Electrochemical Cell
A galvanic cell is an electrochemical cell that converts chemical energy of a spontaneous redox reaction into electrical energy. It consists of two half cells, the half cell in which oxidation takes place is called as anode and the other half cell where reduction takes place is called as cathode. Anode has negative potential with respect to the solution whereas cathode has positive potential with respect to the solution. As soon as the switch is in the on position, the electrons flow from the negative electrode to positive electrode and the direction of current flow is opposite to that of electron flow. The potential difference between two electrodes is called cell potential and is measured in volts. When no current is drawn from the cell, then cell potential is termed as cell electromotive force. The cell potential is given by Nernst equation.
Arrange the following metals in the decreasing order of their reducing strength (the reduction potential values in volts, for the corresponding redox couples are given with metals)
A. Li(s) (Li+ + e− → Li(s); E0 = −3.05)
B. Mg(s) (Mg2+ + 2e− → Mg(s); E0 = −2.36)
C. Cr(s) (Cr3+ + 3e− → Cr(s); E0 = −0.74)
D. Au(s) (Au3+ + 3e− → Au(s); E0 = 1.40)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A, B, C, D
The standard reduction potential E° measures how readily a species gains electrons. A metal acts as a reducing agent by losing electrons, the reverse change. So:
The more negative E°(Mn+/M), the more easily the metal is oxidised and the stronger it is as a reducing agent.
The more positive E°, the weaker the metal is as a reducing agent (and the stronger its ion is as an oxidising agent).
Arranging the given values from most negative to most positive:
Li (−3.05 V) < Mg (−2.36 V) < Cr (−0.74 V) < Au (+1.40 V)
Reducing strength: Li > Mg > Cr > Au
Shortcut: Lithium, with the most negative E°, is the strongest reducing agent. Only Option (3) starts with A.
NCERT (2026-27 edition):
Measurement of Electrode Potential – “… Lithium has the lowest electrode potential indicating that lithium ion is the weakest oxidising agent while lithium metal is the most powerful reducing agent in an aqueous solution. It may be seen that as we go from top to bottom in Table 2.1 the standard electrode potential decreases and with this, decreases the oxidising power of the species on the left and increases the reducing power of the species on the right hand side of the reaction.”