Chemistry Revision Notes of Oxidation, Reduction and Electrochemistry for NDA Part-5
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Battery

The arrangement of one or more cells in a series connection is called battery. It is basically a galvanic cell.

There are two types of battery :

(i) Primary battery : It is non-rechargeable and can be used only once. e.g. dry cell (Lechlanche cell), mercury cell.

(ii) Secondary battery : It rechargeable and can be reused again and again. This battery can act both as galvanic and electrolytic cell e.g. lead storage battery, nickel-cadmium battery.

Fuel Cell

It is the type of galvanic cell in which energy released by the combustion of fuels (hydrogen, methane, methanol) is directly converted into electrical energy is called fuel cell. e.g. fuel cell using `H_2` and `O_2` to form water in order to produce electricity. Efficiency of these cells is 70%. This cell was used first-time in Apollo Space Programme.

Electrolysis

`=>` The process of decomposition of an electrolyte on passing an electric current through its aqueous solution or in the fused state is called electrolysis.

`=>` Cations move towards cathode, gain electrons and become neutral atoms.

`=>` Similarly anions move towards anode, lose electrons and become neutral atoms.

`=>` In electrorefining of metals, impure metals act as anode and a strip of pure metal acts as cathode.

Faraday's Laws of Electrolysis

Faraday gave following two laws to explain the relation between amount of substance produced with current or its equivalent weight.

First Law of Electrolysis : It states that "the quantity (mass) of any substance deposited or liberated at any electrode is directly proportional to the quantity of electricity passed through electrolyte."

`omega prop Q => omega = ZQ = Zit`

`because ` Charge (Q) = current (i) x time (t)

Z = Electrochemical equivalence = `E/(96500)`
(where, E = Equivalent weight)

Second Law of Electrolysis : It states, "when same quantity of electricity is passed through solutions of different electrolytes connected in series, the weight of substance produced at the electrodes is directly proportional to their equivalent weigh:".

i.e. `(text(weight of Cu deposited) ( omega_1))/(text(Weight of Ag deposited) (omega_2))`

`= (text(Equivalent weight of ) Cu (E_1))/(text(Equivalent weight of ) Ag (E_2))`

Electrochemical Series

`=>` The arrangement of elements in order of their increasing standard reduction potential values is known as electrochemical series or activity series.

`K < Ca < Na < Mg < Al < Zn < Cr < Fe < Ni < Sn < H_2 < Cu < Hg < Ag < Pd < Pt < Au` [as reactivity follows the reverse order].

`=>` Gold is least reactive while potassium is most reactive element.
`=>` `text(Reducing power) prop 1/text(reduction potential)`

`=>` Only those metals can liberate hydrogen from the acid which have negative values of reduction potentials or which are placed before `H`-atom
`=>` The more reactive metals of the activity series displaces the less reactive metals from their salt solutions.

`Cu +FeSO_4 → ` No reaction

`Fe + undersettext(Blue coloured solution)(CuSO_4) → undersettext(Colourless ) (FeSO_4) + Cu`

Corrosion

`=>` It is a process of oxidative deterioration of a metal as a result of its reaction with air and moisture present in the environment.

`=>` Corrosion of iron is called rusting. Rust is hydrated iron (III) oxide.

`=>` Similarly, in presence of moist air, copper acquires a green coating of basic copper carbonate, `[ Cu(OH)_2 * Cu CO_3]`, silver acquires a black coating of silver sulphide `[Ag_2S]` and aluminium acquires a dull coating of aluminium oxide (a protective layer).

`=>` A sliced apple turns brown if left open for sometime due to the oxidation of iron present in it as a nutrient.

`=>` Anodisation (an electrolytic process) is the process of forming a thick oxide layer on the surface of metal. This oxide layer is protective and prevents the metal from further oxidation.

`=>` Formation of a layer of `Al_2O_3` on aluminium surface protects the metal from further corrosion.

`=>` The rusting of iron can be prevented by painting, oiling, greasing, galvanising (coating of iron with zinc) and chrome plating.

 
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