The reaction Cl-(aq) + H2(g) is not a spontaneous redox reaction in a voltaic cell. In order for a redox reaction to be spontaneous in a voltaic cell, the reduction potential of the oxidizing agent must be greater than the reduction potential of the reducing agent. The standard reduction potential of Cl- is -1.36 V, while the reduction potential of H2 is 0 V. Since the reduction potential of Cl- is lower than that of H2, the reaction is not spontaneous in a voltaic cell.
