Hydrogen peroxide-Hydrogen-chemistry-notes

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                          Hydrogen peroxide

Laboratory Preparation: 

(i) From Sodium peroxide: It is prepared in the laboratory by adding small amounts of sodium peroxide to ice cold water.

Na2O2 + 2H2O ¾® 2NaOH + H2O2

In place of cold water, dilute sulphuric acid (20%) can be used. Sodium peroxide in small amounts at a time is added to cold 20% sulphuric acid.

Na2O2 + H2SO4 ¾® Na2SO4 + H2O2

The crystals of hydrated sodium sulphate (Na2SO4·10H2O) are removed by cooling the solution. The solution of hydrogen peroxide obtained always consists some dissolved sodium sulphate. The concentration of H2O2 is about 3%.

BaO2·8H2O + H2SO4 = BaSO4 + H2O2 + 8H2O

The barium sulphate formed is filtered off and a solution of H2O2 is obtained.

The use of sulphuric acid has a disadvantage as it catalyses the decomposition of hydrogen peroxide formed. In place of sulphuric acid, weak acids like orthophosphoric acid, carbonic acid are preferred. Barium peroxide on treatment with orthophosphoric acid gives a precipitate of barium phosphate and H2O2 which goes into the solution.

3BaO2 + 2H3PO4 ¾® Ba3(PO4)2 + 3H2O2

                                                                             ppt.

Insoluble barium phosphate is filtered off. The filterate consists of hydrogen peroxide.

The barium phosphate is decomposed by dilute sulphuric acid.

Ba3(PO4)2 + 3H2SO4  ¾® 3BaSO4 + 2H3PO4

BaSO4 is removed by filtration and orthophosphoric acid is used again.

Merck’s process: H2O2 can be obtained by passing a current of CO2 through a cold pasty solution of barium peroxide in water.

BaO2 + CO2 + H2O ¾® BaCO3 + H2O2

Barium carbonate being insoluble is filtered off.

Manufacture of Hydrogen Peroxide

(i) Auto oxidation of  2-butyl anthraquinol

(ii) By Oxidation of isopropyl alcohol 

(iii) Electrolytic process: In this process, the electrolysis of 50% sulphuric acid is carried out at low temperature using platinum electrodes and a current of high density. Proxy disulphuric acid is formed.

2H2SO4  2H+ + 2HSO4–

                    2HSO4– ¾® H2S2O8 + 2e                (anode)

                                                                        peroxy

                                                                                   disulphuric acid       

Peroxy disulphuric acid is distilled. Hydrolysis occurs and a distillate containing about 30% H2O2 is obtained.

H2S2O8 + 2H2O ¾® 2H2SO4 + H2O2

In place of  50% H2SO4, ammonium hydrogen sulphate dissolved in excess of sulphuric acid can be used for  electrolysis.

NH4HSO4 NH4SO4– + H+

At anode                                  2NH4SO4– ¾® (NH4)S2O8 + 2e

                                                                    Ammonium peroxy

                                                                                           disulphate  

At cathode                                2H+ + 2e  ¾®  H2(g)

The solution containing ammonium peroxy disulphate is heated at 43 mm pressure when its hydrolysis occurs forming hydrogen peroxide.

(NH4)2S2O8 + 2H2O ¾® 2NH4·HSO4 + H2O2

Hydrogen peroxide along with water distils over. Ammonium bisulphate can be used again. This method gives 30–40% aq. solution of hydrogen peroxide.

Properties of H2O2

Physical Properties: (i) Pure anhydrous hydrogen peroxide is a syrupy liquid. It is colourless but gives a bluish tinge in thick layers. It is odourless.

(ii) It is soluble in water, alcohol and ether.

(iii) It has bitter taste. It is injurious to skin (blisters are formed).

Chemical Properties: (i) Stability: It is unstable in nature. It decomposes on standing and heating. It is an example of auto oxidation-reduction reaction.

2H2O2 = 2H2O + O2

(ii) Acidic nature: The pure liquid has weak acidic nature but its aqueous solution is neutral towards litmus. It reacts with alkalies and carbonates to give their corresponding peroxides.

H2O2 + 2NaOH = Na2O2 + 2H2O

H2O2 + Ba(OH)2 = BaO2 + 2H2O

H2O2 + Na2CO3 = Na2O2 + CO2 + H2O

(iii) Oxidising nature: It is a powerful oxidising agent. It is a electron acceptor in acidic and alkaline solutions.

                                      H2O2 + 2H+ + 2e = 2H2O (In acidic solutions)

                                      H2O2 + 2e = 2OH-           (In alkaline solutions)

The reactions are generally slow in acid solutions but fast in alkaline solution.

Oxidising nature of hydrogen peroxide can be interpreted on account of the possession of label oxygen atom. The potential equation for its oxidising nature can be written as,

H2O2 ¾® H2O + O

the following examples show the oxidising nature of H2O2:

(a) It oxidises black lead sulphide (PbS) to which lead sulphate (PbSO4).

