Chemistry · Redox Equilibrium · NEET
The oxidising agent is REDUCED. This is the point that confuses most students. The oxidising agent oxidises the other substance, but to do that it must accept the electrons that come off the other substance. Accepting electrons means its own oxidation number goes DOWN, which is reduction. So the oxidising agent is always the one that gets reduced. Same logic in reverse: a reducing agent is always OXIDISED itself.
An oxidising agent GAINS (accepts) electrons. A reducing agent LOSES (donates) electrons. Link it to oxidation number: gaining electrons lowers the oxidation number (reduction), losing electrons raises it (oxidation). So oxidising agent = electron acceptor = oxidation number decreases; reducing agent = electron donor = oxidation number increases. For NEET, spotting who gains and who loses electrons is the fastest way to label both agents.
Use a 3-step method. (1) Write the oxidation number of each element on both sides. (2) Find the element whose number INCREASED - the substance that contains it is the reducing agent (it was oxidised). (3) Find the element whose number DECREASED - the substance that contains it is the oxidising agent (it was reduced). Example: Zn + Cu2+ gives Zn2+ + Cu. Zn goes 0 to +2 (up) so Zn is the reducing agent; Cu goes +2 to 0 (down) so Cu2+ is the oxidising agent.
Because electrons cannot disappear. In a redox reaction, the electrons lost by one substance must be gained by another. The oxidising agent is the substance that grabs those electrons. Grabbing electrons IS reduction. So the very act of oxidising someone else forces the oxidising agent to be reduced. NEET often tests this exact idea by asking whether an agent is oxidised or reduced - the answer is the opposite of its name.
Yes, if the element is in a middle oxidation state, it can go up or down depending on the other reactant. Example: SO2 has sulphur in +4. With a stronger oxidiser like KMnO4, SO2 loses electrons (S goes +4 to +6) and acts as a REDUCING agent. With a stronger reducer like H2S, SO2 gains electrons and acts as an OXIDISING agent. An element already in its highest state (like S in H2SO4, +6) can only act as an oxidising agent, and an element in its lowest state can only reduce.
For oxoacids of phosphorus, reducing power depends on the number of P-H bonds - more P-H bonds means stronger reducing agent. H3PO2 has 2 P-H bonds (strongest reducer), H3PO3 has 1, and H3PO4 has none. In general, metals low on the reactivity series (like Na, K, Al) are strong reducing agents because they lose electrons easily. Non-metals like F2 and O2 are strong oxidising agents because they grab electrons easily.
Name the gas that can readily decolourise acidified KMnO4 solution.
It is because of inability of ns2 electrons of the valence shell to participate in bonding that:
Which one of the following statements is correct when SO2 is passed through acidified K2Cr2O7 solution?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An oxidising agent gains electrons and is reduced; a reducing agent loses electrons and is oxidised. Each agent does the opposite action to itself compared to its name.
An oxidising agent is an electron ACCEPTOR. A reducing agent is an electron DONOR. This single fact lets you label both agents in any redox reaction.
Common oxidising agents: KMnO4, K2Cr2O7, O2, F2, concentrated H2SO4. Common reducing agents: SO2, H2S, H3PO2, active metals like Na, K, Al, and H2. NEET reuses these examples often.
Track each element's oxidation number. The element whose number goes UP is oxidised, so its substance is the reducing agent. The element whose number goes DOWN is reduced, so its substance is the oxidising agent.
NEET asks about redox nearly every year, and many questions test whether an agent is oxidised or reduced, or which substance is the oxidising/reducing agent. Getting the name-flip right earns easy marks and helps in balancing redox equations later.
