Chemistry Project on Electrolysis of Potassium Iodide (KI)

Study the Electrolysis of Products of Potassium Iodide (KI)

Name: Devender Kumar Singh Class: XII - C School: Cambridge School Year: 2009-2010

I would like to express my sincere gratitude to my chemistry mentor Mrs. Anita Sharma, for her vital support, guidance and encouragement without which this project would not have come forth. I would also like to express my gratitude to the lab assistant Mr. Mangeram for his support during the making of this project.

Certificate: This is to certify that Devender Kumar Singh of class XII - C has successfully completed his project “To study the electrolysis of Potassium Iodide”, under my observation.

Signature: Mrs. Mamta Rishi (H.O.D) and Mrs. Anita Sharma (Mentor)

Basic Aim

Aim: To study the electrolysis of products of Potassium Iodide (KI).

Apparatus Used

Apparatus: U-tube stand, electrodes, KI solution, battery eliminator.

Theory

Electrolysis

It is defined as a process of decomposition of an electrolyte by the passage of electricity through its aqueous solution or molten (fused) state.

Mechanism of Electrolysis

Whenever an electrolyte is dissolved in water or is taken in the molten state, the electrolyte dissociates to produce positively and negatively charged ions. On passing electric current, the positively charged ions move towards the cathode and hence are called cations, whereas the negatively charged ions move towards the anode and hence are called anions. On reaching their respective electrodes, ions lose their charge and become neutral. The cations accept electrons from the cathode to become neutral species. Thus, oxidation occurs at the anode while reduction takes place at the cathode. The conversion of ions into neutral species at their respective electrodes is called Primary change. The product formed as a result of primary change may be collected as such or it may undergo a Secondary change to form the final products.

Quantitative Aspects of Electrolysis

Michael Faraday was the first scientist who described the quantitative aspects of electrolysis.

Faraday’s Laws of Electrolysis

First Law: The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).

Second Law: The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights (atomic mass of metal divided by number of electrons required to reduce the cation).

Products of Electrolysis

Products of electrolysis depend on the nature of material being electrolysed and the type of electrodes being used. If the electrode is inert, it does not participate in the chemical reaction and acts only as source or sink for electrons. On the other hand, if the electrode is reactive, it participates in the electrode reaction. Thus, the products of electrolysis may be different for reactive and inert electrodes. The products of electrolysis depend on the different oxidizing and reducing species present in the electrolytic cell and their standard electrode potentials. Moreover, some of the electrochemical processes although feasible, are so slow kinetically that at lower voltages these do not seem to take place and extra potential (called overvoltage) has to be applied, which makes such processes more difficult to occur.

Reactions Involved

In the electrolysis of an aqueous solution of KI, I- ions are oxidized at the anode preferentially to water molecules. Possible reactions at anode are as follows:

2 I-(aq) -> I2(g) + 2e-
2 H2O(l) -> 4 H+(aq) + O2 + 4e-

Reaction (1) occurs in preference to reaction (2) due to standard electrode potential values:

I2(g) + 2e- -> 2 I-(aq), E°/volt = +0.53 V
4 H+(aq) + O2(g) + 2e- -> 2 H2O(l), E°/volt = +1.53 V

Possible cathode reactions are:

K+(aq) + e- -> K(s), E°/volt = -2.92 V
2 H2O(l) + 2e- -> H2(g) + 2 OH-(aq), E°/volt = -0.83 V

The E° value of reduction reaction (5) is much smaller than that of reaction (6). Thus, reaction (6) occurs preferentially over reaction (5) at the cathode. The violet colour at the anode is due to formation of iodine and its subsequent reaction with starch. The pink colour at the cathode is due to formation of OH- ions which also render the solution alkaline. OH- ions give pink colour with phenolphthalein.

Procedure

Procedure:

Prepare 0.1M solution of potassium iodide.
Fix a U-shaped tube in a stand and insert two graphite electrodes into both ends of the U-tube through the corks.
Assemble the apparatus as shown in the figure.
Take about 30 ml of 0.1M solution of potassium iodide in a 100 ml beaker, add five or six drops of phenolphthalein solution and five to six drops of freshly prepared starch solution.
Stir the solution and transfer it into the electrolysis tube fitted with graphite electrodes.
Pass electric current through the electrolyte and observe the appearance of colour.
A pink colour appears at the cathode and a violet colour appears at the anode. Bubble formation also occurs on the surface of the cathode.

Observations

Test Solutions	Observations	Inference
Aqueous solution of potassium iodide with five drops of phenolphthalein and five drops of starch solution.	At the anode: violet colour. At the cathode: (i) pink colour, (ii) formation of bubbles.	Free iodine is evolved. (i) OH- ion is formed, (ii) hydrogen is evolved.

Precautions

Both the electrodes should be loosely fixed into the U-tube so as to allow the escape of evolved gases.
Electrodes should be cleaned before use.

Conclusion

In the electrolysis of an aqueous solution of potassium iodide, I- ions are oxidized at the anode preferentially to water molecules. Violet colour at the anode is due to iodine. Pink colour at the cathode is due to formation of OH- ions which renders the solution alkaline. OH- ions give pink colour with phenolphthalein.

Bibliography

Experiments on formal level topics in chemistry.
Chemistry - a textbook for class XII.
Chemistry teacher - Mrs. Anita Sharma.