CHEMICAL EFFECTS OF ELECTRIC CURRENT

INTRODUCTION: -

• Solid and molten metals are good conductors of electricity due to free electrons. When current flows through metals, only heating occurs and no chemical effect is observed.
• Most liquids have no free electrons and hence do not conduct electricity, e.g., water.
• When an acid, base or an inorganic salt is added to water it dissociates into positive and negative ions which conduct electricity. The solutions which conduct electric current are called electrolytes and the vessel containing it along with electrodes is called an electrolytic cell.
• Inorganic salts like NaCl and KCl conduct electricity in molten form.
• Silver iodide ( AgI ) conduct electric current even in solid form.
• Normally, the solutions of organic compounds are non-conductors.
• In NaCl crystal, Na+ and Cl- ions are bound to each other due to electrostatic attraction. About 7.9 eV [ 1 eV = 1.6 × 10exp(- 19) joule ] energy is required to separate them. Only 0.03 eV thermal energy is available at room temperature which is insufficient to break NaCl crystal. When NaCl is added to water, polar water molecules get arranged in space between the ions which reduce attraction between them. Also, due to a specific distribution of charge inside them, some water molecules stick to each other forming a cluster around the ions. Each cluster is electrically polarized which reduces the strength of electric field between the ions. For this reason, dielectric constant, K, of water is very high which reduces the electric field between Na+ and Cl- ions to 1 / K times and they get dissociated. Due to its high dielectric constant, water acts as a good solvent. Ions dissociated this way participate in conduction of electric current. 
• At room temperature, electrical conductivity of electrolytes is 10exp(- 5) to 10exp(- 6) times that of metals because ( i ) the number density of ions is less as compared to the number density of electrons in metals, ( ii ) viscosity of solution increases electrical resistance and ( iii ) drift velocity of ions is less compared to electrons due to their larger mass.