If E∘(Mn+|M) is negative,H+will be reduced to H2 by the metal MIf E∘(Mn+|M) is positive,Mn+ will be reduced to M by H2In a cell,Mn+|M assembly is attached to hydrogen-half cell.To produce spontaneous cell reaction,metal M will act as negative electrode if the potential Mn+|M is negative.It will serve as positive electrode,if Mn+|M has a positive cell potential.Compounds of active metals (Zn,Na,Mg) are reducible by H2 whereas those of noble metals (Cu,Ag,Au) are not reducible.