Batteries

These are source of electrical energy which may have one or more cells connected in series.

For a good quality battery it should be reasonably light, compact and its voltage should not vary appreciably during its use.

Primary batteries

In primary batteries, the reaction occurs only once and after use over a period of time battery becomes dead and cannot be reused.

(i) Dry cell or Leclanche cell

The cell consists of a zinc container that also acts as anode and the cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon.

The space between the electrodes is filled by a moist paste of ammonium chloride (NH₄Cl) and zinc chloride (ZnCl₂).

The electrode reactions can be written approximately as,

Anode: Zn(s) → Zn²⁺ + 2e ^–

Cathode: MnO₂ + NH₄⁺ + e^– → MnO(OH) + NH₃

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 Zn²⁺ to give [Zn(NH₃)₄]²⁺.

The cell has a potential of nearly 1.5V.

(ii) Mercury cell

Mercury cell, suitable for low current devices like hearing aids, watches, etc. consists of zinc – mercury amalgam as anode and a paste of HgO and carbon as the cathode.

The electrolyte is a paste of KOH and ZnO.

The electrode reactions for the cell are,

Anode: Zn(Hg) + 2OH^–→ ZnO(s) + H₂O + 2e^–

Cathode: HgO + H₂O + 2e^– → Hg(l) + 2OH^–

The overall reaction:

Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)

Cell potential for mercury cell is 1.35 V

Secondary batteries

These cells can be recharged and can be used again and again

(i) Lead storage battery

Used in automobiles and invertors.

Anode-Spongy lead

Cathode-Grid of lead packed with PbO₂

Electrolyte - 38% H₂SO₄ by mass

The cell reactions when the battery is in use or discharge reaction

Anode: Pb(s) + SO₄^2–(aq) → PbSO₄(s) + 2e^–

Cathode: PbO₂(s) + SO₄^2–(aq) + 4H⁺(aq) + 2e^– → PbSO₄ (s) + 2H₂O (l)

Overall cell reaction consisting of cathode and anode reactions is:

Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l)

When recharged the cell reactions are reversed.

(ii) Nickel cadmium storage cell

Has longer life than the lead storage cell but more expensive to manufacture.

Anode-Cadmium

Cathode-Metal grid containing NiO₂

Electrolyte - KOH solution

Reactions during discharge

Anode: Cd(s) + 2OH^-(aq) → Cd(OH)₂(s) + 2e^-

Cathode: NiO₂(s) + 2H₂O(l) + 2e^- → Ni(OH)₂(s) + 2OH^-(aq)

Overall reaction,

Cd (s) + 2Ni(OH)₃ (s) → CdO (s) + 2Ni(OH)₂ (s) + H₂O (l )

Cell potential for Nickel cadmium storage cell is 1.4 V