284 CHEMISTRY

UNIT 9

After studying this unit, you will be

able to

••

• present informed opinions on the

position of hydrogen in the

periodic table;

••

• identify the modes of occurrence

and preparation of dihydrogen on

a small and commercial scale;

describe isotopes of hydrogen;

••

• explain how different elements

combine with hydrogen to form

ionic, molecular and non-

stoichiometric compounds;

••

• describe how an understanding of

its properties can lead to the

production of useful substances,

and new technologies;

••

• understand the structure of water

and use the knowledge for

explaining physical and chemical

properties;

••

• explain how environmental water

quality depends on a variety of

dissolved substances; difference

between 'hard' and 'soft' water and

learn about water softening;

••

• acquire the knowledge about

heavy water and its importance;

••

• understand the structure of

hydrogen peroxide, learn its

preparatory methods and

properties leading to the

manufacture of useful chemicals

and cleaning of environment;

••

• understand and use certain terms

e.g., electron-deficient, electron-

precise, electron-rich, hydrogen

economy, hydrogenation etc.

HYDROGEN

Hydrogen, the most abundant element in the universe and the

third most abundant on the surface of the globe, is being

visualised as the major future source of energy.

Hydrogen has the simplest atomic structure among all the

elements around us in Nature. In atomic form it consists

of only one proton and one electron. However, in elemental

form it exists as a diatomic (H

) molecule and is called

dihydrogen. It forms more compounds than any other

element. Do you know that the global concern related to

energy can be overcome to a great extent by the use of

hydrogen as a source of energy? In fact, hydrogen is of

great industrial importance as you will learn in this unit.

9.1 POSITION OF HYDROGEN IN THE PERIODIC

TABLE

Hydrogen is the first element in the periodic table.

However, its placement in the periodic table has been a

subject of discussion in the past. As you know by now

that the elements in the periodic table are arranged

according to their electronic configurations.

Hydrogen has electronic configuration 1s

. On one

hand, its electronic configuration is similar to the outer

electronic configuration (ns

) of alkali metals , which belong

to the first group of the periodic table. On the other hand,

like halogens (with ns

configuration belonging to the

seventeenth group of the periodic table), it is short by one

electron to the corresponding noble gas configuration,

helium (1s

). Hydrogen, therefore, has resemblance to

alkali metals, which lose one electron to form unipositive

ions, as well as with halogens, which gain one electron to

form uninegative ion. Like alkali metals, hydrogen forms

oxides, halides and sulphides. However, unlike alkali

metals, it has a very high ionization enthalpy and does not

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285HYDROGEN

possess metallic characteristics under normal

conditions. In fact, in terms of ionization

enthalpy, hydrogen resembles more

with halogens,

∆

H of Li is 520 kJ mol

–1

, F is

1680 kJ mol

–1

and that of H is 1312 kJ mol

–1

Like halogens, it forms a diatomic molecule,

combines with elements to form hydrides and

a large number of covalent compounds.

However, in terms of reactivity, it is very low as

compared to halogens.

Inspite of the fact that hydrogen, to a

certain extent resembles both with alkali

metals and halogens, it differs from them as

well. Now the pertinent question arises as

where should it be placed in the periodic table?

Loss of the electron from hydrogen atom

results in nucleus (H

) of ~1.5×10

–3

pm size.

This is extremely small as compared to normal

atomic and ionic sizes of 50 to 200pm. As a

consequence, H

does not exist freely and is

always associated with other atoms or

molecules. Thus, it is unique in behaviour and

is, therefore, best placed separately in the

periodic table (Unit 3).

9.2 DIHYDROGEN, H

9.2.1 Occurrence

Dihydrogen is the most abundant element in

the universe (70% of the total mass of the

universe) and is the principal element in the

Property Hydrogen Deuterium Tritium

Relative abundance (%) 99.985 0.0156 10

–15

Relative atomic mass (g mol

–1

) 1.008 2.014 3.016

Melting point / K 13.96 18.73 20.62

Boiling point/ K 20.39 23.67 25.0

Density / gL

–1

0.09 0.18 0.27

Enthalpy of fusion/kJ mol

–1

0.117 0.197 -

Enthalpy of vaporization/kJ mol

–1

0.904 1.226 -

Enthalpy of bond

dissociation/kJ mol

–1

at 298.2K 435.88 443.35 -

Internuclear distance/pm 74.14 74.14 -

Ionization enthalpy/kJ mol

–1

1312 - -

Electron gain enthalpy/kJ mol

–1

–73 - -

Covalent radius/pm 37 - -

Ionic radius(H

–

)/pm 208

solar atmosphere. The giant planets Jupiter

and Saturn consist mostly of hydrogen.

