Chemistry DRY CELL AND LEAD ACCUMULATOR

Batteries :

Any cell or Battery (more than one cells connected in series) that we use daily as a source of electricity in basically a galvanic cell in which chemical energy of spontaneous redox reactions is converted into electrical energy.

An ideal battery should have following desirable characters..

`*` It should be reasonably light

`*` It should be compact

`*` Its voltage should not vary much during its life i.e., period of use.

There are mainly two types of batteries which we use today

`*` Primary Batteries

`*` Secondary batteries

`text(Primary Batteries or Voltaic Cells (Dray Cells))`

`*` In this type of cell, once the chemicals have been consumed, further reaction is not possible.

`*` After use over a period of time battery becomes dead and cannot be reused again

`*` It cannot be regenerated by reversing the current flow through the cell using an external direct current source of electrical energy.

`*` The most common example of this type is dry cell.

Dry Cell :

See fig.1.

`*` The container of the dry cell is made of zinc which also serves as one of the electrodes.

`*` The other electrode is a carbon rod in the centre of the cell.

`* `The zinc container is lined with a porous paper.

`*` A moist mixture of ammonium chloride, manganese dioxide, zinc chloride and a porous inert filler occupy the space between the paper lined zinc container and the carbon rod.

`*` The cell is sealed with a material like wax.

`*` As the cell operates, the zinc is oxidised to Zn2+

`*` Anode reaction: `Zn → Zn^(2+) + 2e^(-)`

`*` The electrons are utilized at carbon rod (cathode) as the ammonium ions are reduced.

`*` Cathode Reaction: `2NH^(4+)+2e →2NH_3 + H_2`

`*` The cell reaction is: `Zn^(+) 2 NH_4^(+) → Zn^(2+) + 2NH_3 + H_2`

`*` Hydrogen is oxidized by `MnO_2` in the cell: `2MnO_2 + H_2 →2MnO(OH)`

`*` Ammonia produced at cathode combines with zinc ions to form complex ion. `Zn^(2+) + 4NH_3 →[Zn(NH_3)_4]^(2+)`

`*` `E_(text(cell))` is 1.6 volt

See Table 1.

See fig.2.

`text(Alkaline dry cell)`

`*` Alkaline dry cell is similar to ordinary dry cell. It contains potassium hydroxide.

`*` `E_(text(cell))` is 1.5 volt.

`*` The reaction in alkaline dry cell are :

See Table 2.

Secondary Batteries or Voltaic Cells (Lead Storage Battery) :

See fig.1.

`*` The cell in which original reactants are regenerated by passing direct current from external source, i.e., it is re-charged, is called secondary cell.

`*` Lead storage battery is the example of this type.

`text(Lead Storage Battery)`

`*` Lead storage battery consists of a group of lead plates bearing compressed spongy lead, alternating with a group of lead plates bearing leaf dioxide, PbO2.

`*` These plates are immersed in a solution of about 30% `H_2SO_4` .

`*` When the cell discharge; it operates as a voltaic cell.

`*` The spongy lead is oxidized to Pb2+ ions and lead plates acquire a negative charge.

Anode Reaction: `Pb → Pb^(2+) + 2e^(-)`

`*` `Pb^(2+)` ions combine with sulphate ions to form insoluble lead sulphate, `PbSO_4,` which begins to coat lead electrode.

`*` The electrons are utilized at `PbO_2` electrode.

`*` Cathode Reaction: `PbO_2 + 4H^(+) + 2e^(-)→ Pb^(2+) +2H_2O`

`*` Precipitation: `Pb^(2+) + SO_4^(2-) → PbSO_4`

`*` Overall cell reaction is: `Pb + PbO_2 + 4H^(+) + 2 SO_4^(2-)→ 2PbSO_4 + 2H_2O`

`*` `E_(text(cell))` is `2.041` volt.

`*` When a potential slightly greater than the potential of battery is applied, the battery can be re-charged. `2PbSO+4 + 2H_2O → Pb + PbO_2 + 2H_2SO_4`

`*` After many repeated charge-discharge cycles, some of the lead sulphate falls to the bottom of the container, the sulphuric acid concentration remains low and the battery cannot be recharged fully

See Table.

See fig.2