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H2O2 has an open-book (non-planar) structure with O-O single bond and oxygen in -1 oxidation state. Dihedral angle: 111.5 degrees (gas), 90.2 degrees (solid). This intermediate -1 state allows H2O2 to act as both oxidiser and reducer.

As oxidiser (O: -1 → -2): H2O2 + 2$Fe^{2+}$ + 2$H^+$ → 2$Fe^{3+}$ + 2H2O; PbS(black) + 4H2O2 → PbSO4(white) + 4H2O (restores paintings). As reducer (O: -1 → 0): 2$MnO4^-$ + 5H2O2 + 6$H^+$ → 2$Mn^{2+}$ + 5O2 + 8H2O (decolourises KMnO4 — used in volumetric analysis).

Decomposition: 2H2O2 → 2H2O + O2. Catalysed by MnO2, Pt, catalase enzyme. Stabilised by phosphoric acid and acetanilide. Stored in dark, wax-coated bottles.

Volume strength = 11.2 x Molarity = 5.6 x Normality. "x volume" means 1 mL solution releases x mL O2 at STP. The n-factor is 2 in both oxidiser and reducer roles, giving equivalent weight = 17.

Key applications: bleaching agent, disinfectant (3% solution), rocket propellant (high concentration), restoring old paintings, environmental remediation, and volumetric analysis with KMnO4.