However, due to its light nature, it is much less

abundant (0.15% by mass) in the earth’s

atmosphere. Of course, in the combined form

it constitutes 15.4% of the earth's crust and

the oceans. In the combined form besides in

water, it occurs in plant and animal tissues,

carbohydrates, proteins, hydrides including

hydrocarbons and many other compounds.

9.2.2 Isotopes of Hydrogen

Hydrogen has three isotopes: protium,

deuterium,

H or D and tritium,

H or T. Can

you guess how these isotopes differ from each

other ? These isotopes differ from one another

in respect of the presence of neutrons. Ordinary

hydrogen, protium, has no neutrons,

deuterium (also known as heavy hydrogen) has

one and tritium has two neutrons in the

nucleus. In the year 1934, an American

scientist, Harold C. Urey, got Nobel Prize for

separating hydrogen isotope of mass number

2 by physical methods.

The predominant form is protium.

Terrestrial hydrogen contains 0.0156% of

deuterium mostly in the form of HD. The

tritium concentration is about one atom per

atoms of protium. Of these isotopes, only

tritium is radioactive and emits low energy

–

particles (t

, 12.33 years).

Table 9.1 Atomic and Physical Properties of Hydrogen

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286 CHEMISTRY

Since the isotopes have the same electronic

configuration, they have almost the same

chemical properties. The only difference is in

their rates of reactions, mainly due to their

different enthalpy of bond dissociation (Table

9.1). However, in physical properties these

isotopes differ considerably due to their large

mass differences.

9.3 PREPARATION OF DIHYDROGEN, H

There are a number of methods for preparing

dihydrogen from metals and metal hydrides.

9.3.1 Laboratory Preparation of

Dihydrogen

(i) It is usually prepared by the reaction of

granulated zinc with dilute hydrochloric

acid.

Zn + 2H

→ Zn

+ H

(ii) It can also be prepared by the reaction of

zinc with aqueous alkali.

Zn + 2NaOH

→ Na

ZnO

+ H

Sodium zincate

9.3.2 Commercial Production of

Dihydrogen

The commonly used processes are outlined

below:

(i) Electrolysis of acidified water using

platinum electrodes gives hydrogen.

(

)

(

)

(

)

Electrolysis

Traces of acid / base

2H O l 2Hg Og

→ +

(ii) High purity (>99.95%) dihydrogen is

obtained by electrolysing warm aqueous

barium hydroxide solution between nickel

electrodes.

(iii) It is obtained as a byproduct in the

manufacture of sodium hydroxide and

chlorine by the electrolysis of brine

solution. During electrolysis, the reactions

that take place are:

at anode: 2Cl

–

(aq)

→ Cl

(g) + 2e

–

at cathode: 2H

O (l) + 2e

–

→ H

(g) + 2OH

–

(aq)

The overall reaction is

2Na

(aq) + 2Cl

–

(aq) + 2H

O(l)

↓

(g) + H

(g) + 2Na

(aq) + 2OH

–

(aq)

(iv) Reaction of steam on hydrocarbons or coke

at high temperatures in the presence of

catalyst yields hydrogen.

+ → + +

1270K

n 2n 2 2 2

C H nH O nCO (2n 1)H

e.g.,

(

)

(

)

(

)

(

)

1270K

42 2

CH g H O g CO g 3H g

+ → +

The mixture of CO and H

is called water

gas. As this mixture of CO and H

is used for

the synthesis of methanol and a number of

hydrocarbons, it is also called synthesis gas

or 'syngas'. Nowadays 'syngas' is produced

from sewage, saw-dust, scrap wood,

newspapers etc. The process of producing

'syngas' from coal is called 'coal gasification'.

