Why Hydrogen Peroxide Can Both Oxidise and Reduce

Water dissolves a huge range of substances, stays liquid over a wide range of temperatures and expands when it freezes. Replace its hydrogen with deuterium and you get heavy water, which helps run nuclear reactors; add one more oxygen atom and you get hydrogen peroxide, a bleach and antiseptic that can both oxidise and reduce.

This lesson covers water and heavy water, hydrogen peroxide as an oxidising and reducing agent, and the volume strength of hydrogen peroxide.

**A water molecule is bent, with an H-O-H angle of 104.5° and polar bonds, so water molecules form extensive hydrogen bonds; heavy water, ${\mathrm{D}}_2\mathrm{O}$, has the same structure but contains deuterium, which gives it slightly different physical properties.

Structure of water:**

- Oxygen is $\mathrm{s}{\mathrm{p}}^3$ hybridised, with two bonding pairs and two lone pairs
- The lone pairs squeeze the bond angle to 104.5°
- The molecule is polar, with a partial negative charge on oxygen

Hydrogen bonding and its effects:

- Each water molecule can form hydrogen bonds with up to four neighbours
- High boiling point for such a small molecule, plus a high specific heat
- Excellent solvent for ionic and polar substances, because of its high dielectric constant
- Ice is less dense than water — hydrogen bonds hold the molecules of ice in an open, cage-like structure

**Heavy water, ${\mathrm{D}}_2\mathrm{O}$:**

- Obtained by prolonged electrolysis of ordinary water, since ${\mathrm{H}}_2\mathrm{O}$ is electrolysed faster and ${\mathrm{D}}_2\mathrm{O}$ is left behind
- Boils at 101.4 °C and freezes at 3.8 °C, and is denser than ordinary water
- Reactions involving deuterium are slower, because O-D bonds are harder to break than O-H bonds
- Used to make deuterium compounds: $\mathrm{Ca}{\mathrm{C}}_2+2{\mathrm{D}}_2\mathrm{O}\to {\mathrm{C}}_2{\mathrm{D}}_2+\mathrm{Ca}(\mathrm{OD}{)}_2$
- Used as a tracer to follow reaction mechanisms and as a moderator in nuclear reactors

An everyday example. Pressurised heavy water reactors in India, such as those at Kakrapar and Rawatbhata, use heavy water to slow down neutrons so that a chain reaction can run on natural uranium fuel.

The substance. Heavy water is not radioactive — deuterium is a stable isotope; it is tritium, the third isotope of hydrogen, that is radioactive.

Hydrogen peroxide is prepared by acidifying a metal peroxide or by the auto-oxidation of an organic compound, it has a non-planar open-book structure, and because its oxygen is in the -1 oxidation state, it can be reduced to water or oxidised to oxygen gas.

Preparation:

- From barium peroxide: $\mathrm{Ba}{\mathrm{O}}_2\cdot 8{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to \mathrm{BaS}{\mathrm{O}}_4+{\mathrm{H}}_2{\mathrm{O}}_2+8{\mathrm{H}}_2\mathrm{O}$, filtering off the insoluble barium sulphate
- Industrially by auto-oxidation of 2-ethylanthraquinol in air, which gives hydrogen peroxide and a quinone that is reduced back with hydrogen and reused

Structure. The molecule has an open-book shape: the two O-H bonds lie in different planes, at about 111.5° to each other in the gas phase and 90.2° in the solid.

As an oxidising agent — oxygen goes from -1 to -2:

- $2F{\mathrm{e}}^{2+}+2{\mathrm{H}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2\to 2F{\mathrm{e}}^{3+}+2{\mathrm{H}}_2\mathrm{O}$
- $\mathrm{PbS}+4{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{PbS}{\mathrm{O}}_4+4{\mathrm{H}}_2\mathrm{O}$ — black lead sulphide becomes white lead sulphate

As a reducing agent — oxygen goes from -1 to 0:

- $2Mn{\mathrm{O}}_4^{-}+6{\mathrm{H}}^{+}+5{\mathrm{H}}_2{\mathrm{O}}_2\to 2M{\mathrm{n}}^{2+}+8{\mathrm{H}}_2\mathrm{O}+5{\mathrm{O}}_2$
- $\mathrm{A}{\mathrm{g}}_2\mathrm{O}+{\mathrm{H}}_2{\mathrm{O}}_2\to 2Ag+{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$

Storage and uses. It slowly decomposes, $2{\mathrm{H}}_2{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$, faster in light and with traces of metal, so it is stored in wax-lined coloured plastic bottles with a stabiliser such as urea. It bleaches textiles, paper pulp and hair, serves as a mild antiseptic and helps treat industrial effluents.

An everyday example. Dilute hydrogen peroxide dabbed on a scraped knee at a clinic fizzes because catalase, an enzyme in blood, rapidly breaks it down into water and bubbles of oxygen.

The substance. Whether hydrogen peroxide oxidises or reduces depends on its partner — faced with a stronger oxidising agent such as permanganate, it is forced to act as the reducing agent.