Hydrogen peroxide Reactions, Physical Properties an Uses| H₂O₂

Hydrogen peroxide is a very common chemical compound in the people. It is used as a disinfectant in cleaning injuries.

Physical Properties of hydrogen peroxide

• When hydrogen peroxide exists as pure, viscous at liquid state, it is colourless.
• When at solid state, hydrogen peroxide is a white crystal.
• Also at higher temperature, H₂O₂ may be explosive.
• When exposes to the sunlight, hydrogen peroxide decomposes to oxygen gas and water.
• Boiling point: 150⁰C
• Has weak acidic characteristics
• Completely miscible with water, alcohol, and ether in all proportions. Hydrogen peroxide forms a hydrate with water as H₂O₂.H₂0.

Why boiling point of hydrogen peroxide is higher than water?

Boiling points of hydrogen peroxide and water are 150⁰C and 100⁰C respectively. We know both hydrogen peroxide and water are able to make hydrogen bonds.

Following factors will increase boiling points of compounds

• Ability to make how many hydrogen bonds can be formed?
• When molecular mass high, boiling point increases.

Molecular masses of H₂O₂ and H₂O are 34 and 18 respectively. Also hydrogen peroxide molecules have the ability to make more hydrogen bond than water molecules. Due to both greater molecular mass and number of hydrogen bonds, boiling point of hydrogen peroxide is higher than water.

Acidic nature of hydrogen peroxide

Pure hydrogen peroxide is a weak acid dissociates as follow. It's Ka value is 1.55 * 10^-12 mol dm^-3 at 298K.

• First dissociation forms hydronium ion and hydroperoxide ion.
• Second dissociation forms again hydronium ion and peroxide ion.

Storage of hydrogen peroxide

Hydrogen peroxide is stored in wax-lined glass or plastic vessels in the presence of stabilizers such as urea. Also it should be kept away from dust because dust can induces explosive decomposition.

Hydrogen Peroxide Preparing

Alkali metal peroxide and water reaction

Alkali metals peroxides ( Na₂O₂, K₂O₂ ) will react with water and produce hydrogen peroxide and metal hydroxide.

Reactions of hydrogen peroxide

Hydrogen peroxide can react in different ways.

• As an oxidizing agent
• As a reducing agent

Decomposition of Hydrogen Peroxide

Hydrogen peroxide decomposes into water and oxygen gas (O₂) when temperature increases or exposing to the sunlight. This reaction can be catalyzed by minute traces of transition metal ions. In the presence of potassium iodide, MnO₂ aqueous NaOH will also catalyze the decomposition of H₂O₂. Also decomposition of H₂O₂ is an oxidizing-reducing reaction.

2H₂O₂ → 2H₂O + O₂

Enthalpy change for the reaction is -196.0 kJ

But decomposition will slow down when acids are present. ( H₂SO₄ ).

This reaction is a disproportionation process. Two half reactions (oxidation and reduction) are occurred.

As an oxidizing agent

Hydrogen peroxide is reduced to water and other compound is oxidized.

Potassium iodide and hydrogen peroxide reaction in acidic medium | KI + H₂O₂ = I₂ + H₂O

Potassium iodide is oxidized to iodine and hydrogen peroxide is reduced to water. Also in the presence of excess potassium iodide produced iodine combines with iodide ion give red - brown I₃^-.

With Lead sulfide | PbS + H₂O₂ = PbSO₄ + H₂O

Lead sulfide is a black precipitate and reacts with hydrogen peroxide and produce lead sulfate ( PbSO₄ ) and water as products. PbSO₄ is a white precipitate.

Importance of PbS and H₂O₂ reaction

PbCO₃ is a white inorganic compound. It is used as a pigment. But due to presence of H₂S, PbCO₃ is turned to black colour PbS. To take back the white colour, hydrogen peroxide is reacted with lead sulfide. Then lead sulfate, a white precipitate is given.

With ferrous ion in acidic medium | Fe²⁺ + H₂O₂ = Fe³⁺ + H₂O

Ferrous ion ( iron(ii) ) is oxidized to ferric ion ( iron(iii) ) and H₂O₂ is reduced to water. Green colour of Fe²⁺ solution is changed to brown - yellow colour due to production of Fe³⁺.

