Chemistry · Electrochemistry · NEET
When you dip a metal rod (say zinc) into a solution of its own ions (zinc sulphate), two things happen at the same time. Some metal atoms lose electrons and go into the solution as ions (Zn to Zn2+ + 2e-). Some ions in the solution can also gain electrons and stick back on the rod as metal. This tug-of-war leaves a small charge on the metal and a small opposite charge in the solution near it. That tiny voltage across the metal-solution boundary is the electrode potential. In short, it is the tendency of an electrode to lose or gain electrons.
It arises because a metal has a natural tendency to release electrons and go into solution as ions, and its ions have a tendency to take electrons back. These two opposite tendencies do not fully cancel. So a thin layer of charge builds up at the surface where metal meets solution (called the electrical double layer). This charge separation is exactly what we call a potential. Different metals have different pulls for electrons, so each electrode has its own value.
Reduction potential measures the tendency to GAIN electrons (get reduced). Oxidation potential measures the tendency to LOSE electrons (get oxidised). They are equal in size but opposite in sign for the same electrode. IMPORTANT for NEET: by modern IUPAC and NCERT convention, 'electrode potential' and 'standard electrode potential' always mean REDUCTION potential. So if a question just says electrode potential, treat it as reduction potential. A more positive value means the species is reduced more easily.
A negative standard electrode potential (like Zn2+/Zn = -0.76 V) means the metal loses electrons easily and does NOT like to stay reduced. It is a good reducing agent. A positive value (like Cu2+/Cu = +0.34 V) means the ion gains electrons easily and gets reduced readily, so the metal is a poor reducing agent. NCERT reference point: hydrogen (H+/H2) is fixed at 0 V. Negative means a stronger reducing agent than hydrogen; positive means weaker than hydrogen.
No. Electrode potential belongs to ONE half-cell (one electrode in its solution). Cell potential (or EMF) is the DIFFERENCE between the electrode potentials of the two electrodes in a full cell: E_cell = E_cathode - E_anode (both as reduction potentials). You cannot measure a single electrode potential on its own. You always measure it against a reference, the standard hydrogen electrode (SHE), which is set to zero.
A redox couple is the oxidised form and the reduced form of the SAME element written together, oxidised form first, like Zn2+/Zn, Cu2+/Cu or Fe3+/Fe2+. Every electrode is really a redox couple. The electrode potential is the value assigned to that couple. NEET has directly asked this: the correct description of a redox couple is 'both the reduced and oxidised forms involve the same element' (NEET 2023).
The standard electrode potential (E-degree) values of Al3+/Al, Ag+/Ag, K+/K and Cr3+/Cr are -1.66 V, 0.80 V, -2.93 V and -0.74 V, respectively. The correct decreasing order of the reducing power of the metals is:
The correct option for a redox couple is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. A single half-cell potential cannot be measured alone because you always need two electrodes to complete a circuit. We measure it relative to a reference electrode, the standard hydrogen electrode (SHE), whose potential is fixed at 0 V. This gives the electrode potential of the other half-cell.
By NCERT and IUPAC convention, electrode potential always means REDUCTION potential unless stated otherwise. Table values in NCERT are all reduction potentials. If a question gives 'electrode potential', use it as reduction potential.
It is the electrode potential measured under standard conditions: 1 M concentration for ions, 1 bar pressure for gases, and usually 298 K, measured against the SHE (0 V). It lets us compare different electrodes fairly on one scale.
Because no single electrode potential can be measured alone, chemists picked one electrode as a reference. The standard hydrogen electrode was chosen and assigned exactly 0 V at all temperatures. Every other electrode potential is measured with respect to it.
It is the base for many high-yield topics: order of reducing/oxidising power, electrochemical series, predicting if a redox reaction is feasible, cell EMF, corrosion and galvanization. Getting the sign convention right (negative = strong reducer) prevents most silly mistakes.
