Chemistry Batteries and It's Types
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### Topics Covered :

● Battery
● Primary Battery
● Secondary Battery

### Batteries :

color{green}text(Definition) : Any battery (actually it may have one or more than one cell connected in series) or cell that we use as a source of electrical energy is basically a galvanic cell where the chemical energy of the redox reaction is converted into electrical energy.

color{green}text(Characteristics of a Battery) : For a battery to be of practical use it should be reasonably light, compact and its voltage should not vary appreciably during its use.

There are mainly two types of batteries :

### Primary Batteries :

color{green}text(Definition) : In the primary batteries, the reaction occurs only once and after use over a period of time, battery becomes dead and cannot be reused again.

color{green}text(Examples) : (i) Dry cell (known as Leclanche cell after its discoverer) which is used commonly in our transistors and clocks.

color{green}text(Composition and Working) :

=> color{green}("Anode") : a zinc container.

=> color{green}("Cathode") : A carbon (graphite) rod surrounded by powdered manganese dioxide and carbon (Fig.3.8).

=> The space between the electrodes is filled by a moist paste of ammonium chloride (NH_4Cl) and zinc chloride (ZnCl_2).

=> The electrode reactions are complex, but they can be written approximately as follows :

color{green}("Anode") : color{red}(Zn (s) → Zn^(2+) + 2 e^(-))

color{green}("Cathode") : color{red}(MnO_2+NH_4^(+) + e^(-) → Mn O ( OH) + NH_3)

=> In the reaction at cathode, manganese is reduced from the + 4 oxidation state to the +3 state.

=> Ammonia produced in the reaction forms a complex with Zn^(2+) to give [Zn (NH_3)_4]^(2+).

=> The cell has a potential of nearly 1.5 V.

(ii) Mercury cell, (Fig. 3.9) suitable for low current devices like hearing aids, watches, etc.

color{green}text(Composition and Working) :

=> color{green}("Anode") : Zinc–mercury amalgam.

=> color{green}("Cathode") : A paste of HgO and carbon.

=> color{green}("Electrolyte") : Paste of KOH and ZnO.

=> The electrode reactions for the cell are given below :

color{green}("Anode") : color{red}(Zn (Hg) +2 OH^(-) → ZnO(s) +H_2O + 2 e^(-))

color{green}("Cathode") : color{red}(HgO +H_2O +2 e^(-) → Hg (l) +2OH^(-))

The overall reaction is represented by :

color{red}(Zn (Hg) +HgO (s) → ZnO(s) +Hg(l))

=> The cell potential is approximately 1.35 V and remains constant during its life as the overall reaction does not involve any ion in solution whose concentration can change during its life time.

### Secondary Batteries :

color{green}text(Definition) : A secondary cell after use can be recharged by passing current through it in the opposite direction so that it can be used again.

=> A good secondary cell can undergo a large number of discharging and charging cycles.

color{green}text(Examples) : (i) The lead storage battery is commonly used in automobiles and invertors.

color{green}text(Composition and Working) :

=> color{green}("Anode") : Lead.

=> color{green}("Cathode") : A grid of lead packed with lead dioxide (PbO_2 ).

=> color{green}("Electrolyte") : A 38% solution of sulphuric acid.

=> The cell reactions when the battery is in use are given below :

color{green}("Anode "): color{red}(Pb (s) + SO_4^(2-) (aq) → PbSO_4(s) + 2e^(-))

color{green}("Cathode") : color{red}(PbO_2 (s) + SO_4^(2-) (aq) + 4 H^(+) (aq) + 2e^(-) → PbSO_4 (s) + 2H_2O(l))

Therefore, overall cell reaction consisting of cathode and anode reactions is :

color{red}(Pb(s) +PbO_2 (s) +2H_2SO_4 (aq) → 2PbSO_4 (s) +2H_2O (l)).

=> On charging the battery the reaction is reversed and PbSO_4(s) on anode and cathode is converted into Pb and PbO_2 respectively.

=> It has longer life than the lead storage cell but more expensive to manufacture.
=> We shall not go into details of working of the cell and the electrode reactions during charging and discharging.
=> The overall reaction during discharge is :
color{red}(Cd (s) + 2Ni (OH)_3 (s) → CdO (s) + 2Ni (OH)_2 (s) + H_2O(l))