Chemistry · Redox Equilibrium · NEET
The old (classical) idea said oxidation = adding oxygen. But NCERT widened it. Oxidation also means removing an electropositive element (a metal). Potassium (K) is a very electropositive metal. When H2O2 removes K from potassium ferrocyanide, that removal itself is oxidation, even though no oxygen joins the iron complex. This is exactly the point NCERT uses this reaction to teach.
2K4[Fe(CN)6](aq) + H2O2(aq) -> 2K3[Fe(CN)6](aq) + 2KOH(aq). Read it slowly: each K4[Fe(CN)6] loses one K and becomes K3[Fe(CN)6]. Two K atoms leave in total, and they end up in 2KOH. So potassium (electropositive) is removed from the ferrocyanide, which is why we call it oxidation.
Ferrocyanide is K4[Fe(CN)6], where iron is in the +2 state. Ferricyanide is K3[Fe(CN)6], where iron is in the +3 state. Going from +2 to +3 means iron lost one electron, so iron is oxidised. Fewer K atoms are needed (4 -> 3) because iron's charge went up. Memory tip: ferrO = lower (+2), ferrI = higher (+3).
Here H2O2 acts as an oxidising agent. It causes the ferrocyanide to be oxidised (iron +2 to +3). H2O2 is special because it can act BOTH ways depending on the reaction, but in this NCERT example it oxidises the ferrocyanide. Do not memorise H2O2 as always one type; look at what happens in the reaction.
Classical rule: oxidation = removing hydrogen (an electropositive element). NCERT simply extends this. Since hydrogen is electropositive, chemists said removing ANY electropositive element (like the metal K) is also oxidation. So the K-removal in ferrocyanide is just a bigger version of the same idea.
NEET tests whether you understand the DEFINITION of oxidation, not just memorised facts. This reaction is the textbook proof that oxidation can happen with no oxygen and no obvious electron picture, purely by removing an electropositive metal. If you can explain this, you truly understand the classical definition of oxidation.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Potassium ferrocyanide is oxidised because the electropositive element potassium is removed from it (iron goes from +2 to +3). H2O2 is reduced; its oxygen goes from the -1 state in peroxide to the -2 state in KOH.
Iron is +2 in potassium ferrocyanide K4[Fe(CN)6] and +3 in potassium ferricyanide K3[Fe(CN)6]. The rise from +2 to +3 confirms iron is oxidised.
No. This is the key NEET lesson. Oxidation can also happen by adding any electronegative element or by removing hydrogen or any electropositive element (like the metal potassium here). No oxygen needs to touch iron for it to be oxidised.
Potassium easily loses its one outer electron to form K+. Elements that lose electrons and become positive ions are called electropositive. Metals like K, Na, and Ca are strongly electropositive.
