In this article, we will be studying the effect of variation in concentration of iodide ions in the reaction rate between iodide ions and hydrogen peroxide at room temperature.
The aim of this experiment – Studying the effect of variation in concentration of iodide ions in the reaction rate between iodide ions and hydrogen peroxide at room temperature.
The reaction of iodide ions and hydrogen peroxide takes place in an acidic medium. It is represented as follows:
2I–(aq) + H2O2(l) + 2H+(aq) → I2(g) + 2H2O(l)
In the above reaction, iodide ions are oxidised to molecular iodine by hydrogen peroxide. When sodium thiosulphate is added in the presence of starch solution to the above reaction mixture, the iodine liberated, reacts with thiosulphate ions immediately after it is formed and is reduced back to iodide ions as long as all the thiosulphate ions oxidise to tetrathionate ions.
I2(g) + 2S2O32–(aq) → S4O62–(aq) + 2I–(aq)
Once all the thiosulphate ions are consumed, the concentration of liberated iodine increases swiftly to a point where an intense blue complex is formed. The time for the colour to appear is recorded, therefore, the reaction is sometimes known as a clock reaction.
The apparatus and materials required for this experiment are as follows:
- Five conical flask of volume 250 mL
- One conical flask of volume 500 mL
- Measuring cylinder of volume 100 mL
- Starch solution
- 0.1 M Potassium Iodide solution
- 2.5 M Sulphuric acid solution
- 0.04M Sodium thiosulphate solution
- 3% Hydrogen peroxide solution
- Take a conical flask of volume 500 mL.
- Add 25 mL of starch solution (freshly prepared), 25 mL of 3% hydrogen peroxide, 195 mL distilled water, and 25 mL of 2.5 M H2SO4 solution. Stir it well and label the flask as X.
- Place X in a water bath which is maintained at room temperature.
- Take four conical flasks of volume 250 mL each and label them as P, Q, R and S.
- Take the Na2S2O3 solution, KI solution, and distilled water in the flasks P, Q and R in a proportion mentioned in the below steps. keep the flask S aside for carrying out the reaction.
- Take 10 mL of 0.04 M Na2S2O3 solution, 10 mL of 0.1 M KI solution and 80 mL of distilled water in the conical flask marked P. Shake the contents of the flask well and keep it in a water bath.
- Take 10 mL of 0.04 M Na2S2O3 solution, 20 mL of 0.1M KI solution and 70 mL of distilled water in the conical flask marked Q. Shake the resulting solution well and place it in the same water bath.
- Take 10 mL of 0.04 M Na2S2O3 solution, 30 mL of 0.1 M KI solution and 60 mL of distilled water in the conical flask marked R. Shake the solution well and keep it in water bath.
- Take conical flask S. With the help of measuring cylinder pour 25 mL solution from flask X into it. Now add 25 mL of solution P into it with constant stirring. Start the stop-watch when half of the solution from flask P has been transferred. Keep flask S in the water bath to maintain a constant temperature and record the time required blue colour to appear.
- In exactly the same manner, repeat the experiment with the solutions of flasks Q and R separately.
- Repeat the experiment with solutions of flasks P, Q and R twice and calculate the average time.
- Record your observations as given in the below table.
Observation and result
|Composition of the system
|Time taken for appearance of the blue colour
|1.||25 mL solution from flask X + 25 mL solution from flask P|
|2.||25 mL solution from flask X + 25 mL solution from flask Q|
|3.||25 mL solution from flask X + 25 mL solution from flask R|
Precautions to be taken during the experiment:
- Keep the concentration of Na2S2O3 solution less than that of KI solution.
- Use starch solution which is freshly prepared.
- Use fresh samples of KI (potassium iodide) and hydrogen peroxide (H2O2).
- Use the same measuring cylinders to measure solutions in two different sets of observations. And clean it before using.
- Record the time immediately once the blue colour appears.
1. What is the concentration of KI solution used for this experiment?
2. What is the concentration of Na2S2O3 solution used for this experiment?
Ans: 0.04 M.
3. Number of conical flasks required for this experiment?
4. What is the percentage of hydrogen peroxide used.
5. What is the colour of the complex formed?