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Hydrogen peroxide (H 2O 2) as a nonplanar molecule

Hydrogen peroxide is thermodynamically unstable and decomposes to form water and oxygen with a ΔHo of −98.2 kJ/mol and a ΔS of 70.5 J/(mol·K):

{\displaystyle {\ce {2H2O2 -> 2H2O + O2}}} {\displaystyle {\ce {2H2O2 -> 2H2O + O2}}}

The rate of decomposition increases with rising temperature, concentration and pH, with cool, dilute, acidic solutions showing the best stability. Decomposition is catalysed by various compounds, including most transition metals and their compounds (e.g. manganese dioxide, silver, and platinum).[28] Certain metal ions, such as Fe2+
or Ti3+
, can cause the decomposition to take a different path, with free radicals such as (HO·) and (HOO·) being formed. Non-metallic catalysts include potassium iodide, which reacts particularly rapidly and forms the basis of the elephant toothpaste experiment. Hydrogen peroxide can also be decomposed biologically by the enzyme catalase. The decomposition of hydrogen peroxide liberates oxygen and heat; this can be dangerous, as spilling high-concentration hydrogen peroxide on a flammable substance can cause an immediate fire.

Redox reactions
Hydrogen peroxide exhibits oxidizing and reducing properties, depending on pH.

In acidic solutions, H
2O
2 is one of the most powerful oxidizers known and is stronger than chlorine, chlorine dioxide, and potassium permanganate. When used for removing organic stains from laboratory glassware it is referred to as Piranha solution. Also, through catalysis, H
2O
2 can be converted into hydroxyl radicals (·OH), which are highly reactive.

Oxidant Reduced
product Oxidation
potential
(V)
F2 HF 3.0
O3 O2 2.1
H2O2 H2O 1.8
KMnO4 MnO2 1.7
ClO2 HClO 1.5
Cl2 Cl− 1.4
In acidic solutions Fe2+
is oxidized to Fe3+
(hydrogen peroxide acting as an oxidizing agent):

2 Fe2+
(aq) + H
2O
2 + 2 H+
(aq) → 2 Fe3+
(aq) + 2 H
2O(l)
and sulfite (SO2−
3) is oxidized to sulfate (SO2−
4). However, potassium permanganate is reduced to Mn2+
by acidic H
2O

  1. Under alkaline conditions, however, some of these reactions reverse; for example, Mn2+
    is oxidized to Mn4+
    (as MnO
    2).

In basic solution, hydrogen peroxide can reduce a variety of inorganic ions. When it acts as a reducing agent, oxygen gas is also produced. For example, hydrogen peroxide will reduce sodium hypochlorite and potassium permanganate, which is a convenient method for preparing oxygen in the laboratory:

NaOCl + H
2O
2 → O
2 + NaCl + H
2O