6.hydrogen its compound

8/10/2019 6.Hydrogen Its Compound

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6. HYDROGEN AND ITS COMPOUNDS

Synopsis :

Hydrogen is the most abundant element in the universe.It is the 9thmost abundant element in the earth's crust.

It occurs only in the combined state except in volcanic gases where it occurs in free state.

It is the first element in the periodic table.

It is the lightest of all the elements.

It is the only element without neutrons i.e. Protium.

It is the element with ambiguious position in the periodic table, as it resembles both I-A andVII-A group elements.

Hydrogen should belong to the s-block as its electron configuration is 1s1.

The properties in which hydrogen resembles alkali metals area) Like alkali metals it has one electron in s-orbital of valence shell.

b) Like Alkali metals, it forms H+ion.

c) Like Alkali metals it forms monoxides and peroxides.

d) Like Alkali metals, it acts as reductant.

The properties in which Hydrogen resembles Halogens area) Like Halogens it exists as diatomic molecule (H2)

b) Like Halogens it gains one electron to attain inert gas configuration

c) The I.P value of "H" is similar to that of Halogens.

d) Like halogens it forms univalent anion i.e H. Hydrogen has 3 isotopes. They are

Protium 11H

Deuterium 21H

Tritium 31H

Theoritically the number of possible Hydrogen molecules is six. They are H2, D2, T2, HD, HTand DT.

Ordinary Hydrogen contains only 0.02% Deuterium.

Hydrogen and Deuterium can be separated by gas diffusion principle.

Hydrogen is more reactive than Deuterium.

All the isotopes of Hydrogen havea) same chemical properties

b) same atomic radii

c) same bond lengths

The isotopes of Hydrogen differ widely due to large difference in mass numbers.a) Number of neutrons b) Chemical reactivity


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Hydrogen and its compounds

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c) Physical properties d) Nuclear radii

e) Bond energy

The properties or physical constants which are lower for hydrogen than that of deuterium arei) Molecular weight ii) Boiling point

iii) Melting point iv) Latent heat of fusionv) Latent heat of vaporization

vi) Bond energy vii) Activation energy

Chemical reactivity is higher for hydrogen than that of Deuterium.

On electrolysis of water, H2is released six times faster than that of D2.

Ordinary Hydrogen contains Tritium and protium in the ratio 1 : 1017.

Tritium can be obtained in the nuclear transformations such as

7N14+ on

16C12+ 1T

3(occurs in nature)

3Li6+ on

12He4+ 1T

3(takes place in nuclear reactors)

The radioactive isotope of Hydrogen is Tritium. It is a beta emitter and its half life is 12.26years

1T32He

3+ - 1e0(- particle)

Tritium is harmless, as it emits only low energetic -radiation and will not emit harmful -rays.

In the study of mechanism of various chemical reactions, Deuterium and Tritium are used astracers.

As the mass number increases the chemical reactivity decreases. So chemically least reactiveisotope of Hydrogen is Tritium.

Uses of Hydrogen

Hydrogen is usedi) in the manufacture of chemicals

ii) in metallurgy

iii) as a source of atomic energy

iv) as a fuel.

a) Synthesis of Ammonia by Haber's process.

N2(g)+ 3H2(g) 2 NH3(g)

b) Preparation of HCl

( ) ( ) ( )acid

aqwater

g)g(2g2 HCl2HCl2ClH +

c) Synthesis of Methyl alcohol:

( )( )catalyst

CrO.ZnO

gaswater22

3HCOH ++ CH3OH

d) In the production of vanaspathi or Margarine:

Synthetic petrol is prepared by Fischer-Tropsch process. Here Iron oxide is used to removesulphur from a mixture of water gas and Hydrogen.

Synthetic petrol is obtained by passing a mixture of water gas and Hydrogen free from sulphurover Cobalt catalyst.

The heat of combustion of H2gas is high (242 KJ/mole). Hence it is used as an industrial fuel.


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Liquid Hydrogen is used as fuel in the rocket. In SATURN-V" that took Neil Armstrong to themoon liquid hydrogen is used.

OXY hydrogen torch produces a very high temperature of 28000C. It is used for weldingpurpose and for melting platinum and Quartz.

