Question 1

What is the relationship between Ecell, ΔG and spontaneity?

Accepted Answer

The relationship is ΔG = −nFEcell. For the represented reaction, Ecell > 0 gives ΔG < 0 and indicates spontaneity, while Ecell < 0 gives ΔG > 0 and means the reaction is non-spontaneous as written. At equilibrium, Ecell = 0, ΔG = 0 and Q = K.

Question 2

Why does the Nernst equation use Q for the Zn-Cu cell?

Accepted Answer

For Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), pure solids have unit activity, so the reaction quotient is Q ≈ [Zn²⁺]/[Cu²⁺]. At 298 K, E = E° − (0.0591/2) log Q, which means increasing the product concentration lowers the cell potential.

Question 3

Which side is the cathode in a Zn concentration cell?

Accepted Answer

The side with the higher Zn²⁺ concentration is the cathode. The lower-concentration side is the anode, where Zn oxidizes and loses mass; Zn²⁺ is reduced to Zn(s), which deposits on the electrode at the high-concentration cathode. The cell has E°cell = 0 and K = 1.

Question 4

Why is an inert platinum electrode used in the Fe³⁺/Fe²⁺ half-cell?

Accepted Answer

Both Fe³⁺ and Fe²⁺ are ions in solution, so there is no conducting Fe metal surface to make the electrode. Platinum provides an inert conducting surface where Fe³⁺ + e⁻ → Fe²⁺ can occur without being consumed.

Question 5

How does pH affect the hydrogen electrode?

Accepted Answer

At 298 K and hydrogen pressure of 1 atm, the hydrogen electrode reduction reaction is 2H⁺ + 2e⁻ → H₂(g). Its potential follows E = −0.0591 pH, so the potential changes from 0 V at pH 0 to approximately −0.414 V at pH 7.

Cell Potential & Thermodynamics — E°, K, ΔG and the Nernst Dial

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