(

)

(

)

(

)

(

)

1270K

2 2

C s HO g CO g H g

+ → +

The production of dihydrogen can be

increased by reacting carbon monoxide of

syngas mixtures with steam in the presence of

iron chromate as catalyst.

(

)

(

)

(

)

(

)

673 K

2 22

catalyst

COg HOg COg Hg

+    → +

This is called water-gas shift reaction.

Carbon dioxide is removed by scrubbing with

sodium arsenite solution.

Presently ~77% of the industrial

dihydrogen is produced from petro-chemicals,

18% from coal, 4% from electrolysis of aqueous

solutions and 1% from other sources.

9.4 PROPERTIES OF DIHYDROGEN

9.4.1 Physical Properties

Dihydrogen is a colourless, odourless,

tasteless, combustible gas. It is lighter than

air and insoluble in water. Its other physical

properties alongwith those of deuterium are

given in Table 9.1.

9.4.2 Chemical Properties

The chemical behaviour of dihydrogen (and for

that matter any molecule) is determined, to a

large extent, by bond dissociation enthalpy.

The H–H bond dissociation enthalpy is the

highest for a single bond between two atoms

of any element. What inferences would you

draw from this fact ? It is because of this factor

that the dissociation of dihydrogen into its

atoms is only ~0.081% around 2000K which

increases to 95.5% at 5000K. Also, it is

relatively inert at room temperature due to the

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287HYDROGEN

high H–H bond enthalpy. Thus, the atomic

hydrogen is produced at a high temperature

in an electric arc or under ultraviolet

radiations. Since its orbital is incomplete with

electronic configuration, it does combine

with almost all the elements. It accomplishes

reactions by (i) loss of the only electron to

give H

, (ii) gain of an electron to form H

–

, and

(iii) sharing electrons to form a single covalent bond.

The chemistry of dihydrogen can be

illustrated by the following reactions:

Reaction with halogens: It reacts with

halogens, X

to give hydrogen halides, HX,

(

)

(

)

(

)

2 2

H g X g 2HX g (X F,Cl, Br,I)

+→ =

While the reaction with fluorine occurs even in

the dark, with iodine it requires a catalyst.

Reaction with dioxygen: It reacts with

dioxygen to form water. The reaction is highly

exothermic.

(g) + O

(g) 2H

O(l);

∆



= –285.9 kJ mol

–1

Reaction with dinitrogen: With dinitrogen

it forms ammonia.

(

)

(

)

(

)

673 K ,200atm

22 3

3H g N g 2NH g ;

92.6 kJ mol

−

+ →

∆ =−H



This is the method for the manufacture of

ammonia by the Haber process.

Reactions with metals: With many metals it

combines at a high temperature to yield the

corresponding hydrides (section 9.5)

(g) +2M(g) → 2MH(s);

where M is an alkali metal

Reactions with metal ions and metal

oxides: It reduces some metal ions in aqueous

solution and oxides of metals (less active than

iron) into corresponding metals.

(

)

(

)

(

)

(

)

() () () ()

2 xy 2

H g Pd aq Pd s 2H aq

yH g M O s xM s yH O l

+ +

+ →+

Reactions with organic compounds: It

reacts with many organic compounds in the

presence of catalysts to give useful

hydrogenated products of commercial

importance. For example :

(i) Hydrogenation of vegetable oils using

nickel as catalyst gives edible fats

(margarine and vanaspati ghee)

(ii) Hydroformylation of olefins yields

aldehydes which further undergo

reduction to give alcohols.

2 2 22

H CO RCH CH RCH CH CHO

++ = →

2 22 222

H RCH CH CHO RCH CH CH OH

+ →

Problem 9.1

Comment on the reactions of dihydrogen

with (i) chlorine, (ii) sodium, and (iii)

copper(II) oxide

Solution

(i) Dihydrogen reduces chlorine into

chloride (Cl

–

) ion and itself gets oxidised

to H

ion by chlorine to form hydrogen

chloride. An electron pair is shared

between H and Cl leading to the formation

of a covalent molecule.

