
of the reaction between potassium iodate and sodium sulphite using starch solution as indicator. Materials Required: molar potassium iodate solution, 0.005 molar
sodium sulphite solution, 1 molar sulphuric acid, freshly prepared starch solution, four
250 ml conical flasks, measuring cylinders and stopwatch. Procedure: Take four 250 ml (250 milliliters) conical
flasks labeled as A, B, C and D. Measure 2 ml (2 milliliters) of 0.01 molar
potassium iodate solution in a measuring cylinder and transfer it into flask A.
Then add 4 ml (4 milliliters) of potassium iodate solution to flask B, 6 ml (6 milliliters)
to flask C and 8 ml (8 milliliters) to flask D using the measuring cylinder.
Using another measuring cylinder, add 10 ml (10 milliliters) of 1 molar sulphuric acid
to the flasks A, B, C and D. Now using a measuring cylinder, add 88 ml
(88 milliliters) of distilled water to flask A, 86 ml (86 milliliters) to flask B, 84 ml
(84 milliliters) to flask C and 82 ml (82 milliliters) to flask D to make the volume
of solution 100 ml in each flask. Add 5 ml (5 milliliters) of starch solution
to the flasks A, B, C and D using a measuring cylinder.
Now measure 10ml (10 milliliters) of 0.005 molar sodium sulphite solution in a measuring
cylinder and pour it into flask A and start the stop watch immediately.
Stop the stop-watch when the blue colour just appears and note down the time required for
the appearance of blue colour. Repeat the experiment by adding 10 ml (10
milliliters) of 0.005 molar sodium sulphite solution to flasks B, C and D and note the
time required in each case for the blue colour just appears.
Record the observations in a tabular column. Plot a graph between 1/ Vol. of KIO3 (* read
as 1 by volume of potassium iodate) along the ordinate and time for the appearance of
blue colour along the abscissa. The graph should be a straight sloping line. From the graph it is clear that the rate of
reaction between potassium iodate and sodium sulphite increases with increase in concentration
of potassium iodate. Precautions: Always use freshly prepared sodium sulphite
solution because it is easily oxidized by air.
Concentration of potassium iodate should be higher than that of sodium sulphite solution.
Use a freshly prepared starch solution.