The fuel gases containing H2is the important constituent of many fuel gases such asa) Coal gas b) Water gas

c) Carburetted water gas d) Semi water gas

Coal gas:It is obtained by the destructive distillation of coal

The composition of Coal gas isH2= 45 - 55% CH4= 25-35% CO = 4 - 11%

Coal gas has highest calorific value

Water gas: It contains H2and CO in 1 : 1 ratio by volumes. It is prepared by passing steamover white hot coke or coal.

Carburetted water gas is obtained by adding gaseous Hydrocarbons to water gas. The calorific value of carburetted water gas is more than that of water gas due to the presence

of Hydrocarbons.

Semi water gas: It is obtained by passing a mixture of air and steam over red hot coke.

It is the mixture of CO, H2, N2.% by volume : N2> CO > H2

The calorific value of semi water gas is low. It is used as a fuel in steel industry.

Fuel cells:- In these cells heat energy produced by burning of fuel gases is converted intoelectrical energy.

The fuel cell which involves the burning of H2and O2is used in Apollo series of rockets. Thewater formed in the combustion of Hydrogen is condensed and used as drinking water byastronauts.

In Hydrogen - oxygen fuel cell, carbon electrodes and NaOH electrolyte is used.Electrode reactions:

At anode : 2(H2+2OH2H2O + 2e

-)

At cathode : O2+ 2H2O e4 4OH

Cell reaction : 2H2+ O22H2O

The efficiency of fuel cells is very high because the heat energy is directly converted into

electrical energy. Theoretically the efficiency of fuel cell should be 100 % but practically anefficiency of60 - 70% has been achieved so far.

Hydrogen is used as a reducing agent in the extraction of heavier metals like Molybdenum andTungsten.

WO3+ 3H2W + 3H2O

The energy emitting from sun and stars is due to nuclear fusion reactions involving Hydrogennuclei.

Fusion reaction

1H2+ 1H

22He4

Energy released

23 108KJ / mole of


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1H2 + 1H

32He4

+ 0n1

41H1 2He

4 + 21e

0

"He"

17.2 108KJ / mole of"He"

26 108KJ / mole of

"He" Hydrogen bomb involves hydrogen nuclei fusion.

The binary compounds of hydrogen with other elements are called hydrides.

The hydrides have the formula MHxor (MmHn) These are formed byi) all main group elements except the noble gases and probably Indium and thallium.

ii) All lanthanides and Actinides.

iii) Transition elements such as Sc, Y, La, Ac, Tc, Zr, Hf and to lesser extent V, Nb, Ta, Cr, Cuand Zn

On the basis of their physical and chemical properties and types of bonding they are classified

into the following four types.1) Ionic hydrides (or) saline hydrides.

2) Covalent (or) molecular hydrides.

3) Metallic (or) Interstitial hydrides.

4) Polymeric hydrides.

IONIC HYDRIDES (or) SALINE HYDRIDES :

These are formed by elements of group IA, IIA (except Be and Mg) and lanthanides by heatingthe metal in hydrogen.

2Na + H2573K 2NaH

2K + H2673K

2KHCa + H2

1073K CaH2

Sr + H21123K SrH2

These are white colourless solids (crystalline) having high melting point and boiling pointeasily decomposed by water, alcohol, CO2(or) SO2

CaH2+ 2H2O Ca(OH)2+ 2H2

CaH2+ 2C2H5OH Ca(OC2H5)2+ 2H2

CaH2+ 2CO2 (HCOO)2Ca

Except LiH. all ionic hydrides decompose at400C-500C (on strong heating) into their constituent elements.

Electrolysis of these molten hydrides produce hydrogen gas at anode.

(molten)2H H2(g)+ 2e

This reaction provides chemical evidence for the presence of Hin these hydrides

This type of hydrides are formed only by elements with electronegativity value that is less than2.1.

Hydrogen compounds of high electropositive metals may be regarded as the metal hydride.

The density of these hydrides is greater than that of the metal from which they are formed.

These ionic hydrides are stoichiometric and have high heats of formation.


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They act as powerful reducing agents especially at high temperature.Ex.