(ii) Dihydrogen is reduced by sodium to

form NaH. An electron is transferred from

Na to H leading to the formation of an ionic

compound, Na

–

(iii) Dihydrogen reduces copper(II) oxide

to copper in zero oxidation state and itself

gets oxidised to H

O, which is a covalent

molecule.

9.4.3 Uses of Dihydrogen

• The largest single use of dihydrogen is in

the synthesis of ammonia which is used in

the manufacture of nitric acid and

nitrogenous fertilizers.

• Dihydrogen is used in the manufacture of

vanaspati fat by the hydrogenation of

polyunsaturated vegetable oils like

soyabean, cotton seeds etc.

• It is used in the manufacture of bulk

organic chemicals, particularly methanol.

(

)

(

)

(

)

cobalt

catalyst

CO g 2H g CH OH l

+ →

• It is widely used for the manufacture of

metal hydrides (Section 9.5)

• It is used for the preparation of hydrogen

chloride, a highly useful chemical.

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288 CHEMISTRY

• In metallurgical processes, it is used to

reduce heavy metal oxides to metals.

• Atomic hydrogen and oxy-hydrogen

torches find use for cutting and welding

purposes. Atomic hydrogen atoms

(produced by dissociation of dihydrogen

with the help of an electric arc) are allowed

to recombine on the surface to be welded

to generate the temperature of 4000 K.

• It is used as a rocket fuel in space research.

• Dihydrogen is used in fuel cells for

generating electrical energy. It has many

advantages over the conventional fossil

fuels and electric power. It does not produce

any pollution and releases greater energy

per unit mass of fuel in comparison to

gasoline and other fuels.

9.5 HYDRIDES

Dihydrogen, under certain reaction conditions,

combines with almost all elements, except

noble gases, to form binary compounds, called

hydrides. If ‘E’ is the symbol of an element then

hydride can be expressed as EH

(e.g., MgH

)

(e.g., B

The hydrides are classified into three

categories :

(i) Ionic or saline or saltlike hydrides

(ii) Covalent or molecular hydrides

(iii) Metallic or non-stoichiometric hydrides

9.5.1 Ionic or Saline Hydrides

These are stoichiometric compounds of

dihydrogen formed with most of the s-block

elements which are highly electropositive in

character. However, significant covalent

character is found in the lighter metal hydrides

such as LiH, BeH

and MgH

. In fact BeH

and

MgH

are polymeric in structure. The ionic

hydrides are crystalline, non-volatile and non-

conducting in solid state. However, their melts

conduct electricity and on electrolysis liberate

dihydrogen gas at anode, which confirms the

existence of H

–

ion.

(

)

(

)

anode–

2Hmelt Hg 2e

−

   → +

Saline hydrides react violently with water

producing dihydrogen gas.

(

)

(

)

(

)

(

)

2 2

NaH s H O aq NaOH aq H g

+→ +

Lithium hydride is rather unreactive at

moderate temperatures with O

or Cl

. It is,

therefore, used in the synthesis of other useful

hydrides, e.g.,

8LiH + Al

→ 2LiAlH

+ 6LiCl

2LiH + B

→ 2LiBH

9.5.2 Covalent or Molecular Hydride

Dihydrogen forms molecular compounds with

most of the p-block elements. Most familiar

examples are CH

, NH

, H

O and HF. For

convenience hydrogen compounds of non-

metals have also been considered as hydrides.

Being covalent, they are volatile compounds.

Molecular hydrides are further classified

according to the relative numbers of electrons

and bonds in their Lewis structure into :

(i) electron-deficient, (ii) electron-precise,

and (iii) electron-rich hydrides.

An electron-deficient hydride, as the name

suggests, has too few electrons for writing its

conventional Lewis structure. Diborane (B

)

is an example. In fact all elements of group 13

will form electron-deficient compounds. What

do you expect from their behaviour? They act

as Lewis acids i.e., electron acceptors.

Electron-precise compounds have the

required number of electrons to write their

conventional Lewis structures. All elements of

group 14 form such compounds (e.g., CH

)

which are tetrahedral in geometry.

Electron-rich hydrides have excess

electrons which are present as lone pairs.

