WBCHSE Class 11 Chemistry Notes For Hydrogen Peroxide

Hydrogen Peroxide And Its Preparation

Hydrogen peroxide (H₂O₂) is another hydride of oxygen. It was discovered by the French chemist Thenard in 1818. Unlike water, it is very unstable and so, it does not occur as such in nature. Hydrogen peroxide can be prepared by the following methods.

From Sodium Peroxide (Merck’s Process)

An aqueous solution of H₂O₂ may be prepared by the action of 20% dilute sulphuric acid, cooled to 0°C by freezing mixture, on a calculated amount of Na₂O₂.

Sodium sulfate produced in the reaction separates out as crystals of Glauber’s salt (Na₂SO₄.10H₂O). The filtrate, when distilled under reduced pressure, produces a 30% H₂O₂ solution. This solution is known as Merck’s perhydrol.

⇒ \(\mathrm{Na}_2 \mathrm{O}_2+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Na}_2 \mathrm{SO}_4+\mathrm{H}_2 \mathrm{O}_2\)

From Barium Peroxide (Laboratory Preparation)

Principle: The laboratory preparation of hydrogen peroxide involves reacting an ice-cold thin paste of hydrated barium peroxide (BaO₂-8H₂O) with an ice-cold solution of 20% H₂SO₄.

Procedure: In a beaker, a thin paste of hydrated barium peroxide is prepared by adding a small amount of water to it. In another beaker, 20% dilute H₂SO₄ is taken. Both the beakers are well-cooled by placing them in a freezing mixture.

• Then the cold paste is slowly added to the cold acid solution, and kept in a freezing mixture, with constant stirring. The addition of paste is stopped when the mixture remains still slightly acidic.
• As a result of the reaction between barium peroxide and dilute sulphuric acid, H₂O₂ is produced and white insoluble BaSO₄ is precipitated. Solid BaSO₄ is then separated by filtration when a dilute aqueous solution (5%) of H₂O₂ is obtained as the filtrate.

Limitations: Hydrogen peroxide prepared by this method contains appreciable amounts of Ba₂+ ions (in the form of dissolved barium persulphate) which may catalyze the decomposition of H₂O₂. Therefore, H₂O₂ prepared by this method cannot be preserved for a long time.

1. The reaction should be carried out at low temperatures because H₂O₂ is a very unstable compound that dissociates easily into H₂O and O₂ if the temperature is increased.
2. Dilute H₂SO₄ instead of concentrated H₂SO₄ is to be used because H₂O₂ may decompose by the heat evolved when cone. H₂SO₄ reacts with BaO₂.
3. A thin paste of hydrated barium peroxide instead of anhydrous barium peroxide is to be used. This is because the heat generated by the addition of H₂SO₄ on anhydrous BaO₂ decomposes H₂O₂ to H₄O and O₂.
   Moreover, if anhydrous Ba02 is used, an insoluble layer of BaSO₄ is formed on it which prevents further reaction. However, there is no evolution of heat when a thin paste of BaO₂ is used. Also, BaO₂-8H₂O exists as fine particles and this prevents the formation of insoluble BaSO₄, and the reaction proceeds.
4. The paste of BaO₂ is poured into dilute H₂SO₄. If dilute H₂SO₄ is added to the paste, the concentration of BaO₂ at the beginning of the reaction becomes sufficiently higher as compared to that of the acid. As a consequence, the mixture becomes basic in nature.
   In this basic medium, the decomposition of H₂O₂ becomes very fast. On the other hand, if the paste is poured into the ice-cold dilute acid, the mixture always contains an excess of the acid and this prevents the decomposition of H₂O₂.
5. At the end of the reaction, the mixture should contain a small amount of surplus acid. This is because the excess acid acting as a negative catalyst decelerates the rate of decomposition of H₂O₂.
6. In this process, neither HCl nor HNO₃ can be used. HCl is not used because it reacts with BaOz to form water-soluble BaCl₂ and the separation of H₂O₂ from the mixture becomes quite difficult HNO₃ (an oxidizing agent) is not used as it oxidizes H₂O₂: H₂O₂ 2HNO₃→ 2H₂O + 2NO₂ + O₂. Moreover, due to the formation of water-soluble Ba(NO₃)₂, the separation of H₂O₂ becomes difficult.
7. It is better to use syrupy phosphoric acid instead of dilute sulphuric acid. This is because in the reaction of barium peroxide with dilute H₂SO₄, water-soluble barium persulphate (BaS208) is produced which decomposes H₂O₂. However, when syrupy phosphoric acid is used, the phosphate ions \(\left(\mathrm{PO}_4^{3-}\right)\) separate the heavy metal impurities like Pb²⁺ (which promotes the decomposition of H₂O₂) present in BaO₂ as insoluble phosphates. Therefore, although both H₂SO₄ and H₃PO₄ slow down the dissociation of H₂O₂, it is better to use H₃PO₄.

The Reaction Of CO₂ And Barium Peroxide

When a stream of CO₂ is passed through a thin paste of barium peroxide in ice-cold water, H₂O₂ and BaCO₃ are produced. The insoluble BaCO₃ is separated by filtration when a dilute solution of H₂O₂ is obtained.