PbS + 4H2O2  ¾® PbSO4 + 4H2O

(b) It oxidises nitrites to nitrates.

NaNO2 + H2O2  ¾® NaNO3 + H2O

(c) It oxidises sulphites into sulphates.

Na2SO3 + H2O2 ¾®  Na2SO4 + H2O

(d) It oxidises arsenites into arsenates

Na3AsO3 + H2O2 ¾® Na3AsO4 + H2O

(e) It liberates iodine from potassium iodide

2KI + H2O2 ¾® 2KOH + I2

(f) It oxidises H2S into sulphur

H2S + H2O2 ¾® 2H2O + S

(g) It oxidises acidified ferrous sulphate to ferric sulphate

2FeSO4 + H2SO4 + H2O2 ¾® Fe2(SO4)3 + 2H2O

(h) It oxidises acidified potassium ferrocyanide to potassium ferricyanide

2K4Fe(CN)6 + H2SO4 + H2O2 ¾® 2K3Fe(CN)6 + K2SO4 + 2H2O

(i) It oxidises formaldehyde to formic acid. This oxidation occurs in presence of pyrogallol and in alkaline medium.

HCHO + H2O2 ¾® HCOOH + H2O

or          HCHO + H2O2 ¾® 2HCOOH + H2

(j) Benzene in presence of ferrous sulphate is oxidised to phenol.

(k) It dissolves the chromic hydroxide precipitate present in NaOH solution forming a yellow solution of sodium chromate.

2Cr(OH)3 + 4NaOH + 3H2O2 ¾® 2Na2CrO4 + 8H2O

(l) A solution of chromic acid in sulphuric acid or acidified potassium dichromate is oxidised to blue peroxide of chromium (CrO5) which is unstable, however, it is soluble in ether and produces blue coloured solution.

K2Cr2O7  + H2SO4 + 4H2O2 ¾® 2CrO5 + K2SO4 + 5H2O

Peroxide of chromium decomposes to form chromic sulphate and oxygen.

4CrO5 + 6H2SO4 ¾® 2Cr2(SO4)3 + 6H2O + 7O2

Peroxide of chromium is represented as

                                                                  

(m) In acidic solution, mercury is oxidised to mercuric oxide.

Hg + H2O2HgO + H2O

(n) Bleaching action: Due to its oxidising nature, it acts as a bleaching agent.

Coloured material + O ¾® Colourless

It bleaches materials like silk, hair, ivory, cotton, wool, etc.

(iv) Reducing nature: It can also act as a reducing agent towards powerful oxidising agents.

H2O2 ¾® 2H+ + O2 + 2e–

In alkaline solution, however, its reducing action is more effective.

H2O2 + 2OH– ¾® 2H2O + O2 + 2e–

The potential equation when H2O2 acts as a reducing agent can be expressed as,

H2O2 + O ¾® H2O + O2

(a) Ag2O is reduced to silver

Ag2O + H2O2 ¾® 2Ag + H2O + O2

(b) It reduces ozone to oxygen.

H2O2 + O3 ¾® H2O  + 2O2

(c) Manganese dioxide in acidic medium is reduced to manganous salt.

MnO2 + H2SO4 ¾® PbO + H2O + O2

(e) Red lead in presence of HNO3 is reduced to plumbous salt.

Pb3O4 + 6HNO3 + H2O2 ¾® 3Pb(NO3)2 + 4H2O + O2

(f) Chlorine and bromine are reduced to corresponding hydracids.

Cl2 + H2O2 ¾® 2HCl + O2

This reaction can be shown in following steps:

Cl2 + H2O2 ¾® 2HCl + O2

Similarly, Br2 + 2O2 ¾® 2HBr + O2

(g) It reduces acidified KMnO4 solution, i.e., acidified KMnO4 is decolorised by H2O2.

2KMnO4 + 3H2SO4 + 5H2O 2 ¾® K2SO4 + 2MnSO4 + 8H2O + 5O2 

(h) Potassium ferricyanide (alk. solution) is reduced to potassium ferrocyanide.

2K3Fe(CN)6 + 2KOH + H2O2 ¾® 2K4Fe(CN)6 + 2H2O + O2

(i) Hypohalites are reduced to halides

NaOCl + H2O2 ¾® NaCl + H2O + O2

(j) KIO4 is reduced to KIO3

KIO4 + H2O2 ¾® KIO3 + H2O + O2

Uses:

(a) for restoring colour of old paintings.

(b) as ‘antichlor’ to remove traces of chlorine and hypochlorite.

(c) Highly concentrated solution (about 40%) of H2O2 is used to oxidise petrol, alcohol, and hydrozine hydrate for the propelling of rockets and torpedoes.

