Electrochemistry- Electrolysis, Batteries, Fuel Cells

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Electrolysis Cells– Electrolytic cells are those cells in which electrical energy is used to carry out non spontaneous chemical reaction and the reaction taking place in an electrolytic cell is called electrolysis. During this process oxidation takes place at anode and reduction occurs at cathode. Thus positive ions get deposited at cathode and negative ions at anode.

First law electrolysis – According to first law of Faraday, the amount of material discharged at the electrode is proportional to the quantity of electricity passes through the solution.

$W \alpha Q$ or $W=ZQ= Zit$

Where, z = electrochemical constant

Second law of Faraday – According to second law of Faraday, when current is passed through different electrolyte different substance deposited by the same quantity of electricity are proportional to their chemical equivalent weight.

i.e. $\frac { { w }_{ 1 } }{ { E }_{ 1 } } =\frac { { { w }_{ 2 } } }{ { E }_{ 2 } } =\frac { { w }_{ 3 } }{ { E }_{ 3 } } ....$

Where, ${w}_{1}$  is the mass of substance one deposited and even is its Equivalent weight and so on

Product of electrolysis depend upon the nature of electrolyte being electrolysed and the nature of the electrodes. If electrodes are inert  like Pt or Au, they do not take part in chemical reaction. Electrolysis also depend upon the electrode potential of oxidising and reducing species.

Batteries

On the basis of reversible, cell are of following two types

1. Primary cell– In primary cell, electrode reaction occurs only once & after sometimes cell becomes dead, i.e. cell cannot be recharged. E.g. dry cell, mercury cell, etc.

2. Secondary cell– In these cells, electrode reaction can be reversed by an external source of energy, i.e. in these cells, by passing electric current they can be recharged again. In these cells current can be passed thus they are known as storage cell or accumulative cell. E.g. lead storage cell, nickel-cadmium cell, etc.

Lead storage battery- Lead storage battery consists of a lead anode and a grid of lead packed with lead dioxide as cathode. A 38% solution of sulphuric acid acts as the electrolyte.

Cathode  $PbO_{ 2 }$ (soluble lead)

Electrolyte- $37%\quad { H }_{ 2 }{ SO }_{ 4 }$

The overall reaction is $Pb(s)+PbO_{ 2 }(s)+{ 2SO }_{ 4 }^{ 2- }(aq)+4H^{ + }(aq)\longrightarrow 2PbSO_{ 4 }(s)+2H_{ 2 }O(I)$

Fuel cells– Fuel cells are galvanic cells that are designed to convert the energy of combustion of fuels like hydrogen, methane, methanol directly into electrical energy, porous carbon electrodes into concentrated KOH solution.

The electrode reactions of ${H}_{2}-{O}_{2}$ fuel cell are

At Anode ${ 2H }_{ 2 }(g)+{ 4OH }^{ - }(aq)\longrightarrow { 4H }_{ 2 }O(l)+{ 4e }^{ - }$

At Cathode ${ O }_{ 2 }+{ 2H }_{ 2 }O(l)+{ 4 }e^{ - }\longrightarrow { 4OH }^{ - }(aq)$

The overall reaction is ${ O }_{ 2 }(g)+{ O }_{ 2 }(g)\longrightarrow { 2H }_{ 2 }O(l)$

Corrosion– Wear and tear of metal surface by air and chemical slowly with time is known as corrosion. E.g. copper utensil turns green, rusting of iron, silver gets black in colour.