Primary cells





Primary cells become dead after some time; as electrode reactions cannot be reversed, i.e., redox reaction takes place only once hence non-rechargeable.

E.g.

Dry cell, mercury cell

 

Dry cell :

It is a compact form of Lechlanche cell. In brief it can be summarised as:

Anode \to Zinc Container

Cathode \to Graphite rod surrounded by MnO_2 and carbon .

Electrolyte \to Apaste of NH_4Cl \hspace{2mm} \text{and} \hspace{2mm} ZnCl_2 filled between two electrodes.

Cell Potential \to1.25 to 1.5 Volt

Relation: At anode \to Zn(s) \to Zn^{+2}(aq.) + 2e^-

At cathode \to 2MnO_2(s) + 2NH_4^+(aq.) + 2e^- \to Mn_2O_3(s) + 2NH_3(g) + H_2O

 

\line(1,0){250} \\ \text{Net Reaction} \to Zn(s) + 2MnO_2(s) + 2NH^+_4(aq.) \to

 

Zn(aq.) + Mn_2 O_3 ( s ) + 2N H_3 + H_2 O \\ \line(1,0){250}

 

Here Zn^{+2} \text{and} NH_3 combine to give [Zn(NH_3)_4]^{2+} complex ion .

[Zn^{+2} + 4Anh_3] \to [Zn(NH_3)_4]^{+2}

 

This cell has a very short life duration and cell may get corroded even when it is not in use as NH_4Cl being acidic corrode Zn-container.

 

Mercury cell:

Here reducing agent is Zn(Hg), i.e., anode while oxidising agent, i.e., cathode is graphite. Electrolyte is HgO + KOH or Zn(OH)_2

Cell potential is 1.35 volt and it does not change during life time since, no ion is involved in the reaction whose concentration may change.

At anode: Zn(Hg) + 2OH^- \to ZnO(s) + H_2O + 2e^-

At cathode: HgO+ H_2O + 2E^- \to Hg(I) + 2OH^-

It is used in hearing aids and watches.



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