1) NaH reduces CO and CO2to formates on heating

2CO + NaH HCOONa + C

CO2+NaH HCOONa + C (Sodium formate)

2) Reduces sulphates to sulphide

PbSO4+ 2CaH2 PbS + 2Ca(OH)2

3) Reduces some halides and oxides

SiCl4+ 4NaH SiH4+ 4NaCl

4) Reduces organic acids to alcohols

RCOOH + 2LiH RCH2OH + Li2O

USES OF IONIC HYDRIDES :

Metal hydrides are used in laboratory to remove traces of water from solvents and inert gasessuch as N2and Ar.

Alkali metal hydrides are used to prepare other hydride compoundsLike - LiAlH4, NaBH4etc.

Since they evolve hydrogen when heated and therefore ignite spontaneously they are used assolid fuels.

MOLECULAR (OR) COVALENT HYDRIDES :

These hydrides are formed by the combination of elements of comparatively higherelectronegatively as p-block elements. (IIIA to VIIA groups)

Ex. NH3, HCl, CH4, H2O, HF, B2H6, AsH3

Their molecular formula can be written as MHn(or) MH8nn = group number of element in short form of periodic table

The molecular hydrides are classified into three categories.

Electron precise compounds:In these compounds all the electrons of the atom are involvedin bond formation.

Ex. CH4(methane), C2H6(ethane)

Electron deficient compounds: In these compounds the available number of valenceelectrons are less than the number required for covalent bond formation (or) writing the Lewisstructure of the molecule.

Ex. B2H6, (AlH3)n Electron rich compounds:In these compounds the valence electrons on the central atom are

more than that are required for bond formation. i.e. lone pair present on the central atom.

Ex. NH3, H2O

The bonds present in this hydrides are mostly covalent in character.

Covalent hydrides like HF, HCl are partly ionic in nature.

The molecules are held together by weak van der Waals forces.PREPARATION OF COVALENT HYDRIDES :

1) By direct combination of elements

H2+ F2 2HF 3H2+ N2 2NH3


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2H2+ O2 2H2O H2+Cl2 2HCl

2) By the hydrolysis of metal borides, carbides, nitrides, phosphides etc.

Mg3B2+ 6H2O 3Mg(OH)2+ 2 6Diborane

B H

Al4C3+ 12H2O 4Al(OH)3+ 3CH4Ca3P2+ 6H2O 2 3

phosphine

3Ca(OH) 2PH+

3) By reduction of certain compounds by nascent hydrogen (or) by using reducing agent likeLiAlH4(ether solution)

AsCl3+ 6(H) AsH3+ 3HCl

SnCl4+ LiAlH4ether LiCl + AlCl3+ 4

stannane

SnH

Nomenclature and classification of covalent hydrides

The systematic (or) IUPAC name of these hydrides are usually formed from the name of theelement and the suffix ane

Name of the hydrideElement

Group No. inthe P.T.

Formula of thehydride IUPAC Common

B III (13) B2H6 Diborane (6) Diborane

C IV (14) CH4 Methane Methane

N V (15) NH3 Azane Ammonia

P V (15) PH3 Phosphane Phosphine

As V (15) AsH3 Arsane Arsine

Sb V (15) SbH3 Stibane Stibine

O Vi (16) H2O Oxidane Water

S VI (16) H2S Sulphane Hydrogen sulphide

F VII (17) HF Hydrogen Fluoride Hydrogen fluoride

Cl VII (17) HCl Hydrogen chloride Hydrogen chloride

Molecular hydrides are soft, have low melting point and low boiling point.

Molecular hydrides are volatile in nature have low electrical conductivity.METALLIC HYDRIDES:

Most of the d-block elements and f block elements from metallic hydrides on reacting withhydrogen.

Ex. CrH, CrH2, ZnH2, ThH2

Metallic hydrides are normally prepared by heating the metal with hydrogen under highpressure.

Most of the metallic hydrides are metallic conductors and have variable composition.

These hydrides have properties similar to those of parent metals.

Metallic hydrides are hard, have metallic lustre and have magnetic properties.

The density of these hydrides is less than that of the parent elements.


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Metallic hydrides exhibit metallic properties and are powerful reducing agents.

Metallic hydrides are non-stoichiometric compounds and their composition varies withtemperature and pressure.

Ex. (BeH2)n, (MgH2)n, (AlH3)n

Be and Mg of s-block and aluminium also form metallic hydrides. In these hydrogen exist in the atomic rather than ionic form.

