Subject: Chemistry (Electrochemistry)

Question Analysis: Identification of commercial electrochemical cells based on their specific applications.

Step 1 (Identification of Cells): [0.5 Mark per sub-part]

• (a) Apollo Space Programme: The cell used is the Hydrogen-Oxygen Fuel Cell. It provides both electricity and potable water for astronauts.
• (b) Automobiles and Inverters: The cell used is the Lead Storage Battery. It is a secondary cell capable of being recharged.
• (c) Hearing Aids and Watches: The cell used is the Mercury Cell. It provides a constant potential throughout its life.
• (d) Transistors (Non-steady potential): The cell used is the Leclanché Cell (Dry Cell). Its potential drops as the reaction proceeds.

Step 2 (Chemical Basis / Governing Principles): [0.5 Mark per sub-part]

The selection of these cells is based on their specific electrochemical characteristics:

• (a) Fuel Cell: Operates on the combustion reaction: $$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$$
• (b) Lead Storage: Involves the reversible reaction: $$Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightleftharpoons 2PbSO_4(s) + 2H_2O(l)$$
• (c) Mercury Cell: The overall reaction involves no ions in the solution whose concentration changes, hence $E_{cell}$ remains constant: $$Zn(Hg) + HgO(s) \rightarrow ZnO(s) + Hg(l)$$
• (d) Dry Cell: The potential is not steady because the concentration of $NH_4^+$ ions changes during the discharge process.

Step 3 (Final Concluded Answer): [Total 2 Marks]

💡 EL10 Concept Insight: Think of these cells like different types of engines: some are built for long-distance space travel (Fuel Cells), some are heavy-duty for cars (Lead Storage), some are tiny and super steady for your watch (Mercury), and some are simple, cheap, and "get the job done" for basic electronics (Dry Cells).