Chemistry · Redox Equilibrium · NEET
It is a pair of two forms of the SAME element: the oxidised form and the reduced form. NCERT defines it as having together the oxidised and reduced forms of a substance taking part in an oxidation or reduction half reaction. Example: Zn2+ (oxidised form) with Zn metal (reduced form) makes the couple Zn2+/Zn. The element (zinc) is the same on both sides; only its oxidation state changes.
By convention the oxidised form (higher oxidation state) is written on the LEFT and the reduced form (lower oxidation state) on the RIGHT, like Cu2+/Cu. This matches the way we write a reduction: oxidised form + electrons -> reduced form. NEET options often flip it to test you, so remember: LEFT = oxidised, RIGHT = reduced.
It is NOT a divide sign. The slash or vertical line just represents an interface (a boundary), such as solid/solution. So Zn2+/Zn means the Zn2+ ions in solution meet the solid Zn metal at a boundary. You can write it as Zn2+/Zn or Zn2+ | Zn; both mean the same thing.
They are closely linked. NCERT says the two portions of a Daniell cell are also called half-cells or redox couples. A half-cell is the physical setup (electrode dipped in electrolyte); the redox couple is the chemical pair (like Cu2+/Cu) that describes what is happening in that half-cell. In NEET language they are often used interchangeably.
Because a couple describes ONE half reaction where an element gains or loses electrons. Zn2+/Zn works because zinc changes from +2 to 0. Something like Zn2+/Cu is NOT a couple because they are two different elements, that would be a whole cell, not one couple.
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.
Zinc: Zn2+/Zn. Copper: Cu2+/Cu. In each, the oxidised ion is on the left and the neutral metal on the right.
Yes. Fe3+/Fe2+ is a valid couple because both are iron in different oxidation states (+3 oxidised form, +2 reduced form). A couple does not need a solid metal; it just needs the same element in two states.
No. The slash is only an interface line between the two forms, not a division. Read it as the couple of Zn2+ and Zn.
Standard electrode potentials are always given for a couple, like E of Cu2+/Cu = +0.34 V. You need to read the couple correctly (which form is oxidised) to write half reactions and find cell EMF.