These are interstitial compounds as the hydrogen atom occupy interstitial positron in the metallattices.

This is due to small size of hydrogen atoms compared to metal atom.

The finely divided metals obtained by thermal decomposition of these hydrides are used ascatalyst and in metallurgy.

The formation of the metallic hydrides and their capacity to release hydrogen at highertemperature are utilized in purification of H2((Pd Ag alloys is used) and for storing hydrogen.

POLYMERIC HYDRIDES : These are solids containing molecules linked together in two (or) three dimensions by hydrogen

bridge bonds. (BeH2)n, Mg(H2)n and (AlH3)n

HYDROGEN AS FUEL :

Hydrogen readily burns in air gives large amount of heat 242 kJ mol1.

It does not give any polluting bi products. Therefore it is used as fuel.

Hydrogen is potential fuel for automobiles than gasoline because it can provide large amount ofenergy per unit mass without producing biproducts such as CO2, CO, SO2 and unburnthydrocarbons which causes pollution.

Hydrogen is an important component of many industrial fuels like coal gas, water gas,carburetted water gas and semiwater gas.

Hydrogen burnt with O2 gives a flame temperature of 2800C. this flame is called oxyhydrogen blow torch.

Oxy hydrogen torch used for welding works and to melt substances like pt, quartz etc.

Atomic hydrogen torch used as oxy-hydrogen blow torch.INDUSTRIAL FUEL GASES CALORIFIC VALUES:

Coal gas:Preparation : destructive distillation of coal

Composition : H2 45 55%

CH4 25 35%CO 3 11%

Calorific value : 21,000 KJ m3

Water gas:Preparation : passing steam over red hot coke (1000C) or coal


CO 40 50%Small amount of CO2and N2

Calorific value : 13,000 KJ m3


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Semiwater gas:Preparation : A mixture of steam and air passed over red hot layers of coal.


CO 25 28%

CO2 4 5%

N2 50 55%

CH4 1 2%Calorific value : 75.24 KJ mol1

Carburetted water gas:Preparation : Hydrocarbons from the cracking of petroleum oils are mixed with water gas.

Composition : water gas + hydrocarbons

Calorific value : 15,048 KJ mol1

FUEL CELLS:

In the combination of fuel gases like H2, CO and methane energy produced is converted intoelectrical energy in the fuel cells.

A fuel cell consist of porous carbon electrodes suspended in NaOH solution. H2and O2gasesare bubbled at the surface of electrodes.

Electrodes are embedded with suitable catalyst.

The electrode reaction in hydrogen fuel cell

At cathode : O2(g)+ 2H2O(l)4e+ 4OH(aq)

At anode : 2[H2(g)+ 2 OH

(aq)2e 2H2O(aq)

2H2(g)+ O2(g) 2H2O(l)

In fuel cells the heats of combustion of fuel materials are directly converted into electricalenergy.

In the traditional methods of generating electricity by H2 (or) C by nuclear reactor there is aloss of energy in such process.

The fuel cell have more efficiency and prevents the atmospheric pollution.

In spite of the fuel value of hydrogen (heat combustion of H2= 242 KJ mol1). It is not used

frequently due to the following reasons.

1) Hydrogen elements is not present in the free state in nature.

2) The storing and Transportation of hydrogen gas is dangerous and involves risks.

WATER :

Water is called as universal solventbecause it is an excellent solvent for ionic compounds andpolar covalent compounds.

3/4th of earth's surface is covered by water.

Purest form of natural water is rain water.

Potable water means which is for drinking.

Hardness of water:

Chemically soap is sodium stearate.

Water which gives good lather readily with soap is known as soft water.


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Water which does not give lather readily with soap but precipitates soap is known as hardwater.

When soap is added to hard water Na+ of soap are replaced by Ca2+ or Mg2+ ions andprecipitates as Ca or Mg soap.

The disadvantages of using hard water area) It produces boiler scales

b) It causes wastage of soap in laundries

Hardness of water is expressed in terms of ppm of CaCO3. That means the number of grams ofCaCO3(or) its equivalent part present in 1 million grams of water is known as hardness.

Hardness is of two types. They are1) Temporary hardness

2) Permanent hardness

Temporary hardness of water is due to the presence of the bicarbonates of Ca and Mg.

