Sunday, February 3, 2008

Revision - Electrochemistry


In eletrolytic conduction, ions carry electric charge. An increase in temperature increases electrolytic conductance because as the temperature is increased, the ions move faster.

In electrolytic cell, electric energy is used to cause a chemical reaction to take place.

For example, in a eletrolytic cell molten sodium chloride is the electrolyte. Two electrodes anode and cathode are placed into the molten sodium chloride and the an electri current passed through it.

The cation (Na+) moves toward the electrode called the cathode. At this electrode Na+ ions accept electrons to form sodium metal.

Na+ + e- → Na

This is reduction (gain of electrons)

At the other electrode, called the anode, the anion (Cl-) loses electrons to form chlorine gas.

2Clˉ → Cl-2 + 2eˉ

Therefore, oxidation (loss of electrons) takes place at the anode.

In the cell, electrons move from the anode through the external circuit to the cathode.

The reaction at anode and the reaction at cathode separately are called half-reactions.

Various types of half reactions are possible at anode (but all are oxidation only)

1. oxidation of an anion to an element
2Clˉ → Cl-2 + 2eˉ

2. Oxidation of an anion or cation in solution to an ion of higher oxidation state
Fe^2+ → Fe^3+ + e‾
3. Oxidation of a metal anode,
Cu(s) → Cu^2+ + 2e‾
4. Oxidation of H-2O to produce oxygen:
2H-2O → O-2 +4H^+ +4e‾

Similarly at Cathode various types of reactions occur (But all are reduction reactions)

1. Reduction of a cation to a metal

2. Reduction of an anion or cation in solution to an ion of lower oxidation state.

3. Reduction of a nonmetal to an anion.

4. Reduction of water to produce hydrogen.

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