Electrochemistry

Introduction

Electrochemistry deals with the inter-conversion of electrical energy and chemical energy. A flow of electricity through a substance may produce a chemical change (redox reaction) and also a chemical change (redox reaction) may cause a flow of electricity through some external circuit. The former involves the study of electrolysis and conductance while the latter, the measurement of electromotive force.

Electrolysis

The phenomenon of electrolysis involves the breaking of electrolytes when electric current is passed through it. The apparatus used to carryout electrolysis is known as electrolytic cell.

The electrolysis of molten salts produces substances which are characteristic of the salt. When aqueous salt solutions are electrolysed, water may be also be involved in the electrode reaction rather than the ion derived from the solute only

The reaction involved during electrolysis of water are follows:

2H2O –> O2 + 4H+ + 4e (oxidation)

2H2O + 2e –> H2 + 2OH (reduction)

The following figure shows the layout of an electrolytic cell used for commercial production of Mg metal from molten MgCl2.

As in a galvanic cell, oxidation occurs at the anode and reduction occurs at the cathode, electrons travel through the external wire from anode to cathode, cations move through the electrolyte toward the cathode, and anions move toward the anode. Unlike the process in a galvanic cell, source drives electrons through the wire in a pre-determined direction, forcing oxidation to occur at one electrode and reduction at the other:

Anode reaction : 2Cl(l) –> Cl2(g) + 2e

Cathode reaction: Mg2+ (l) + 2e –> Mg(l)

A rechargeable battery functions as galvanic cell when it is doing work and as an electrolytic cell when it is being recharged

Preferential Discharge Theory: If more than one type of ion is attracted towards a particular electrode, then the ion discharged is the one which requires the least energy.

The decreasing order of the discharge potential or the increasing order of deposition for some of the cations and anions is as follows.

Cations: K+, Na+, Ca2+, Mg2+, Al3+, Zn2+, H+, Cu2+, Ag+, Au3+

Anions: SO42−, NO3, OH, Cl, Br, I

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