Permanent hardness of water is due to the presence of the chlorides and sulphates of Ca, Mg,Fe.

Temporary hardness of water can be removed by1) Boiling 2) Clark's Method

Boiling removes the temporary hardness as soluble bicarbonates convert into insolublecarbonates

Ca(HCO3)2 CaCO3+ H2O + CO2

Mg(HCO3)2 MgCO3+ H2O + CO2

In Clark's method temporary hardness can be removed by adding a calculated quantity of milk

of lime. Ca(HCO3)2+ Ca(OH)22CaCO3+ 2H2OIf excess of lime is added it will cause permanent hardness.

Using Na2CO3both temporary and permanent hardness can be removed.

The methods used for removing permanent hardness area) Permutit method

b) Ion exchange method

c) Calgon method

a) Permutit is an artificial zeolite. Chemically permutit is hydrated sodium aluminum orthosilicate. Its formula is Na2Al2Si2O8. x H2O.

b) Gan's permutit process is also known as Base exchange process.Na2Al2Si2O8.xH2O + Ca

2+ CaAl2Si2O8.xH2O + 2Na+

The exhausted permutit is regenerated by treating 10% NaCl solution [Brine solution]

Calgon is sodium hexameta phosphate. Its formula is Na2[Na4(PO3)6)] or (NaPO3)6

Calgon removes hardness by forming complex compounds with the Ca2+andMg2+ions of hard

water or by adsorption.

Ion exchange method:

Deionised water is obtained in ion exchange method. It is free from all ions. It can be used inplace of distilled water.

Cation exchange resin is RCOOH or RSO3H and anion exchange resin is RNH3OH.


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In cation exchange resin, the cations of hard water get replaced by H+ ions.

2RCOOH+ Ca2+(RCOO)2Ca + 2H

+

Resin (from hard water)

In anion exchange resin the anions of hard water get replaced by OHions.

(RNH3)OH + Cl (RNH3)Cl + OHResin (from Hard water)

The exhausted cation exchange resin can be regenerated by moderately concentrated H2SO4solution.

Degree of hard ness:

It is expressed in PPM in terms of CaCO3.

It is the number of parts by weight of CaCO3in one million parts by weight of water

Degree of hardness in PPM = 632

weight of CaCO10

weight of H O

100g CaCO3 = 162 g Ca(HCO3)2 ; = 111 g of CaCl2= 136 g of CaSO4; = 95 g of MgCl2

= 120 g of MgSO4;

= 146g of Mg(HCO3)2

Heavy water:

Heavy water was discovered by Urey.

Heavy water is Deuterium oxide

Source of heavy water is normal water

Natural water contains I part of heavy water in 6000 parts of ordinary water.

Heavy water is also found in Himalayan snow melting areas and in the leaves of banyan trees,rain water.

Theoretically possible number of water molecules by using all the 3 isotopes of hydrogen andall the 3 isotopes of oxygen (O16, O17, O18) are 18.

Theoretically possible number of heavy water molecules are six.

They are D2O16, D2O

17, D2O18, HDO16, HDO17, HDO18. T2O is super heavy water.

Preparation of D2O

Preparation of D2O can be obtained by two methods. They are

a) Exhaustive electrolysis of alkaline water.

b) Exchange process. Heavy water is prepared by the exhaustive electrolysis of water containing N/2 NaOH.

In the preparation of heavy water by electrolytic method, the cathode is steel vessel and anodeis a perforated cylindrical Nickel sheet.

30 litres of water on electrolysis gives 1 ml. of D2O. Electrolysis occurs in 7 stages.

At the end of 1st stage, volume becomes 1/6. Alkali present in the water is neutralised bypassing CO2gas.

During electrolysis bonds in H2O are broken 18 times faster and H2gas is released about sixtimes faster than that of D2.

In the electrolysis, H2and D2are liberated at cathode and O2is liberated at anode.


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At the end of 7thstage, 99% D2O is obtained.

D2O can be prepared by exchange reactions. When H2S gas is passed through hot water, thehydrogen atoms in H2S exchange with deuterium from D2O present in water. Thus H2S

becomes D2S. On passing D2S through cold water, the deuterium from D2S and Hydrogen from

H2O exchange and the cold water becomes richer in D2O. The process is repeated. The reaction in which interchange of the isotopes of an element take place between two

compounds is known as an isotopic exchange reaction. Isotopic exchange reactions occur morereadily with compounds containing active or labile Hydrogens.

