Q.The button cell used in watches functions as follows: $\text{Zn(s)} + \text{Ag}_2\text{O(s)} + \text{H}_2\text{O(l)} \rightleftharpoons 2\text{Ag(s)} + \text{Zn}^{2+}\text{(aq)} + 2\text{OH}^{-}\text{(aq)}$ The half-cell potentials are $\text{Ag}_2\text{O(s)} + \text{H}_2\text{O(l)} + 2e^{-} \rightarrow 2\text{Ag(s)} + 2\text{OH}^{-}\text{(aq)}$, $E^{o} = 0.34$ V, and $\text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2e^{-}$, $E^{o} = 0.76$ V. The cell potential will be

A secondary cell can be recharged; the lead storage battery (lead-acid) is the classic rechargeable (secondary) cell.

Primary cells (e.g. Leclanche cell, dry cell) cannot be recharged. Secondary cells can be recharged by passing current in the reverse direction; examples are the lead storage battery and the nickel-cadmium cell. A concentration cell is not a storage/rechargeable cell in this sense.

The mercury (button) cell is a primary cell used in low-current devices like watches and hearing aids; it uses a zinc-mercury amalgam anode and a paste of HgO + carbon as cathode.

Anode: Zn(Hg) + 2OH− -> ZnO(s) + H2O + 2e−

Cathode: HgO(s) + H2O + 2e− -> Hg(l) + 2OH−

A notable feature of this cell is that its cell potential remains constant during its life, because the overall reaction does not involve any ion in solution whose concentration changes with time.

The cell commonly used in wrist watches (and hearing aids) is the mercury cell.

The mercury cell consists of a zinc anode (amalgamated with mercury) and a cathode of a paste of $HgO$ and carbon, in a moist paste of $KOH$ and $ZnO$ as electrolyte. Its key advantage for miniature devices like wrist watches is that its cell potential remains constant throughout its life (about 1.35 V), since the overall reaction does not involve any ion in solution whose concentration changes.

Anode: $Zn(Hg) + 2OH^- \rightarrow ZnO(s) + H_2O + 2e^-$

Cathode: $HgO + H_2O + 2e^- \rightarrow Hg(l) + 2OH^-$

A primary cell undergoes a one-way (irreversible) electrode reaction and cannot be recharged; the mercury cell is a classic example.

Checking each option:

• "Electrode reactions can be reversed" - false; this describes a SECONDARY (rechargeable) cell, not a primary cell.

• "It can be recharged" - false; primary cells are used once and discarded (or their reaction is not reversible on recharging).

• "An example of a primary cell is a mercury cell" - TRUE; the mercury cell (Zn-HgO cell) is a standard primary cell used in hearing aids, watches, etc., and is not rechargeable.

• "An example of a primary cell is nickel-cadmium storage cell" - false; the Ni-Cd cell is a rechargeable SECONDARY (storage) cell.

Primary cells are non-rechargeable, one-use galvanic cells; among the four listed, the Leclanche cell and the Mercury cell are primary, while Nickel-Cadmium and Lead storage batteries are secondary (rechargeable) cells.

A primary cell can supply current only until its reactants are used up, after which it must be discarded — the electrode reactions are not efficiently reversible. A secondary (storage) cell, by contrast, can be recharged by passing an external current through it in the reverse direction, regenerating the original reactants. (i) Leclanche cell (the common dry cell, Zn-$MnO_2$/$NH_4Cl$ paste) — primary. (ii) Nickel-Cadmium cell — a rechargeable battery — secondary. (iii) Lead storage battery (used in vehicles, Pb/$PbO_2$/$H_2SO_4$) — rechargeable — secondary. (iv) Mercury cell (Zn-HgO, used in hearing aids/watches, gives a very steady voltage) — primary, cannot be recharged. So the two primary cells among the list are (i) and (iv).

In the nickel–cadmium storage cell, cadmium metal is the anode and is oxidised during discharge.

The cadmium–nickel (Ni–Cd) cell is a rechargeable (secondary) cell with cadmium metal as the anode (negative electrode), nickel oxide-hydroxide NiO(OH) as the cathode, and aqueous KOH as the electrolyte.

During discharge, oxidation occurs at the anode — cadmium loses electrons and reacts with hydroxide ions from the electrolyte:

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

(For reference, the cathode reaction is: 2NiO(OH)(s) + 2H2O(l) + 2e⁻ → 2Ni(OH)2(s) + 2OH⁻(aq).)

Primary cells cannot be recharged by passing current in the reverse direction — once used up they are discarded — option (b).

In a primary cell, the electrochemical reaction occurs only once; when the reactants are consumed, the cell stops producing current and cannot be reused by recharging (the reaction is essentially irreversible in practice). Examples include the dry (Leclanché) cell and the mercury cell. This is unlike secondary cells (e.g. the lead storage battery, Ni-Cd cell) which can be recharged by passing current in the opposite direction, reversing the cell reaction.