Q.Two metals $M_1$ and $M_2$ have reduction potential values of $-x$ V and $+y$ V respectively. Which will liberate $\text{H}_2$ from $\text{H}_2\text{SO}_4$?

The electromotive force (emf) of a cell is the force driving electrons from the anode, where a spontaneous reaction pushes them out, to the cathode, where the reaction pulls them in; it is measured in volts, with one volt releasing one joule of energy per coulomb of charge moved. Emf genuinely depends on the electrode materials, the electrolyte concentrations, and the temperature — only under standard conditions (1 M, 1 atm, 25°C) does it equal the fixed, tabulated value $E^{o}_{cell}$.

A single, isolated electrode's potential can never be measured directly — a voltmeter only ever reads a potential DIFFERENCE between two electrodes. The universal fix is the Standard Hydrogen Electrode (SHE): platinum in 1 M HCl with H2 gas bubbled at 1 atm, assigned $E^{o} = 0$ V by international convention. Every other electrode's standard potential is obtained by building a cell against the SHE and measuring the resulting emf — for instance, pairing zinc against the SHE gives a measured cell emf of 0.76 V, which (since the SHE contributes exactly zero) is entirely zinc's own oxidation potential; reversing the half-reaction and its sign gives the standard REDUCTION potential most commonly tabulat …