Sulfurous acid and Hydrogen Peroxide Reaction | H₂SO₃ + H₂O₂ = H₂SO₄ + H₂O

Sulfurous acid is oxidized to sulfuric acid. So oxidation number of sulfur is increased from +4 to +6. As other product, water is given.

Sulfurous acid is a weak acid and sulfuric acid is a strong acid.

Chromium +3 ion and Hydrogen Peroxide Reaction in Basic Medium | Cr³⁺ + H₂O₂ = CrO₄^2- + H₂O

• Aqueous Cr³⁺ solution is green colour and oxidized to yellow colour CrO₄^2-.
• H₂O₂ is reduced to water.

Hydrogen peroxide and hydrogen sulfide reaction

Hydrogen sulfide is oxidized to sulfur and hydrogen peroxide is reduced to water.

Hydrogen peroxide and NaOH reaction

NaOH reacts with hydrogen peroxide and produce sodium hydroperoxide which is an acidic salt. With excess NaOH, hydrogen peroxide forms sodium peroxide which is a normal salt.

Oxidizing reactions in alkaline medium

Hydrogen peroxide can accept electrons in the alkaline medium and behave as an oxidizing agent.

Manganese sulfate and hydrogen peroxide reaction in alkaline medium

Manganese sulfate is oxidized to manganese dioxide while hydrogen peroxide is reduced to water. Manganese dioxide is a red-brown precipitate.

Metal nitrites, sulphites and arsenites are oxidized to nitrate, sulphates and arsenates respectively in alkaline medium.

Hydrogen peroxide as a reducing agent

Hydrogen peroxide can behave as a reducing agent in both acidic and basic mediums.

Potassium permanganate and hydrogen peroxide in acidic medium

Acidified potassium permanganate is reduced to colourless manganese sulfate and hydrogen peroxide is oxidized to oxygen gas. Acidified potassium permanganate solution is purple colour.

Potassium dichromate and hydrogen peroxide reaction in acidic medium

Acidified orange colour potassium dichromate solution is oxidized to green colour chromium salt while hydrogen peroxide is oxidized to oxygen gas.

Manganese dioxide and hydrogen peroxide reaction

In acidified medium, brown-black colour manganese dioxide is reduced to manganese sulfate. Oxygen is given by the oxidation of hydrogen peroxide.

Ozone reduction

Ozone is reduced to dioxygen molecule.

With hypochlorous acid

Hypochlorous acid is reduced to chloride ion in acidic medium.

Potassium permanganate reduction in alkaline medium

Potassium permanganate is reduced to manganese dioxide while oxygen is oxidized to oxygen gas.

With ferric ion in alkaline medium

Ferric ions are reduced to ferrous ion by hydrogen peroxide which is oxidized to dioxygen molecule.

Potassium ferricyanide reduction

When potassium ferrycyanide reacts with hydrogen peroxide in a alkaline medium, potassium ferrocyanide is given as the reduction product.

Oxides of metals reduction to metals or metal oxide

Some metal oxides can be reduced to metal or higher oxidation state metal oxides can be reduced to lower oxidation state metal oxide.

• Silver oxide is reduced to silver
• Lead dioxide is reduced to lead oxide

Uses of hydrogen peroxide

• As a bleaching agent for textiles, paper, pulp, leather, oils, fats and more
• As a mild disinfectant
• To produce inorganic compounds such as sodium perborates, sodium percarbonates which are important constituents of high quality detergents.
• As pollution control agent. Oxidizing cyanides and restoration of aerobic conditions to sewage wastes.
• As an antiseptic for washing wounds
• To get back the white colour of lead paintings. White colour of lead carbonate will turn to black when occurs the contact with hydrogen sulfide due to formation of lead sulfide. Then lead sulfide reacts with hydrogen peroxide and forms lead sulfide, white colour compound.
• In food industry - tartaric acid production
• In pharmaceuticle industry - cephalosoporin

Summary of hydrogen peroxide reactions