peroxides

Meaning: A peroxide contains the peroxide ion O₂²⁻ or an O–O single bond (as in H₂O₂). Each oxygen is at −1, midway between O₂ (0) and oxide (−2). Because of this intermediate state, oxygen in a peroxide can be reduced to −2 or oxidized to 0, so H₂O₂ can act as an oxidizing agent or as a reducing agent, depending on its partner. The weak O–O bond also makes H₂O₂ prone to disproportionation: 2H₂O₂ → 2H₂O + O₂. One oxygen goes from −1 to −2 (reduced) and the other from −1 to 0 (oxidized).

Equation: Σ(oxidation numbers) = overall charge. For H₂O₂: 2(+1) + 2(x) = 0, so x = −1. Here x is the oxidation number of oxygen, a dimensionless number with no SI unit.

On the MCAT: Oxidation-number questions use peroxides to test whether you apply the exception. You may be asked for the oxidation state of oxygen in Na₂O₂ or H₂O₂, or to identify which species is oxidized and which is reduced in H₂O₂ decomposition. A passage may add a catalyst, such as MnO₂ or the enzyme catalase, to speed this up.

Common confusion: Students assign oxygen −2 automatically and get the wrong metal or hydrogen oxidation state. If oxygen is bonded to another oxygen, check for a peroxide. Superoxides such as KO₂ are different, with oxygen at −½.