Elements of group 15-17 form such

compounds. (NH

has 1- lone pair, H

O – 2

and HF –3 lone pairs). What do you expect from

the behaviour of such compounds ? They will

behave as Lewis bases i.e., electron donors. The

presence of lone pairs on highly electronegative

atoms like N, O and F in hydrides results in

hydrogen bond formation between the

molecules. This leads to the association of

molecules.

Problem 9.2

Would you expect the hydrides of N, O

and F to have lower boiling points than

the hydrides of their subsequent group

members ? Give reasons.

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289HYDROGEN

Solution

On the basis of molecular masses of NH

O and HF, their boiling points are

expected to be lower than those of the

subsequent group member hydrides.

However, due to higher electronegativity

of N, O and F, the magnitude of hydrogen

bonding in their hydrides will be quite

appreciable. Hence, the boiling points

, H

O and HF will be higher than the

hydrides of their subsequent group

members.

9.5.3 Metallic or Non-stoichiometric

(or Interstitial ) Hydrides

These are formed by many d-block and f-block

elements. However, the metals of group 7, 8

and 9 do not form hydride. Even from group

6, only chromium forms CrH. These hydrides

conduct heat and electricity though not as

efficiently as their parent metals do. Unlike

saline hydrides, they are almost always non-

stoichiometric, being deficient in hydrogen. For

example, LaH

2.87

, YbH

2.55

, T iH

1.5–1.8

, ZrH

1.3–1.75

0.56

, NiH

0.6–0.7

, PdH

0.6–0.8

etc. In such

hydrides, the law of constant composition does

not hold good.

Earlier it was thought that in these

hydrides, hydrogen occupies interstices in the

metal lattice producing distortion without any

change in its type. Consequently, they were

termed as interstitial hydrides. However, recent

studies have shown that except for hydrides

of Ni, Pd, Ce and Ac, other hydrides of this class

have lattice different from that of the parent

metal. The property of absorption of hydrogen

on transition metals is widely used in catalytic

reduction / hydrogenation reactions for the

preparation of large number of compounds.

Some of the metals (e.g., Pd, Pt) can

accommodate a very large volume of hydrogen

and, therefore, can be used as its storage

media. This property has high potential for

hydrogen storage and as a source of energy.

Problem 9.3

Can phosphorus with outer electronic

configuration 3s

form PH

Solution

Although phosphorus exhibits +3 and +5

oxidation states, it cannot form PH

Besides some other considerations, high

∆

H value of dihydrogen and ∆

H value

of hydrogen do not favour to exhibit the

highest oxidation state of P, and

consequently the formation of PH

9.6 WATER

A major part of all living organisms is made

up of water. Human body has about 65% and

some plants have as much as 95% water. It is

a crucial compound for the survival of all life

forms. It is a solvent of great importance. The

distribution of water over the earth’s surface

is not uniform. The estimated world water

supply is given in Table 9.2

Table 9.2 Estimated World Water Supply

Source % of Total

Oceans 97.33

Saline lakes and inland seas 0.008

Polar ice and glaciers 2.04

Ground water 0.61

Lakes 0.009

Soil moisture 0.005

Atmospheric water vapour 0.001

Rivers 0.0001

9.6.1 Physical Properties of Water

It is a colourless and tasteless liquid. Its

physical properties are given in Table 9.3 along

with the physical properties of heavy water.

The unusual properties of water in the

condensed phase (liquid and solid states) are

due to the presence of extensive hydrogen

bonding between water molecules. This leads

to high freezing point, high boiling point, high

heat of vaporisation and high heat of fusion in

comparison to H

S and H

Se. In comparison

to other liquids, water has a higher specific

heat, thermal conductivity, surface tension,

dipole moment and dielectric constant, etc.

These properties allow water to play a key role

in the biosphere.

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290 CHEMISTRY

The high heat of vaporisation and heat

capacity are responsible for moderation of the

climate and body temperature of living beings.

It is an excellent solvent for transportation of

ions and molecules required for plant and

animal metabolism. Due to hydrogen bonding

with polar molecules, even covalent

compounds like alcohol and carbohydrates

dissolve in water.

