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Chemical Effects of an Electric Current

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Describe in details the chemical changes (chemical effects) that take place when electric current is passed through a liquid.

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This write-up describes in detail the process of electrolysis and Faraday's laws of electrolysis.

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CHEMICAL EFFECTS OF CURRENT - ELECTROLYSIS

1. What is meant by chemical effect of current?

When a current is passed through a solid it produces heating effect, magnetic effects etc. which do not involve any chemical changes. However, when a current is passed through some liquids, under certain conditions, chemical changes take place. These are known as chemical effects of current.

2. Do all liquids show chemical effects when current is passed through them?

No, all liquids do not show chemical effects when current is passed through them. Some liquids do not allow the current to pass through at all i.e. these are insulators (examples : distilled water, transformer oil, vegetable oil). Some liquids act as good conductors and allow the current to pass without any chemical effect (example : mercury). However, there are some liquids notably solutions of salts, acids and bases in water or alcohol (called electrolytic solutions) which show chemical reactions when current is passed through them.

3. What is an electrolyte?

Many substances even in solid state exist in ionized form. In these substances the +ve and -ve ions hold together due to the electrostatic force of attraction. Such substances are called electrolytes. For example in a molecule of NaCl, each Na atom carries a single +ve charge and Cl atom a single -ve charge. As we know charged atoms are called ions. Thus a molecule of NaCl is made up of one positively charged Na ion (designated as Na+) and one negatively charged Cl ion (designated as Cl-), both held together, in solid state, because of the electrostatic force of attraction. Of course there is the thermal agitation of the molecules which tends to dissociate (break up) the ions even in the solid state. However, the thermal agitation energy is not sufficient to provide the energy required to dissociate them For example : in case of NaCl, the average thermal agitation energy per molecule is 0.03 eV and the energy required to dissociate NaCl ions is 7.9 eV per molecule.

Acid, bases and salts are the most common examples of electrolytes.

4. Why do electrolytes dissociate on dissolving in liquids ? What is electrolytic conduction?

As we had learnt in electrostatics, as per Coulomb's law the force between two charges q1 and q2 at a distance r in a medium with dielectric constant K is given by :

(see attached file for equations)

where Є0 is the permittivity of vacuum. Since value of K for vacuum is 1 and for any other medium it is greater than 1, the force of attraction between the ions will reduce when the electrolyte is dissolved in any liquid, extent of reduction depending upon the value of K for the liquid. For example, value of K for water is 81. Thus the force of attraction between Na+ and Cl- ions will be reduced by a factor of 81 when NaCl is dissolved in water. Accordingly the energy required to dissociate the ions also reduces drastically and a large number of ions get dissociated making the solution rich in ions of both the types.

Now if two metal plates (called electrodes) are dipped partially into the electrolytic solution, and connected to a battery (one plate called anode connected to +ve terminal and the other called cathode to the negative terminal), an electric field is set up inside the solution. Under the influence of this field, positively charged ions start moving towards the negatively charged cathode (these ions are called cations) and negatively charged ions move towards the positively charged anode (called anions). Thus the process of conduction of electric current from the anode to the cathode (since the electrons are moving from cathode to anode) starts inside the solution. This process is called electrolytic conduction.

5. What is electrolysis?

The process of dissociating electrolytes and producing continuous chemical activity (such as production of ...

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