It is another important industrial application of electrolysis. Electroplating is an art of coating a layer of costlier metal like gold, silver, etc. over the cheaper metal like iron. The purpose of electroplating is
• protection of cheaper metal like iron from corrosion
• beautification of articles like, earrings, bangles, tings, parts of wrist watches, etc.
• repair of the broken parts of delicate machinary where welding is not possible.
The principle of electroplating is similar to that of purification of metals by electrolysis. The article to be plated, is thoroughly cleaned with aq H2SO 4 and washed with distilled water. It is then made the cathode of the electrolytic cell. The anode is pure sheet of metal to be coated or plated. The electrolyte is a solution of a salt of the metal to be plated. During electrolysis, the metal to be electroplated is transferred from the anode to the cathode. Experimental technique is similar to that discussed in Activity 34.1 D.
(a) Acidified water or very dilute tetraoxosulphate(VI) acid.
(b) Dilute brine solution.
(c) Concentrated brine solution.
(d) Copper(II) tetraoxosulphate(VI) using copper electrodes.
A. Electrolysis of Acidified Water
1. Set up Hoffmann’s voltameter as shown in Fig. 34.3 using either D.C source of power or battery. Fill the voltameter with water acidified with tetraoxosulphate(VI) acid by opening the taps at both the ends. Close the taps after filling.
2. Complete the circuit and let the current flow for five minutes.
3. Note the volumes of the gases at the electrodes
4. Collect the gases in small glass tubes by holding them directly above the taps and closing them with your finger.
5. Test the cathodic gas by a burning splinter and anodic gas by glowing splinter.
Record your observations and try to answer the following:
(i) Name the gases formed at anode and cathode.
(ii) Write equations to represent anodic and cathodic reaction.
(iii) What is then relationship between volumes of two gases, Justify.
(iv) What is the net reaction equation.
B. Electrolysis of Dilute Brine Solution
1. Empty the voltameter, wash it thoroughly, rinse with distilled water and fill with very dilute aqueous sodium chloride solution. Complete the circuit and leave the current flowing for about 10 minutes.
2. Collect the gases, after noting their volumes.
3. After enough collection and noting the final volume of each gas, test the gas at the anode with a glowing splinter and that at cathode with a burning splinter as in experiment A.
Record your observations and answer the following:
(i) Which ions will be present at each electrode and what are their sources?
Which species will be discharged at the anode? And at the cathode?
(ii) Give an explanation in each case.
(iii) Note the relative volume of gas in each arm as the electrolysis proceeds. What is relationship?
(iv) Write an equation to represent the anodic reaction, the cathodic reaction. Combine these equations with a net reaction equation.
(v) What will be the likely observation if the final solution is tested with moist litmus paper?
(vi) Give an explanation for the prediction, and test to verify it.
C. Electrolysis of Concentrated Brine Solution
Repeat the experiment by using concentration brine solution. Record your observations and answer the same questions (i) to (vi)
Note: In this case you will test the gas at anode with litmus paper and gas at cathode with glowing splinter.
D. Electrolysis of Copper(II)tetraoxosulphate(VI) Solution using Copper Electrodes
1. Set up the apparatus as shown in Fig. 34.4.
2. Note the mass of properly cleaned copper plates to be used as electrodes.
3. Close the switch and allow the current to flow for about 15 minutes.
4. Take out the electrodes dry them and again weigh them. Record the change in the mass of electrodes after the experiment. Try to provide answer of the following.
(i) Which electrode loses and which one gains weight? why?
(ii) Write an equation to represent (a) the cathodic reaction (b) the anodic reaction.
(iii) With the help of a diagram, show how you would obtain a large piece of pure copper from a large piece of impure copper and a thin sheet of pure copper