Question

write down one preparation method of hydrogen peroxide . what happens when hydrogen peroxide reacts with ... (1) aqueous KI solution . (2) and acidified KMnO4

Answer 1

From sodium peroxide (Merck’s method)

Calculated amount of sodium peroxide (Na2O2) is gradually added to an ice-cold solution of 20% H2SO4 in small lots with constant stiring.

Na2O2 + H2SO4 → Na2SO4 + H2O2

Upon cooling, crystals of Na2SO4.10H2O separate out and the resulting solution contains about 30% H2O2. The solution also contains some dissolved Na2SO4, but it does not interfere with the reactions of H2O2. A pure sample of  H2O2 may, however, be prepared by vacuum distillation.

2. From barium peroxide-Laboratory method of preparation.

Hydrogen peroxide is prepared from barium peroxide by the following methods:

(a) By the action of dilute sulphuric acid. Apaste of hydrated barium peroxide (BaO2.8H2O)  is prepared in ice – cold water and then added slowly to an ice-cold solution of 20% H2SO4.

BaO2.8H2O + H2SO4 → BaSO4 + H2O2 + 8H2O   

The white precipitate of BaSO4 is removed by filtration leaving behind a dilute solution (5%) of H2O2. In this method, anhydrous barium peroxide can not used since the precipitated  BaSO4 forms a protective layer around unreacted barium peroxide thereby preventing the further reaction

Limitation

Hydrogen peroxide prepared by this method contains appreciable quantities of Ba2+ ions (in the form of dissolved barium persulphate) which catalyse the decomposition of H2O2.Therefore,H2O2 prepared by this method cannot be stored for a long time.

Further, H2SO4also acts as a catalyst for decomposition of H2O2.

(b) By the action of carbon dioxide or carbonic acid. When a rapid stream of CO2 is bubbled through a thin paste of BaO2 in ice-cold water, H2O2 and BaCO3 are produced:



The insoluble barium carbonate is removed by filtration leaving behind a dilute solution of H2O2

(c) By the action of phosphoric acid. Hydrogen peroxide can also be prepared by the action of phosphoric acid on barium peroxide:



This method has the advantage over  method since almost all the heavy metal (e.g. Pb etc) impurities present in BaO2 and which catalyse the decomposition of H2O2 are removed as insoluble phosphates. As a result, the resulting solution of H2O2 has good keeping properties.

Manufacture of Hydrogen Peroxide

1. By electrolysis of 50% H2SO4

Hydrogen peroxide is manufactured by the electrolysis of a cold 50% solutions of H2SO4 in an electrolytic cell using platinum as anode and graphite as cathode. The reactions taking place are:



At cathode:          

At anode : 

Peroxydisulphuric acid formed around anode is withdrawn and then distilled with water under reduced pressure. The low boiling H2O2 distils over along with water leaving behind high boiling H2SO4 which is recovered and recycled.

 

  or, 

Modification

Recently, it has been observed that if instead of 50% H2SO4 an equimolar mixture of H2SO4­ and ammonium sulphate is electrolysed, a more concentrated solution of H2O2 is obtained.  





At cathode: 

At Anode: 

Ammonium persulphate formed around anode is withdrawn and distilled with water to give H2O2



This process is now used only for the laboratory preparation of D2O2, i.e.;


                                      Deutroperoxide

2. By autoxidation of 2-ethylanthraquinol

This is a new method and is widely used in U.S.A. In this process, air is bubbled through a 10% solution of 2-ethylanthraquinol in benzene and cyclohexane when 2-ethylanthraquinol is oxidized to 2-ethylanthraquinone and  is formed according to the following equations.



The  thus formed (about 1%) is extracted with water and the aqueous solution is concentrated by distillation under reduced pressure to give 30% (by weight) H2O2solution.

2 – Ethylanthraquinone so produced can be reduced back to the starting to the material i.e.,2-ethylanthraquinol by hydrogen using palladium catalyst.

Thus, we find that the raw material are H2 and O2 only.

Answer 2

Explanation:

The hydrogen peroxide is prepared by hydrolysis of ammonium persulfate to give hydrogen peroxide and ammonium bi-sulfate.

$(NH_3)_2S_2O_8+2H_2O\rightarrow H_2O_2+2(NH_3)HSO_4$

1) Reaction of $H_2O_2$ with KI.

$H_2O_2+KI\rightarrow H_2O+KOI$

2) Reaction of $H_2O_2$ with acidified $KMnO_4$

$2KMnO_4+5H_2O_2+3H_2SO_4\rightarrow 5O_2+2MnSO_4+K_2SO_4+8H_2O$