To study the rate of reaction between potassium iodate (KIO<sub>3</sub>) and sodium sulphite (Na<sub>2</sub>SO<sub>3</sub>)

In this article, we will be studying the rate of reaction between potassium iodate (KIO3) and sodium sulphite (Na2SO3).

Aim:

The aim of this experiment – Studying the rate of reaction between KIO3 (potassium iodate) and Na2SO3 (sodium sulphite).

Theory:

The reaction between potassium iodate and sodium sulphite indirectly include the formation of iodide ions oxidised in acidic medium. The overall reaction takes place in two steps.

IO3 + 3SO32- → I + 3SO42- ……… (1)

5I + 6H+ + IO3 → 3H2O + 3I2 ………(2)

The evolved iodine generates blue colour with starch solution. This reaction is also called clock reaction.

Materials required:

The apparatus and materials required for this experiment are as follows:

  1. Six conical flask of volume 250 mL
  2. Stop-watch
  3. Trough
  4. Measuring cylinder of volume 100 mL
  5. 2 M Sulphuric acid
  6. 5% Starch solution
  7. 6% Potassium iodate solution
  8. 0.04M Sodium thiosulphate solution
  9. 6% Sodium sulphite solution

Procedure:

  1. Take a 250 mL conical flask and mark it as ‘P’.
  2. Pour 25 mL of 6% potassium iodate solution, 50 mL of distilled water, and 25 mL of 2.0 M H2SO4 into it.
  3. Shake the contents of the flask well.
  4. Keep the flask in temperature bath.
  5. Take five conical flasks of volume 250 mL each. Label them as Q, R, S, T and U.
  6. Take 6% Na2SO3 solution, starch solution and distilled water in flasks Q, R, S and T in the proportion given in the following steps and keep flask U to carry out the reaction.
  7. In the conical flask labelled ‘Q’ take 20 mL of Na2SO3 solution, 75 mL of distilled water, and 5 mL of starch solution. Mix the contents of the flask well and place it in the water bath.
  8. Take the conical flask marked ‘R’, add 15 mL of Na2SO3 solution, 80 mL of distilled water, and 5 mL of starch solution. Stir the mixture and place the flask in water bath.
  9. In conical flask ‘S’, add 10 mL of Na2SO3 solution, 85 mL of distilled water, and 5 mL of starch solution. Mix the contents and place the flask S in water bath.
  10. In conical flask ‘T’, take 5 mL of Na2SO3 solution, 90 mL of distilled water, and 5 mL of starch solution. Stir the contents of the flask T and keep it in the water bath.
  11. Take conical flask ‘U’. Pour 25 mL of the solution from the conical flask labelled ‘P’ and add 25 mL of the solution from conical flask Q. Start the stop-watch when half of the solution from flask Q has been added. Mix thoroughly by continues stirring and keep the flask in the water bath. Record the time required for the blue colour to appear..
  12. Repeat the experiment with the solutions in flasks R, S and T respectively by using 25 mL of the solution as in the experiment with solution from flask Q and record the time required to obtain blue colour.
  13. Record your observations as given in the below table
  14. Observe the details tabulated and find out the relationship between the time blue colour appears and the variation in concentration of Na2SO3.

Observation and result

Sl. no.

Composition of the system

Time taken for the blue colour to appear (t is seconds)

Average time in seconds

First trial

Second trial

1.

25 mL solution from flask P + 25 mL solution from flask Q

2.

25 mL solution from flask P + 25 mL solution from flask R

3.

25 mL solution from flask P + 25 mL solution from flask S

4

25 mL solution from flask P + 25 mL solution from flask T

Precautions to be taken during the experiment:

  • As Na2S2O3 is likely to be easily oxidised in air, hence, always use fresh solution of Na2S2O3.
  • Keep the concentration of KIO3 solution higher than the concentration of Na2S2O3 solution.
  • Use starch solution which is freshly prepared.
  • Start the stop-watch when half of the solution from conical flask Q,R,S or T is added to the conical flask U containing 25 mL solution from flask P.

Viva Voce:

  1. What is the concentration of Sulphuric acid used for this experiment?
  2. Ans: 2M.

  3. What is the percentage of starch solution used for this experiment?
  4. Ans: 5%.

  5. What is the percentage of Potassium iodate solution used for this experiment?
  6. Ans: 6%.

  7. What is the concentration of Sodium thiosulphate solution used for this experiment?
  8. Ans: 0.04M.

  9. What is the percentage of Sodium sulphite solution used for this experiment?
  10. Ans: 6%.

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