Chemistry · Redox Equilibrium · NEET
Oxygen is usually -2, but there is a special rule for peroxides. In H2O2 the structure is H-O-O-H. The two oxygen atoms are joined by an O-O bond. Bonds between the same element are shared equally, so that bond gives no charge shift. Each oxygen only pulls electrons from one hydrogen (+1). So each oxygen is -1, not -2. Check: 2(+1) for H + 2(-1) for O = 0. This -1 peroxide state is the whole reason H2O2 can act in two ways.
It depends on the starting oxidation number. Oxygen in H2O2 is at -1, which is between 0 and -2. If oxygen moves UP to 0, H2O2 has lost electrons, so it is oxidised, which means it acted as a reducing agent. If oxygen moves DOWN to -2, H2O2 has gained electrons, so it is reduced, which means it acted as an oxidising agent. An element that is stuck at its highest or lowest state (like O at -2 in water) has only one direction to move, so it can act only one way.
When H2O2 reacts with an easily oxidised substance, oxygen goes from -1 to -2. Example: 2Fe2+ + H2O2 + 2H+ -> 2Fe3+ + 2H2O. Here iron is oxidised (Fe2+ to Fe3+) and the oxygen of H2O2 is reduced (-1 to -2 in water). So H2O2 is the oxidising agent. Another common one: PbS + 4H2O2 -> PbSO4 + 4H2O (black PbS to white PbSO4).
When H2O2 meets a strong oxidiser, its oxygen goes from -1 UP to 0 (released as O2 gas). Example with acidified KMnO4: 2MnO4- + 5H2O2 + 6H+ -> 2Mn2+ + 5O2 + 8H2O. Manganese is reduced (+7 to +2, the purple colour fades) and oxygen of H2O2 is oxidised (-1 to 0). So here H2O2 is the reducing agent. The oxygen bubbles you see are O2 coming from H2O2.
This is disproportionation, a special case where the SAME element is both oxidised and reduced. NCERT gives it directly: 2H2O2 -> 2H2O + O2. One oxygen at -1 goes up to 0 (in O2) and another goes down to -2 (in H2O). This is only possible because -1 is a middle state with room to move both up and down. It is why H2O2 is stored in dark bottles: light speeds up this self-reaction.
Consider the following compounds: KO2, H2O2 and H2SO4. The oxidation states of the underlined elements (K in KO2, O in H2O2, S in H2SO4) in them are, respectively,
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because oxygen in H2O2 is at -1, a middle state that can move up to 0 (reducer) or down to -2 (oxidiser).
H2O2 and SO2 act as both (their key element is in a middle oxidation state). O3 and HNO3 act only as oxidisers because their elements are already at the top state and can only go down. This is a direct NCERT exercise (7.8).
Yes. In 2H2O2 -> 2H2O + O2 the same oxygen (-1) is both oxidised to 0 and reduced to -2, which is the definition of disproportionation.
Track the oxygen of H2O2. If it ends as O2 (state 0), H2O2 was the reducing agent. If it ends as H2O (state -2), H2O2 was the oxidising agent.
It acts as an oxidising agent more often, since -1 to -2 is easy. It behaves as a reducer only against strong oxidisers like KMnO4, Cl2 or acidified K2Cr2O7.
