In which of the following half cells, electrochemical reaction is pH dependent?

Solution

Chemistry ma’am · 2 steps

Write each half-reaction fully: only the one that needs H⁺ to balance depends on pH.

1. The test. In the Nernst equation, every species in the half-reaction appears in the reaction quotient. A half cell is pH dependent only if H⁺ (or OH⁻) is part of its balanced reaction.

   Half cell	Reduction half-reaction	H⁺ involved?
   (a) Fe³⁺, Fe²⁺	Fe³⁺ + e⁻ → Fe²⁺	No
   (b) MnO₄⁻, Mn²⁺	MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O	Yes, 8 of them
   (c) Ag, AgCl, Cl⁻	AgCl + e⁻ → Ag + Cl⁻	No
   (d) F₂, F⁻	1/2 F₂ + e⁻ → F⁻	No

   E = E° - 0.0591/5 log[Mn²⁺]/[MnO₄⁻][H⁺]⁸

2. So. Only the permanganate electrode has [H⁺]⁸ in its Nernst expression, so its potential changes with pH. The answer is (b).

The idea to keep. Balance the half-reaction before you judge it. An oxoanion that loses oxygen as water, such as MnO₄⁻ or Cr₂O₇²⁻, needs H⁺, and that makes its potential depend on pH.

Answer from the NTA final answer key. Solution by PrepWiser.