Oxidation and Reduction as Electron Transfer (OIL RIG)
Chemistry · Redox Equilibrium · NEET
Oxidation means an atom or ion LOSES electrons. Reduction means it GAINS electrons. Both always happen together in the same reaction, because the electrons one species loses are the electrons another species gains. Memory hook: OIL RIG - Oxidation Is Loss, Reduction Is Gain (of electrons).
In Zn + CuSO4, zinc loses 2 electrons (oxidised, the reducing agent) and Cu2+ gains those same 2 electrons (reduced, the oxidising agent). Same electrons transferred - that is a redox reaction.
Your doubts, answered
Is oxidation loss or gain of electrons? I keep mixing it up.
Oxidation is LOSS of electrons. Reduction is GAIN of electrons. Use the sentence OIL RIG: Oxidation Is Loss, Reduction Is Gain. So if a species gives away electrons, it is oxidised. If it takes electrons, it is reduced. This is the electron-transfer definition that NEET uses.
If oxidation is LOSS of electrons, why does the oxidation number INCREASE?
They agree, not clash. Electrons are negative. When an atom loses negative electrons, it becomes more positive, so its oxidation number goes UP. Example: Zn (0) loses 2 electrons to become Zn2+ (+2). Loss of electrons = oxidation = oxidation number rises. Reduction is the opposite: gain electrons, become more negative, oxidation number falls.
How do I know which species is oxidised and which is reduced in a reaction?
Track the electrons or the oxidation numbers. The species whose oxidation number goes UP has lost electrons, so it is oxidised. The one whose oxidation number goes DOWN has gained electrons, so it is reduced. In Zn + CuSO4 -> ZnSO4 + Cu: Zn goes 0 to +2 (oxidised, loses 2e), Cu goes +2 to 0 (reduced, gains 2e).
Does a reducing agent lose or gain electrons?
A reducing agent LOSES electrons (it gives them away). By giving electrons, it reduces the other species, so it is itself oxidised. An oxidising agent GAINS electrons; it takes them from the other species, so it is itself reduced. Easy rule: the agent does the OPPOSITE to itself of its name suggests for the electron count.
Can oxidation happen alone, without reduction?
No. Electrons cannot just disappear. If one species loses electrons, another species must gain the exact same number. So oxidation and reduction always occur together in one reaction. That combined reaction is called a redox reaction, and this is why NEET treats them as two halves of one event.
What is a redox couple in the electron-transfer picture?
A redox couple is the oxidised form and the reduced form of the SAME element, written as oxidised/reduced, for example Cu2+/Cu or Fe3+/Fe2+. The two forms differ only by electrons. NEET 2023 asked exactly this: a redox couple has both the reduced and oxidised forms of the same element.
⚠️ The NEET trap ✗ A reducing agent gains electrons because it 'reduces' the reaction. ✓ A reducing agent LOSES electrons (donates them). By donating, it reduces something else, and it is itself oxidised. The oxidising agent is the one that gains electrons. 🧠 The AGENT always does the opposite to itself: reducing agent gets oxidised, oxidising agent gets reduced.
Real NEET questions
NEET 2020
What is the change in oxidation number of carbon in the reaction CH4(g) + 4Cl2(g) -> CCl4(l) + 4HCl(g)?
A · -4 to +4 ✓
B · 0 to -4
C · +4 to +4
D · 0 to +4
Solution: In CH4, H is +1, so C = -4. In CCl4, Cl is -1, so C = +4. Carbon goes from -4 to +4. The oxidation number rises by 8, meaning carbon has effectively lost electrons, so carbon is oxidised. This is the electron-transfer idea: loss of electrons = oxidation = oxidation number goes up. Answer (a).
NEET 2023 Phase 2
The correct option for a redox couple is:
A · Both the reduced and oxidised forms involve the same element ✓
B · Cathode and anode together
C · Both are oxidised forms involving the same element
D · Both are reduced forms involving the same element
Solution: A redox couple is the oxidised form and the reduced form of the SAME element, differing only by electrons, for example Cu2+/Cu or Fe3+/Fe2+. So the couple always has one reduced form and one oxidised form of the same element. Answer (a).
NEET 2024
Which of the following reactions is NOT a redox reaction?
A · 2KClO3 + I2 -> 2KIO3 + Cl2
B · H2 + Cl2 -> 2HCl
C · BaCl2 + Na2SO4 -> BaSO4 + 2NaCl ✓
D · Zn + CuSO4 -> ZnSO4 + Cu
Solution: A redox reaction needs electron transfer, shown by a change in oxidation number. In BaCl2 + Na2SO4 -> BaSO4 + 2NaCl, no element changes oxidation state, so no electrons move: it is only a double-displacement (precipitation) reaction, not redox. In (d) Zn 0 to +2 (loses electrons, oxidised) and Cu +2 to 0 (gains electrons, reduced), so that one IS redox. Answer (c).
Solved Redox Equilibrium NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the electron-transfer definition of oxidation and reduction?
Oxidation is loss of electrons and reduction is gain of electrons. This is the modern definition NEET uses. Remember it with OIL RIG.
What does OIL RIG stand for?
OIL RIG = Oxidation Is Loss, Reduction Is Gain (of electrons). It is the fastest way to never mix up the two.
Why must oxidation and reduction happen together?
Electrons cannot vanish. The electrons one species loses must be gained by another species, so both processes occur in the same reaction, called a redox reaction.
Does losing electrons increase or decrease oxidation number?
Losing electrons increases the oxidation number, because the species becomes more positive. That is why oxidation raises the oxidation number.
Is the reducing agent oxidised or reduced?
The reducing agent is oxidised. It donates electrons (loses them) and by doing so reduces the other species. The oxidising agent is reduced because it gains electrons.
