Question

Mercury cells suitable for low current devices like hearing aids, watches, etc. consists of which substance as anode?
1. Zinc-mercury amalgam
2. Zinc coating
3. Ammonia
4. Carbon

Answer 1

Correct Answer - Option 1 : Zinc-mercury amalgam

The correct answer is 1 i.e. Zinc-mercury amalgam.

Primary battery:

• 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 a galvanic cell where the chemical energy of the redox reaction is converted into electrical energy. 
• In the primary batteries, the reaction occurs only once, and after use over some time battery becomes dead and cannot be reused again.
• The most familiar example of this type is the dry cell (known as Leclanche cell after its discoverer) which is used commonly in our transistors and clocks. 
• 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 are complex, but they can be written approximately as follows:
  • 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 1.5 V.
• Mercury cells 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 given below:
  • Anode: Zn(Hg) + 2OH- → ZnO(s) + H2O + 2e-
    Cathode: HgO + H2O + 2e- → Hg(l ) + 2OH-
• The overall reaction is represented by 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 a solution whose concentration can change during its lifetime.