Question

If SHE (Standard Hydrogen Electrode) acts as anode and given metal acts as cathode, what is the sign of reduction potential of the metal $?$

Answer 1

The standard reduction potential of a standard hydrogen electrode is always zero. Here, SHE acts as anode while the given metal will behave like a cathode then we have to find the sign of the reduction potential of the metal. SHE is also used as a reference electrode for finding reduction potential.
${E^ \circ }_{cell}{\text{ = }}{{\text{E}}_{cathode}}{\text{ - }}{{\text{E}}_{anode}}$

Complete answer:
Reduction potential of a metal is defined as the tendency of the metal atom to acquire the electrons. The more positive value of reduction potential of a metal means that it has a high tendency to acquire electrons. Reduction potential is also known as the redox potential. Also we have a formula for calculating standard reduction potential for the electrodes.
${E^ \circ }_{cell}{\text{ = }}{{\text{E}}_{cathode}}{\text{ - }}{{\text{E}}_{anode}}$
All the electrodes have a reduction potential. According to the given question Standard Hydrogen Electrode will act as anode and metal will act as cathode. But we know that Standard potential for SHE is always Zero. So the above equation can be modified as:
${E^ \circ }_{cell}{\text{ = }}{{\text{E}}_{cathode}}{\text{ - 0}}$
${E^ \circ }_{cell}{\text{ = }}{{\text{E}}_{cathode}}{\text{ }}$
Therefore we can see that the reduction potential of the metal comes equal to the reduction potential of a cell. Thus we can say that it is a positive number. Also when metal acts as a cathode and reduction takes place at cathode only, so it is correct. Thus the magnitude of the reduction potential of the metal comes to be positive.

Note:
The potential which is used in the above equation is the reduction potential. If we don’t have reduction potential, we will first convert oxidation potential into reduction potential by applying the negative potential. Also the above equation changes for oxidation potential.