9.6.2 Structure of Water

In the gas phase water is a bent molecule with

a bond angle of 104.5°, and O–H bond length

of 95.7 pm as shown in Fig 9.1(a). It is a highly

Property H

O D

Molecular mass (g mol

–1

) 18.0151 20.0276

Melting point/K 273.0 276.8

Boiling point/K 373.0 374.4

Enthalpy of formation/kJ mol

–1

–285.9 –294.6

Enthalpy of vaporisation (373K)/kJ mol

–1

40.66 41.61

Enthalpy of fusion/kJ mol

–1

6.01 -

Temp of max. density/K 276.98 284.2

Density (298K)/g cm

–3

1.0000 1.1059

Viscosity/centipoise 0.8903 1.107

Dielectric constant/C

/N.m

78.39 78.06

Electrical conductivity (293K/ohm

–1

) 5.7×10

–8

Table 9.3 Physical Properties of H

O and D

polar molecule, (Fig 9.1(b)). Its orbital overlap

picture is shown in Fig. 9.1(c). In the liquid

phase water molecules are associated together

by hydrogen bonds.

The crystalline form of water is ice. At

atmospheric pressure ice crystallises in the

hexagonal form, but at very low temperatures

it condenses to cubic form. Density of ice is

less than that of water. Therefore, an ice cube

floats on water. In winter season ice formed

on the surface of a lake provides thermal

insulation which ensures the survival of the

aquatic life. This fact is of great ecological

significance.

9.6.3 Structure of Ice

Ice has a highly ordered three dimensional

hydrogen bonded structure as shown in

Fig. 9.2. Examination of ice crystals with

Fig. 9.1 (a) The bent structure of water; (b) the

water molecule as a dipole and

molecule.

Fig. 9.2 The structure of ice

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291HYDROGEN

X-rays shows that each oxygen atom is

surrounded tetrahedrally by four other oxygen

atoms at a distance of 276 pm.

Hydrogen bonding gives ice a rather open

type structure with wide holes. These holes can

hold some other molecules of appropriate size

interstitially.

9.6.4 Chemical Properties of Water

Water reacts with a large number of

substances. Some of the important reactions

are given below.

(1) Amphoteric Nature: It has the ability to

act as an acid as well as a base i.e., it behaves

as an amphoteric substance. In the Brönsted

sense it acts as an acid with NH

and a base

with H

(

)

(

)

(

)

(

)

–

H O l NH aq OH aq NH aq



(

)

(

)

(

)

(

)

–

22 3

H O l H S aq H O aq HS aq

+ +



The auto-protolysis (self-ionization) of water

takes place as follows :

(

)

(

)

(

)

(

)

–

22 3

HO l HO l HO aq OH aq

+ +



acid-1 base-2 acid-2 base-1

(acid) (base) (conjugate (conjugate

acid) base)

(2) Redox Reactions Involving Water: Water

can be easily reduced to dihydrogen by highly

electropositive metals.

(

)

(

)

(

)

(

)

2 2

2H O l 2Na s 2NaOH aq H g

+→ +

Thus, it is a great source of dihydrogen.

Water is oxidised to O

during photosynthesis.

6CO

(g) + 12H

O(l) → C

(aq) + 6H

O(l)

+ 6O

(g)

With fluorine also it is oxidised to O

(g) + 2H

O(l) → 4H

(aq) + 4F

–

(aq) + O

(g)

(3) Hydrolysis Reaction: Due to high

dielectric constant, it has a very strong

hydrating tendency. It dissolves many ionic

compounds. However, certain covalent and

some ionic compounds are hydrolysed in water.

(

)

(

)

(

)

4 10 2 34

P O s 6H O l 4H PO aq

+→

(

)

(

)

(

)

(

)

4 2 2

SiCl l 2H O l SiO s 4HCl aq

+ →+

(

)

(

)

(

)

(

)

N s 3H O l NH g 3OH aq

− −

+ →+

(4) Hydrates Formation: From aqueous

solutions many salts can be crystallised as

hydrated salts. Such an association of water

is of different types viz.,

(i) coordinated water e.g.,

()

–

Cr H O 3Cl





(ii) interstitial water e.g.,

BaCl .2H O

(iii) hydrogen-bonded water e.g.,

()

2–

2 42

CuHO SO .HO





CuSO .5H O,

Problem 9.4

How many hydrogen-bonded water

molecule(s) are associated in

CuSO

.5H

Solution

Only one water molecule, which is outside

the brackets (coordination sphere), is

hydrogen-bonded. The other four

molecules of water are coordinated.

