What is the reduction potential of a hydrogen electrode that is still at standard pressure, but has pH = 5.25 , relative to the SHE?

Question

The reduction potential for all other half-reactions are measured relative to the half-reaction

2H⁺ (aq) + 2e^- H₂​(g)

Under standard conditions (pH=0.000, P_H2 = 1 atm) this standard hydrogen electrode (SHE) is defined to have a reduction potential of 0.000 V.

What is the reduction potential of a hydrogen electrode that is still at standard pressure, but has pH = 5.25 , relative to the SHE?

Answer 1

You need to use the Nernst equation for this problem since you are no longer at standard conditions.

At standard conditions the Eo reduction of the SHE is 0.000 V at 25 C (298K)

The Nernst equation is E = Eo - RT/zF ln Q where R = 8.314 J/mol-K T = 298 K z =number of electrons transferred in the reaction = 2 and F is the Faraday constant 96485 C/mol

Q is the reaction quotient for the non standard conditions Q = (P H2)/[H+]^2

Since the pH = 5.25 H+ = 10^-5.25 = 5.62x10^-6 M

So E = 0.000 V - 8.314 J/mol-K * 298 K/ (2 * 96485 C/mol) x ln ( 1 / (5.62x10^-6)^2)

E = 0.000 V - 0.310 V = -0.310 V