Eg.: HCl + D2O HDO + DCl

NaOH + D2O NaOD + HDO

NH4Cl + D2O NH3DCl + HDO

The reaction of salt with D2O is known as Deuterolysis

Eg.: AlCl3+ 3D2O AI (OD)3+ 3DCl

Salts like CuSO4. 5H2O, MgSO4.7H2O are known as deuterated salts or salt deuterates. Heavy water is toxic to micro organisms.

Both H2O and D2O are associated liquids due to Hydrogen (or Deuterium) bonds. But theboiling point of water is less than that of D2O. This is because the molecular weight of D2O isgreater than that of H2O.

Chemical Properties of D2O:

+ 22iselectrolys

OD

2Na

D + NaOD Sodium-deuteroxide

ONa2 NaOD

CaO Ca(OD)2Calcium deuteroxide

42SO

SOD3 Deutero sulphuric acid

52ON DNO3Deutero Nitric acid

( ) 32NMg

NDODMg23 +

Deutero Ammonia

32PCa

PD)OD(Ca23 +

Deutero phosphine

222

CaC

DC)OD(Ca

2

+ Deutero acetylene

43CAl

CD)OD(Al34 +

Deutero methane

( ) 42CBe

CDODBe2 +

Deutero methane

Uses of D2O:

i) D2O is used as a moderator in nuclear reactors to slow down the neutrons.

ii) As tracer in studying reaction mechanisms.

Ex.: Mechanism of electrophilic substitutions of Aromatic compounds, Metabolic processes.

D2O


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iii) The basicity and structures of H3PO2 & H3PO3 are studied by the exchange reactions ofD2O.

iv) D2O is used in the preparation of Deuterium.

HYDROGEN PEROXIDE

H2O2was discovered by Thenard. H2O2is also known as oxygenated water.

(O-- O-) bond is known as Peroxy bond.

H2O2 is a weak dibasic acid. Its molecular weight is 34 and its equivalent weight is 17.

Substances containing peroxy linkage are H2O2, Na2O2, BaO2, NaHO2, H2SO5, H2S2O8, HNO4,H3PO5, CrO5, H2TiO4

Perchloric acid (HClO4), MnO2, PbO2, TeO2, KMnO4etc are not true peroxides as they do notcontain peroxy bond.

Laboratory preparation of H2O2:

i) In the laboratory H2O2 is prepared by the action of ice cold dil. H2SO4 on Na2O2 (or)hydrated Barium Peroxide [BaO2.8H2O].

ii) H2O2can also be obtained by passing CO2gas through a suspension of Barium peroxide inwater.

In the preparation of H2O2 from BaO2, phosphoric acid is preferred to dil H2SO4. This isbecause H3PO4stabilizes the formation of H2O2.

Auto - oxidation process:

In auto oxidation method the starting substance is 2 - ethyl anthraquinone.

2 - ethyl anthraquinone is reduced to2 - ethyl anthraquinol with H2/Pd.

On aerial oxidation 2-ethyl anthraquinol gives H2O2and 2-ethyl anthraquinone back.Electrolytic Method:

H2O2 is manufactured by the electrolysis of 50% H2SO4 (or) a mixture of (NH4)2SO4 + dil.H2SO4using platinum anode and lead cathode.

Electrodes are separated by porous stoneware diaphragm.

The product at anode is H2S2O8and at cathode is H2 gas.

a) 2H2SO4 lonisation 2H++ 4HSO2

At cathode

+

+ ductionRee2H2

H2At anode4HSO2

Oxidation H2S2O8+ 2e-

b) Peroxy disulphuric acid (Marshall's acid) on distillation gives H2O2.

Caro's acid (H2SO5) is intermediate product.

H2S2O8+ H2O H2SO4+ H2SO5(Caros acid)

H2SO5+ H2O H2SO4+ H2O2

During the electrolysis of 50% H2SO4, the pH of the solution increases because theconcentration of acid decreases.


