$\begin{array}{1 1}\text{If }E^{\large\circ}(M^{n+}|M)\text{ is negative,}H^+\text{will be reduced to }H_2\text{ by the metal M}\\\text{If }E^{\large\circ}(M^{n+}|M)\text{ is positive},M^{n+}\text{ will be reduced to M by }H_2\\\text{In a cell,}M^{n+}|M\text{ assembly is attached to hydrogen-half cell.To produce spontaneous cell reaction,metal M will act as negative electrode if the potential }M^{n+}|M\text{ is negative.It will serve as positive electrode,if }M^{n+}|M\text{ has a positive cell potential.}\\\text{Compounds of active metals (Zn,Na,Mg) are reducible by }H_2\text{ whereas those of noble metals (Cu,Ag,Au) are not reducible.}\end{array} $
