Q1(j). Same electrode positive or negative

Answer in brief

Justified: polarity is relative; an electrode is positive against a couple of lower reduction potential and negative against one of higher reduction potential.

Key points

• Only a potential difference is measurable; in a galvanic cell the couple with the higher reduction potential is the cathode (+), the other the anode (−).
• Zn | Zn²⁺ || Cu²⁺ | Cu: Ecell = 0.34 − (−0.76) = 1.10 V; copper is the positive electrode.
• Cu | Cu²⁺ || Ag⁺ | Ag: Ecell = 0.80 − 0.34 = 0.46 V; the same copper electrode is now negative.
• Concentration can also reverse polarity when E° values are close: for Sn²⁺/Sn and Pb²⁺/Pb, Ecell = 0 at [Pb²⁺]/[Sn²⁺] = 0.459.
• Galvanic cell: cathode is +; electrolytic cell: cathode is −; so the sign is not fixed by the electrode itself.