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Concentration of H2O2: H2O2obtained above is very dilute and so it is to be concentrated.Hydrogen peroxide is unstable and decomposes easily. If organic impurities are present, theycatalyze the decomposition of H2O2which may lead to explosion.

H2O2solution is concentrated carefully in the following 3 stages.

The stages present in the concentration of H2O2area) Freezing of water

b) Evaporation of water

c) Distillation under reduced pressure

d) Crystallisation of H2O2

20 - 30% H2O2is obtained by evaporating dilute H2O2on water bath at reduced pressure, usingfractionating column.

H2O2decomposes below it's boiling point if it is distilled at ordinary pressure therefore, theabove 20 - 30% H2O2 is distilled at reduced pressure i.e. 15mm Hg (vaccum distillation at70C) and 90% H2O2is obtained.

100% H2O2 is obtained by crystallisation of 90% H2O2by using a freezing mixture of solidCO2and diethyl ether. (from this needle shaped crystals of 100% pure H2O2separate out).

Physical properties of H2O2:

It is colorless syrupy liquid concentrated H2O2has bluish tinge.

It forms stronger H - bonds than H2O.

It's B.P is higher and M.P is lower then compared with H2O.

It is completely miscible with water.

It is feebly acidic and will not blue litmus to red.

The strength of H2O2can be expressed in

a) Volumes b) Molarityc) Normality

d) Weight - Volume Percentage

The volume of O2gas at S.T.P. obtained by the decomposition of 1 c.c. of H2O2 solution isknown as its volume strength. It is denoted by V.

Sample of H2O2 % strength (w/v) Molarity, M Normality, N

5.6 vol. H2O2 1.7% w/v 0.5 M 1 N

11.2 vol. H2O2 3.4% w/v 1 M 2 N

22.4 vol. H2O2 6.8% w/v 2 M 4 N

10 vol. H2O2 3% w/v 0.89 M 1.78 N

100 vol. H2O2 30% w/v 8.9 M 17.8 N

Perhydol is 30% (w/v). Its Morality is 8.9M Normality is 17.8 N and it contains 300g/lit (or)0.3g/ml.

On long standing or on heating H2O2undergoes decomposition. The equation for thedecomposition of H2O2is

2H2O22H2O + O2


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The substances which retard the decomposition of H2O2are Acetanilide, Glycerol, Urea,Alcohol, H3PO4, sodium stannate Pyrophosphates etc. They act as negative catalysts. They areknown as inhibitors (or) stabilisers.

Alkalies, silica, MnO2, metals (Fe, Mn), graphite, alumina etc. catalyse the decomposition of

H2O2. Dilute aqueous solution of H2O2is fairly stable in acid Medium of alcohol or ester. Alkaline

solution of H2O2are selectively not very stable.

Precautions to be taken in storing H2O2.a) The glass bottle must have wax coating on the inner side, so that the surface is smooth.

b) Plastic bottles are preferred to glass bottles. It is because the Na2O in glass being alkaline,catalyses the decomposition of H2O2

c) A small quantity of acetanilide or urea is added as a stabiliser.

90% H2O2is used as an oxidant for rocket fuel with hydrazine.

The boiling point of H2O2is 1520C.

Anhydrous H2O2 and dilute solutions of H2O2 are neutral to Litmus. They do not turn bluelitmus to red colour. But Conc. H2O2turns blue litmus to red colour.

H2O2exhibits a) Oxidising propertiesb) Reducing properties

c) Bleaching property

d) Acidic Property

It doesnt exhibit dehydrating property.

H2O2has greater reactivity in basic medium. It is because bases catalyses the decomposition ofH2O2.

H2O2is a stronger oxidizing agent and a weaker reducing agent. The antiseptic action and the bleaching action of H2O2involve the oxidizing propertiesChemical properties of H2O2:

Stability: H2O2is unstable and decomposes easily. 2H2O22H2O + O2Metals, graphite, MnO2, silica, alkalies are positive catalysis.

Urea, Glycerol, acetanilide, alcohol, H3PO4, Pyrophosphates are negative catalyses orInhibitors.

Acidic nature: Pure H2O2is weak dibasic acid. It gives two types of salts with alkalies.

H2O2+ NaOH NaHO2+ H2O

H2O2+ 2NaOH Na2O2+ 2H2O During decomposition, H2O2undergoes disproportionation.