BaO₂ + H₂O + CO₂→BaCOgl↓ + H₂O₂

Manufacture Of Hydrogen Peroxide

By the electrolysis of 50% H₂SO₄: H₂O₂ is manufactured by the electrolysis of 50% H₂SO₄ solution which is carried out at low temperature (-20°C) using platinum electrodes and a current of high density. Dihydrogen is liberated at the cathode and peroxodi sulphuric acid (H₂S₂O₈) or Marshall’s acid is liberated at the anode.

⇒ \(2 \mathrm{H}_2 \mathrm{SO}_4 \rightleftharpoons 2 \mathrm{H}^{+}+2 \mathrm{HSO}_4^{-}\)

Cathode: \(2 \mathrm{H}^{+}+2 e \rightarrow 2[\mathrm{H}] \rightarrow \mathrm{H}_2\)

Anode: \( 2 \mathrm{HSO}_4^{-} \longrightarrow \mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_8+2 e\) peroxide sulphuric acid

Peroxodisulphuric acid is collected from the anode chamber and then distilled with water under reduced pressure, when it gets hydrolyzed. The low boiling H₂O₂ distills over along with water leaving behind the high boiling H₂SO₄ in the flask.

⇒ \(\mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_8+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{SO}_5+\mathrm{H}_2 \mathrm{SO}_4\)

⇒ \(\mathrm{H}_2 \mathrm{SO}_5+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{SO}_4+\mathrm{H}_2 \mathrm{O}_2\)

Modification: A recent modification of the above method involves the use of an equimolar mixture of sulphuric acid and ammonium sulfate for electrolysis. Ammonium persulphate [(NH₄)₂S₂O8] formed around the anode is collected and then distilled with water to form H₂O₂.

⇒ \(\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow 2 \mathrm{NH}_4 \mathrm{HSO}_4\)

⇒ \(\mathrm{NH}_4 \mathrm{HSO}_4 \rightleftharpoons \mathrm{NH}_4 \mathrm{SO}_4^{-}+\mathrm{H}^{+}\)

Cathode: \(2 \mathrm{NH}_4 \mathrm{SO}_4^{-}-2 e \rightarrow\left(\mathrm{NH}_4\right)_2 \mathrm{~S}_2 \mathrm{O}_8\)

Anode: 2\(\mathrm{NH}_4 \mathrm{SO}_4^{-}-2 e \rightarrow\left(\mathrm{NH}_4\right)_2 \mathrm{~S}_2 \mathrm{O}_8\)

⇒ \(\left(\mathrm{NH}_4\right)_2 \mathrm{~S}_2 \mathrm{O}_8+2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NH}_4 \mathrm{HSO}_4+\mathrm{H}_2 \mathrm{O}_2\)

Deuteroperoxide (D₂O₂) may be prepared by using K₂SO₄ instead of (NH₄)₂SO₄ in the above process and distilling the resulting potassium persulphate with D₂O.

⇒ \(\mathrm{K}_2 \mathrm{~S}_2 \mathrm{O}_8(s)+2 \mathrm{D}_2 \mathrm{O}(l) \rightarrow 2 \mathrm{KDSO}_4(a q)+\mathrm{D}_2 \mathrm{O}_2(l)\)

By auto-oxidation of 2-ethylanthraquinol: in this process, the air is passed through a 10% solution of 2-ethylanthraquinol in benzene and cyclohexanol when 2-ethylanthraquinol is oxidized by oxygen of air to 2-ethyl anthraquinone and oxygen is reduced to H₂O₂. The resulting H₂O₂ is separated from the organic layer by extraction with deionized water and the aqueous solution is distilled under reduced pressure to give a 30% (by weight) H₂O₂ solution.

2-ethyl-anthraquinone thus obtained is reduced with H₂ in the presence of Pd catalyst to give back 2- ethylanthraquinol which is used again. Therefore, the raw materials required in this process are H₂ and atmospheric oxygen which are inexpensive. Therefore, this modern industrial method of H₂O₂ preparation is the most convenient and economical.

By partial oxidation of 2-propanol:

⇒ \(\underset{\text { 2-propanol }}{\left(\mathrm{CH}_3\right)_2 \mathrm{CHOH}} \underset{\text { [under pressure] }}{\stackrel{\left(\text { little } \mathrm{H}_2 \mathrm{O}_2\right)+\mathrm{O}_2}{\longrightarrow}} \underset{\text { 2-propanone }}{\left(\mathrm{CH}_3\right)_2 \mathrm{C}=\mathrm{O}}+\mathrm{H}_2 \mathrm{O}_2\)

Preparation of H₂O₂ from Us dilute aqueous solution: Hydrogen peroxide produced by any of the above methods is in the form of a dilute solution. The solution is concentrated simply by heating it because H₂O₂ readily decomposes below its boiling point (2H₂O₂—>2H₂O + O₂). The dilute solution of H₂O₂ is concentrated carefully to get pure H₂O₂ by the following steps:

1. The dilute solution of H₂O₂ is heated carefully in a water bath. Slow evaporation of water leads to the formation of a 50% H₂O₂ solution.
2. The 50% solution thus obtained is placed in a vacuum desiccator over concentrated H2S04 when approximately 90% H₂O₂ solution is obtained.
3. The 90% solution is distilled under reduced pressure (10-15 mm) when about 99% pure H₂O₂ is obtained.
4. The 99% solution is finally cooled in a freezing mixture of solid CO₂ and ether when crystals of H₂O₂ separate out. These are removed, dried, and remelted to yield very pure H₂O₂.