To Study the Reaction Rate of the Reaction Between Potassium Iodate and Sodium Sulphite Using Starch Solution as Indicator

Redox Reaction refers to the kind of chemical change where both oxidation and reduction happens simultaneously. Oxidation is the chemical process in a substance when oxygen is added to it. The reaction where electrons are gained by one of the atoms among the two or more chemicals in the reaction is known as a reduction reaction.

Potassium iodate is reduced to iodide by sulphite in an acidic medium. Sulphite ions react with potassium iodate, manufacturing iodide ions. Iodide ions, therefore formed, are oxidized to iodine by reaction with more iodate ions. Iodine reacts immediately with sulphite ions forming iodide ions. When sulphite ions are fully consumed, the liberated iodine will not be consumed and will show blue color if starch is present. 

This is an example of a clock reaction, as the rate of the reaction is calculable by the time taken for the appearance of the blue color. The oxidation of sodium sulphite by air is retarded by alcohol. 

Table of Content

• Aim

• Articles Required

• Theory

• Procedure

• Result

• Precautions

• Viva Questions

• Lab Manual Questions

• Summary

Aim

To study  the reaction rate of the reaction between KIO3 (Potassium Iodate) and Na2SO3 (sodium sulphite).

Articles Required

• 4 Conical flasks (250 ml)

• measuring cylinder

• Burette 

• pipette (25 ml)

• stop-watch

• 0.01 M sodium sulphite solution 

• 0.1 M potassium iodate solution

• starch solution

• 2 M H2SO4.

Theory

The reaction between potassium iodate and sodium sulphite indirectly includes the formation of iodide ions oxidized in an acidic medium. The overall reaction takes place in 2 steps.

$\begin{align} & IO_{3}^{-}+3SO_{3}^{2-}\to {{I}^{-}}+3SO_{4}^{2-}\cdots (1) \\ & 5{{I}^{-}}+6{{H}^{+}}+IO_{3}^{-}\to 3{{H}_{2}}O+3{{I}_{2}}\cdots (2) \\ & {{I}_{2}}+SO_{3}^{2-}+{{H}_{2}}O\to SO_{4}^{2-}+2{{I}^{-}}+2{{H}^{+}}\cdots (3,\text{ very fast)} \\ \end{align}$

The evolved iodine generates blue color with a starch solution. This reaction is additionally known as a clock reaction.

Procedure

1. Take four 250 milliliter conical flasks and label them as A, B, C and D.

2. Add 10 ml, 20 ml, 30 milliliters and 40 milliliters of 0.1 M KIO3 solution to the flasks A, B, C and D, respectively, with the help of burette.

3. Add 10 ml of 2 M H2SO4 to each flask.

4. Add water to form the volume of solution of 100 milliliters in each flask.

5. Add 5 milliliters of freshly prepared starch solution to every flask.

6. Add 25 milliliters of 0.01 M sodium sulphite solution to flask A with the assistance of a pipette and start the stopwatch immediately. Note the time once the blue color just appears.

7. Repeat step 6 with the solutions of flasks B, C and D.

Observation

Result 

The rate of reaction will increase with the increase in the concentration of potassium iodate.

Precautions 

Following precautions must be taken while carrying out the experiment:

• Only use freshly prepared starch solution.

• We have only to use a freshly prepared solution of sodium sulfide because it can simply get oxidised by air.

• The concentration of KIO3 should be more than the concentration of sodium sulphite solution. 

Viva Questions 

Q1.What is the concentration of Sulphuric acid used for this experiment?

Ans: 2M.

2.What's the proportion of sodium sulphite solution used for this experiment?

Ans: 6%.

3. What’s  the percentage of starch solution used for this experiment?

Ans: 5%.

4. What is the proportion of sodium thiosulphate solution used for this experiment?

Ans: 0.04M.

5. What is the percentage of K iodate solution used for this experiment?

Ans: 6%.

Lab Manual Questions

1. Why is the concentration of K iodate solution kept more than the concentration of sodium sulphite solution?

Ans. As Na2SO3 is likely to be easily oxidised in air, always use a fresh solution of  Na2SO3.

Keep the concentration of KIO3 solution more than the concentration of  Na2SO3 solution. Use a starch solution that is freshly prepared.

2. What happens when you combine K iodate and sodium metabisulfite?

Ans. In such a reaction, potassium iodate and sodium metabisulfite react to make iodine. The starch solution is an indicator of the end of the reaction by forming a deep-blue coloured starch–iodine complex.

3. Why is the excess of potassium iodide utilised in iodometric titration?

Ans. Potassium iodide should be added in excess to keep iodine dissolved. Once all the ascorbic acid has been consumed, any excess iodine can remain in the solution. Since aqueous iodine solutions are brown in colour, iodine can act as its own indicator.

4. What does K iodate do in salt?

Ans. The Potassium iodide and the role of iodate ions in salt are used to prevent iodine deficiency and associated thyroid disease.

Summary 

Kinetics is the study of the rates of chemical reactions. As reactants are transformed into products during a chemical change, the number of reactants can decrease and also the amount of product can increase. The speed of the reaction describes how fast the reaction happens.

The larger the speed of the reaction, the less time is required for a selected amount of reactants to be converted to a product. Some factors that will affect the rate of a chemical reaction include temperature, the nature of the reactants, their concentrations, and the presence of a catalyst. Starch as an indicator is used in the experiment performed.