9.6.5 Hard and Soft Water

Rain water is almost pure (may contain some

dissolved gases from the atmosphere). Being a

good solvent, when it flows on the surface of

the earth, it dissolves many salts. Presence of

calcium and magnesium salts in the form of

hydrogencarbonate, chloride and sulphate in

water makes water ‘hard’. Hard water does

not give lather with soap. Water free from

soluble salts of calcium and magnesium is

called Soft water. It gives lather with soap

easily.

Hard water forms scum/precipitate with

soap. Soap containing sodium stearate

COONa) reacts with hard water to

precipitate out Ca/Mg stearate.

(

)

(

)

( ) ()

17 35

2C H COONa aq M aq

C H COO M 2Na aq ; M is Ca / Mg

+→

↓+

It is, therefore, unsuitable for laundry. It is

harmful for boilers as well, because of

deposition of salts in the form of scale. This

reduces the efficiency of the boiler. The

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292 CHEMISTRY

hardness of water is of two types: (i) temporary

hardness, and (ii) permanent hardness.

9.6.6 Temporary Hardness

Temporary hardness is due to the presence of

magnesium and calcium hydrogen-

carbonates. It can be removed by :

(i) Boiling: During boiling, the soluble

Mg(HCO

)

is converted into insoluble Mg(OH)

and Ca(HCO

)

is changed to insoluble CaCO

It is because of high solubility product of

Mg(OH)

as compared to that of MgCO

, that

Mg(OH)

is precipitated. These precipitates can

be removed by filtration. Filtrate thus obtained

will be soft water.

(

)

(

)

Heating

3 2

Mg HCO Mg OH 2CO

→ ↓ + ↑

(

)

Heating

3 32 2

Ca HCO CaCO H O CO

   → ↓ + + ↑

(ii) Clark’s method: In this method calculated

amount of lime is added to hard water. It

precipitates out calcium carbonate and

magnesium hydroxide which can be filtered off.

(

)

(

)

Ca HCO Ca OH 2CaCO 2H O

+ → ↓+

(

)

(

)

()

3 3

Mg HCO 2Ca OH 2CaCO

Mg OH 2H O

→↓

+ ↓+

9.6.7 Permanent Hardness

It is due to the presence of soluble salts of

magnesium and calcium in the form of

chlorides and sulphates in water. Permanent

hardness is not removed by boiling. It can be

removed by the following methods:

(i) Treatment with washing soda (sodium

carbonate): Washing soda reacts with soluble

calcium and magnesium chlorides and

sulphates in hard water to form insoluble

carbonates.

2 23 3

4 23 3 24

MCl Na CO MCO 2NaCl

(M Mg, Ca)

MSO Na CO MCO Na SO

+ → ↓+

(ii) Calgon’s method: Sodium hexameta-

phosphate (Na

), commercially called

‘calgon’, when added to hard water, the

following reactions take place.

[ ]

2–

6 6 18 4 6 18

2 2

4 6 18 2 6 18

Na P O 2Na Na P O

(M Mg, Ca)

M Na P O Na MP O 2Na

−

+ − +

→+

+→ +

The complex anion keeps the Mg

and Ca

ions in solution.

(iii) Ion-exchange method: This method is

also called zeolite/permutit process. Hydrated

sodium aluminium silicate is zeolite/permutit.

For the sake of simplicity, sodium aluminium

silicate (NaAlSiO

) can be written as NaZ. When

this is added in hard water, exchange reactions

take place.

(

)

(

)

(

)

(

)

2NaZ s M aq MZ s 2Na aq

(M Mg, Ca)

+ +

+ →+

Permutit/zeolite is said to be exhausted

when all the sodium in it is used up. It is

regenerated for further use by treating with an

aqueous sodium chloride solution.

