Chemistry · Redox Equilibrium · NEET
No, it is not truly zero. The potential of a single, lone half-cell can never be measured, because a voltmeter always needs two terminals (two electrodes) to read a difference. So we cannot get an absolute value for any electrode. To fix this, chemists AGREED to call the hydrogen electrode's potential exactly 0.00 V, as a reference. Every other potential is then reported relative to it. It is a chosen zero (like sea level), not a measured zero.
Three conditions must all be met: (1) H2 gas pressure = 1 bar, (2) concentration of H+ ions in the solution = 1 mol/L (unit activity), and (3) temperature = 298 K (25 C). A platinum wire coated with platinum black is dipped in the acid and pure H2 is bubbled over it. Miss any one condition and it is only a hydrogen electrode, not the STANDARD one, so E is no longer exactly 0 V.
Hydrogen is a gas, so it cannot itself act as a solid electrode surface. Platinum is an inert (unreactive) metal that does not take part in the reaction; it only provides a surface for the H2 to H+ change and conducts the electrons out. The platinum black coating is finely divided platinum with a huge surface area, which adsorbs H2 gas and makes the electron transfer fast and reversible.
A positive standard reduction potential means that species is reduced more easily than H+ ions — its reduced form is more stable than hydrogen gas, so it is a stronger oxidising agent (e.g. Cu2+/Cu = +0.34 V, Ag+/Ag = +0.80 V). A negative value means the species is oxidised more easily than H2 — its metal is a stronger reducing agent than hydrogen (e.g. Zn2+/Zn = -0.76 V, K+/K = -2.93 V). Reading the sign is the whole skill NEET tests.
By convention the SHE is placed on the LEFT as the reference and the other half-cell on the right. When the SHE acts as the anode (reference half-cell) and the other half-cell as the cathode, the measured cell emf directly gives the standard REDUCTION potential of that other electrode. That is why every value in the electrochemical series is a reduction potential referenced to the SHE.
The standard electrode potential (E) 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 reducing power of the metals is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It can act as either. Its reaction 2H+ + 2e- <=> H2 is reversible. When paired with a stronger oxidiser it acts as the anode (oxidation, H2 to H+); when paired with a stronger reducer it acts as the cathode (reduction, H+ to H2). Its assigned potential stays 0 V under standard conditions.
Standard electrode potential (E) is measured under standard conditions — 1 bar gas, 1 M ions, 298 K. Plain electrode potential (E) applies at any other concentration or pressure and is found using the Nernst equation. NEET Class 11 Redox mostly uses standard values from the series.
A potential is always a difference between two points. A voltmeter has two probes, so it can only read the gap between two half-cells, never one alone. That is exactly why a reference zero (the SHE) is needed.
Yes. The values in the standard series are REDUCTION potentials. If you reverse the half-reaction to an oxidation, you must reverse the sign. Always confirm you are using reduction potentials before comparing, or NEET's ordering questions will trip you.