NH2·NH2 + 2H2O2 ¾® N2 + 4H2O

                                                  Problem for practice

Q.1 A B C D H2O2 can be obtained when following reacts with H2SO4 except with -

            (A) PbO₂                    (B) BaO₂   

            (C) Na₂O₂                   (D) SrO₂

Q.2 A B C D        H₂O₂ ¾® 2H+ + O₂ + 2e¯,

            E₀ = – 0.68 V

            The above equation represent which of the following behaviour of H₂O₂ ?

            (A) Reducing agent     (B) Oxidising agent

            (C) Acidic                   (D) Catalytic

Q.3 A B C D        Which one of the following reactions shows oxidising nature of H₂O₂ ?

            (A) H₂O₂ + 2KI ¾® 2KOH + I₂

            (B) Cl₂ + H₂O₂ ¾® 2HCl + O₂

            (C) H₂O₂ + Ag₂O ¾® 2Ag + H₂O + O₂

            (D) NaClO + H₂O₂ ¾® NaCl + H₂O + O₂

Q.4 A B C D        In which of the following reaction, H2O2 is behaving as a reducing agent -

            (A) C₆H₆ + H₂O₂ ¾® C₆H₅OH + H₂O

            (B) PbS + 4H₂O₂ ¾® PbSO₄ + 4H₂O

            (C) 2I¯ + 2H+ + H₂O₂ ¾® I₂ + 2H₂O

            (D) 2MnO₄¯ + 6H+ + 5H₂O₂ ¾®

                   2Mn2+ + 5O₂ + 8H₂O

Q.5 A B C D        The reaction H₂S + H₂O₂ ¾® S + 2H₂O manifests -

            (A) Acidic nature of H₂O₂

            (B) Alkaline nature of H₂O₂

            (C) Oxidising action of H₂O₂

            (D) Reducing nature of H₂O₂

Q.6 A B C D        Which is true about different forms of hydrogen -

            (A)  Ortho hydrogen has same spins of two nuclei clockwise or anticlockwise

            (B)  Para hydrogen has different spins of two nuclei

            (C)  At absolute zero, there is 100% para form and at high temperature, there is 75% ortho form

            (D)  All are correct

Q.7 A B C D        H₂O₂ ¾® H₂O + O₂

            This represents -

            (A) Oxidation of H₂O₂

            (B) Reduction of H₂O₂

            (C) Disproportionation of H₂O₂

            (D) Acidic nature of H₂O₂ 

Q.8 A B C D 10 Volumes of H₂O₂ has a strength of approximately -

            (A) 3%     (B) 30%     (C) 10%      (D) 5%

Q. 9 A B C D       A given solution of H₂O₂ is 30 volumes. Its concentration in terms of molarity is -

            (A)  9.1 M                   (B) 2.68 M

            (C)  2.5 M                                      (D) 26.8 M

Q. 10 A B C D     When the same amount of zinc is treated separately with excess of sulphuric acid and excess of sodium hydroxide, the ratio of volumes of hydrogen evolved is -

            (A) 1 : 1                                           (B) 1 : 2

                        (C) 2 : 1                                           (D) 9: 4

Q.11 A B C D      The volume of '10 Vol' of H₂O₂ required to liberate 500 mL O₂ at NTP is -

            (A) 50 mL                   (B) 25 mL

            (C) 100 mL                 (D) 125 mL

Q.12 A B C D      Decomposition of H₂O₂ is accelerated by -

            (A) Traces of acids

            (B) Acetanilide

            (C) Finely divided metals

            (D) Alcohol

Q.13 A B C D      Hydrogen peroxide does not -

            (A)  Liberate iodine from KI

            (B)  Turn the titanium salt yellow

            (C)  Give silver peroxide with moist silver oxide

            (D)  Decolourise the violet colour of KMnO4 solution

Q.14 A B C D      Which of the following statements about H₂O₂ is not true -

            (A)  H₂O₂ is used to clean oil paintings

            (B)  H₂O₂ acts as oxidising as well as reducing agent

            (C)  Two hydroxyl groups in H₂O₂ lie in the same plane

            (D)  It retains same structure in liquid and solid form       

Q.15 A B C D      For which purpose H₂O₂ is used -

            (A) In refreshing old oil painting

            (B) In detection of Ti⁴⁺, Cr³⁺ ions

            (C) As water softener

            (D) As rocket fuel

Q.16 A B C D      Hydrogen peroxide is prepared by which of the following acid ?             

            (A) H₂SO₄                   (B) HCl

            (C) HNO₃                    (D) H₃PO₄

Q.17 A B C D      H₂O₂ is obtained when dilute H₂SO₄ reacts
with -                                          [REE-1994]

            (A) BaO₂                                     (B) MnO₂

            (C) Na₂O₂                                    (D) PbO₂

Q.18 A B C D      H₂O₂ is prepared by the action of -

                                                              [REE-1997]

            (A) Cold H₂SO₄ on BaO2

            (B) Cold H3PO₄ on BaO2

            (C) Cold H2SO₄ on MgO2

            (D) Dilute HCl on MnO2

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