While passing electricity through electrolytes, you might have noticed some changes in them. You might also have noticed bubbles of gas at the electrodes as well as changes in the metal electrodes.
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All these indicate that when an electric current is passed through an electrolyte, a chemical change takes place. This chemical change is called electrolysis. The electrolyte and the electrodes used in electrolysis form an electrolytic cell.
When electricity passes through an electrolyte, the positive ions of the electrolyte move towards the cathode (negative electrode), where they gain electrons to become a neutral substance. The negative ions move towards the anode (positive electrode) and give up electrons to become neutral.
If the neutral substance is a gas in its natural state, it is evolved at an electrode. If it is a metal, it is deposited at the cathode. With the formation of neutral substances, the electrolyte is decomposed. So we can say that the decomposition of an electrolyte when electricity is passed through it is called electrolysis.
Take some water in a plastic or glass vessel. Add a pinch of salt or a few drops of an acid such as vinegar to the water. This will make the solution conduct electricity. Break open two cells and use their carbon rods as electrodes.
Since carbon does not react easily, the electrodes do not undergo change during electrolysis. Place the electrodes in the solution and connect them to a battery of 6 volts. Soon you will see bubbles of gas at the electrodes. The gas evolved at the cathode is hydrogen and that at the anode is oxygen. Note that there are more bubbles at the cathode.
The chemical change that takes place in this activity is called the electrolysis of water, in which water gets decomposed into hydrogen and oxygen. First, water splits into hydrogen (H+) and hydroxide (OH–) ions. These lose charge to form hydrogen and oxygen at the electrodes.
The overall reaction can be represented by
It is clear that the volume of hydrogen liberated is twice that of oxygen. That is why you see more bubbles at the cathode. You can use electrolysis to make an electric pen. Dissolve some potassium iodide (KI) in a starch solution.
Soak a piece of stiff paper in this solution and place it on the bared end of a wire. Connect this wire to the negative terminal of a battery. Use a wire connected to the positive terminal of the battery to write on the piece of paper.
In solution, potassium iodide consists of potassium and iodide ions.
KI → K+ + I–
When electricity is passed through the solution, the iodide ions lose their charge to form iodine.
2l– – 2e‑ → I2 (at the anode)
The iodine released produces a blue-black-coloured substance with starch, which acts as the ‘ink’ of the electric pen.
Electroplating is an important use of electrolysis. The electrical process of coating an inexpensive conductor with a metal is called electroplating. Electroplating is done for protection or decoration.
For example, the bumpers of cars are chromium-plated to protect them from corrosion. For the same reason, water taps are plated with nickel or chromium. Iron plated with tin to prevent rusting is used to make ‘tin’ cans. A ‘silver’ spoon is an example of electroplating for decoration.
For electroplating a steel spoon with silver, a solution of a silver salt is taken as the electrolyte. The spoon and a silver bar are dipped into the electrolyte and connected to the negative and positive terminals of a battery respectively. The positively charged silver ions move to the negative electrode (spoon) and form a deposit of silver on it.
You can electroplate a five-rupee coin or a carbon rod with copper by using copper sulphate as the electrolyte. Fix a metal clip on a five-rupee coin and connect it to the negative terminal of a battery. Connect the positive terminal of the battery to a thick copper wire. Place the coin and the copper wire in the copper sulphate solution. Soon the coin will get coated with copper.
A copper sulphate solution consists of copper and sulphate ions.
CuSO4 → Cu2+ + SO42-
When electricity is passed through the solution, the positively charged copper ions move to the cathode (the coin connected to the negative terminal of the battery). There the copper ions gain electrons and get deposited as copper metal.
Cu2+ + 2e– → Cu
The sulphate ions remain in solution. At the copper anode, copper goes into solution by giving up two electrons. Thus the strength of the solution remains unchanged.
Cu → Cu2+ + 2e–
Uses of electrolysis:
1. Electrolysis is used in the extraction of metals from their ores. For example, when a current is passed through molten sodium chloride, sodium is deposited at the cathode and chlorine gas is evolved at the anode. Aluminium and potassium are also extracted by electrolysis.
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2. It is used for refining certain metals such as copper and zinc.
3. Electrolysis is used for the manufacture of chlorine. In submarines, oxygen produced by the electrolysis of water is used for breathing.