When H2O2acts as an oxidising agent it undergoes reduction.

The reaction in which H2O2acts as a reducing agent, there will be the liberation of O2.

H2O22H++ O2+ 2e

-E0= - 0.67V represents the reducing property of H2O2.

The fundamental equation for oxidising property of H2O2is

H2O2H2O+ (O)


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The oxidising properties of H2O2in both acidic and basic media are due to

The standard reduction potential values indicate that H2O2is a strong oxidising agent in acidicmedium and weak oxidising agent in basic medium.

Oxidising properties of H2O2:

42PbSblack

PbSOOH + white

( ) OHSOFe 2342SOHFeSO

saltsFerrous

424

+ +

KOHl2OHKI 2 + +

422teSod.sulphi

SONaSONaOH32 +

32ePot.Nitrit

KNOKNOOH2 +

432Arsenitesod.

AsONaAsONaOH33 +

( )[ ] ( )[ ]632yanidepot.Ferroc

CNFeKCNFeKOH64 +

potassium ferri-cyanide

( ) dchromicacior

OCrKacidified 722 blue coloured CrO5in ether

+ 2pyrogallolalkaline

HCHOHHCOOH

OHOHHC 256FeSO/HC 466 +

The bleaching action of H2O2is due to its oxidising nature. H2O2 H2O + (O)

H2O2is used to bleach silk, wool, ivory and hair.

H2O2is used to bleach black hair to golden yellow colour under the common name Auricome.

The fundamental equation for reducing property of H2O2 is

H2O2+ (O) H2O + O2

In both acidic and basic media, H2O2shows reducing properties as( )

( ) ( )V67.0Ee2H22O

acidic

oxidation2O2H =

+++

( )( )

+++ e2O2H22Obasicoxidation

2O2HOH2 ( )V08.0E +=

H2O2

acidic medium

H2O2Oxidant

basic medium

2H++ 2e

-+ O2(E

0= +1.77v)

O2+ 2H2O + 2e-(E

0=+0.87v)


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The reducing properties of H2O2are

Formation of addition compounds:

It forms addition compounds with some organic and inorganic compounds.

On adding H2O, these addition compounds give back H2O2.

i) H2O2.Na2HPO4

ii) H2O2.(NH4)2SO4iii) H2O2.(NH2)2CO (Hyperol)

Tests for H2O2:

i) H2O2oxidises acidified K2Cr2O7solution to blue CrO5in ether.

ii) H2O2turns acidifed TiO2solution to orange coloured pertitanic acid [H2TiO4]

iii) H2O2turns starch - Iodide paper to blue colour.

iv) It decolorises KMnO4in acid medium.

Bleaching property : It's bleaching action is due to it's oxidising nature.

H2O2H2O + 'O'

'O' + color colorless Disinfectant action: It's antiseptic properties are also due to its oxidising behaviour.

The structure of H2O2. in gaseous statecan be shown as:

1.48A

1110.301

H

H

940.481

Open bookstructure of H-

O

O

22KMnOacidified OMn4 + +

2Cl

OHCl2 +

2Br OHBr2 +

2OAgmoist

OAg2 + ( )[ ] ( )[ ] 2

deFerrocyanipot64

CNFealkalineKOCNFeK63 +

2NaOCl

ONaCl+

2NaOBr

ONaBr+

2PbO

OPbO2 + ( )

22blackMnOacidified OMn2 + +

( ) ( ) 2OFerrous2FesFerricsaltFealkaline 3

++

+

( )OHO 22

ionationdisproportO3 +

H2O


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The H O O bond angle is 94o481

The dihedral angle is 111o301

The O - O bond length is 1.48 and the O - H is 0.97 . In liquid and solid states the bond lengths and bond angles are slightly changed due to

hydrogen bonding.

In crystalline H2O2the dihedral angle is reduced to 90.Uses Of H2O2:

As disinfectant and germicide

In cleaning of wounds

As bleaching agent for textiles, silk, wool, wood pulp etc.

Mixture of H2O2and hydrazine is used as rocket fuel.

To restore color of old oil paintsAs Oxidising agent

Mixture of H2O2and NH3solution is used to bleach humain hair into golden yellow color.

6.hydrogen its compound

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