CBSE Class 10 Physics Chemical Effects of Electric Current Notes Set 04

Electrolytes and Conduction in Solutions

An electrolyte is a substance that conducts electricity in solution or in the molten state by means of free ions. Strong electrolytes (HCl, NaOH, NaCl, K\( _2 \)SO\( _4 \)) ionise completely and conduct well. Weak electrolytes (acetic acid, ammonium hydroxide, phosphoric acid) ionise only partly and conduct poorly. Pure water and solutions of sugar, glucose and alcohol do not conduct because they have almost no free ions. Solid NaCl does not conduct because its ions cannot move.

Electrolytic conduction is the flow of current through a solution due to the movement of positive ions towards the cathode and negative ions towards the anode. Its conductivity is far lower than that of metals because there are fewer free charge carriers, the solution is viscous and the ions are much heavier than electrons. The conductivity of an electrolyte rises with temperature.

Electrolysis

The chemical change produced when an electric current passes through an electrolyte is called electrolysis. It is carried out in an electrolytic cell (voltameter) made of a bath containing the electrolyte and two electrodes joined to a battery.

1. The electrode joined to the positive terminal is the anode; negative ions move to it and are oxidised (lose electrons).
2. The electrode joined to the negative terminal is the cathode; positive ions move to it and are reduced (gain electrons).
3. Direct current is used, not alternating current.
4. Current in the solution is carried by ions, and in the wires by electrons.
5. Electrical energy is converted into chemical energy. In a galvanic cell, chemical energy is converted into electrical energy.

Electrolysis of Copper Sulphate Solution

With copper electrodes in a CuSO\( _4 \) solution, the solution has Cu\( ^{2+} \) and SO\( _4^{2-} \) ions. At the cathode: \( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \), so copper is deposited. At the anode: \( \text{Cu} \rightarrow \text{Cu}^{2+} + 2e^- \), so copper dissolves. Copper is thus transferred from the anode to the cathode and the concentration of the solution stays the same.

Electrolysis of Water

Pure water is a poor conductor, so a little dilute sulphuric acid is added. With platinum electrodes (which do not react), hydrogen is liberated at the cathode and oxygen at the anode, in the volume ratio 2 : 1: \( 2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2 \).

Applications of Electrolysis

1. Electroplating: depositing a thin layer of a metal (for example chromium, nickel, silver) on an object to protect it or make it attractive. The object is the cathode and the plating metal is the anode.
2. Extraction of reactive metals such as sodium and aluminium from their compounds.
3. Refining of metals such as copper: impure metal is the anode and pure metal is deposited on the cathode.
4. Making of chemicals such as chlorine and sodium hydroxide from brine.
5. Cells and batteries: the lead accumulator uses dilute sulphuric acid as the electrolyte and can be recharged; a dry cell and a button cell are other common types.