(

)

(

)

(

)

(

)

2 2

MZ s 2NaCl aq 2NaZ s MCl aq

+ →+

(iv) Synthetic resins method: Nowadays

hard water is softened by using synthetic

cation exchangers. This method is more efficient

than zeolite process. Cation exchange resins

contain large organic molecule with - SO

group and are water insoluble. Ion exchange

resin (RSO

H) is changed to RNa by treating it

with NaCl. The resin exchanges Na

ions with

and Mg

ions present in hard water to

make the water soft. Here R is resin anion.

(

)

(

)

(

)

(

)

2RNa s M aq R M s 2Na aq

+ +

+ →+

The resin can be regenerated by adding

aqueous NaCl solution.

Pure de-mineralised (de-ionized) water free

from all soluble mineral salts is obtained by

passing water successively through a cation

exchange (in the H

form) and an anion-

exchange (in the OH

–

form) resins:

(

)

(

)

(

)

(

)

2RH s M aq MR s 2H aq

+ +



In this cation exchange process, H

exchanges

for Na

, Ca

, Mg

and other cations present

in water. This process results in proton release

and thus makes the water acidic. In the anion

exchange process:

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293HYDROGEN

(

)

(

)

(

)

2 2 3

RNH s H O l RNH .OH s

+−



(

)

(

)

(

)

()

–

3 3

RNH .OH s X aq RNH .X s

OH aq

+ − +−

−



–

exchanges for anions like Cl

–

, HCO

–

, SO

2–

etc. present in water. OH

–

ions, thus, liberated

neutralise the H

ions set free in the cation

exchange.

(

)

(

)

(

)

Haq OHaq HOl

+ −

+→

The exhausted cation and anion exchange

resin beds are regenerated by treatment with

dilute acid and alkali solutions respectively.

9.7 HYDROGEN PEROXIDE (H

)

Hydrogen peroxide is an important chemical

used in pollution control treatment of domestic

and industrial effluents.

9.7.1 Preparation

It can be prepared by the following methods.

(i) Acidifying barium peroxide and removing

excess water by evaporation under reduced

pressure gives hydrogen peroxide.

(

)

(

)

(

)

( ) ()

22 24 4

22 2

BaO .8H O s H SO aq BaSO s

H O aq 8H O l

+ → +

(ii) Peroxodisulphate, obtained by electrolytic

oxidation of acidified sulphate solutions at

high current density, on hydrolysis yields

hydrogen peroxide.

(

)

(

)

() () ()

Electrolysis

4 33

Hydrolysis

4 22

2HSO aq HO SOOSO H aq

2HSOaq 2Haq HOaq

−

−+

→

→ ++

This method is now used for the laboratory

preparation of D

(

)

(

)

(

)

(

)

22 8 2 4 22

K S O s 2D O l 2KDSO aq D O l

+→ +

(iii) Industrially it is prepared by the auto-

oxidation of 2-alklylanthraquinols.

()

( )

O air

H /Pd

2 ethylanthraquinol H O

oxidised product

→

− +

←   

In this case 1% H

is formed. It is

extracted with water and concentrated to ~30%

(by mass) by distillation under reduced

pressure. It can be further concentrated to

~85% by careful distillation under low

pressure. The remaining water can be frozen

out to obtain pure H

9.7.2 Physical Properties

In the pure state H

is an almost colourless

(very pale blue) liquid. Its important physical

properties are given in Table 9.4.

is miscible with water in all

proportions and forms a hydrate H

(mp 221K). A 30% solution of H

is marketed

as ‘100 volume’ hydrogen peroxide. It means

that one millilitre of 30% H

solution will give

100 mL of oxygen at STP. Commercially

marketed sample is 10 V, which means that

the sample contains 3% H

Problem 9.5

Calculate the strength of 10 volume

solution of hydrogen peroxide.

Solution

10 volume solution of H

means that

1L of this H

solution will give 10 L of

oxygen at STP

(

)

(

)

(

)

22 2 2

2HO l O g HO l

→+

2×34 g 22.7 L at STP

68 g

On the basis of above equation 22.7 L of

is produced from 68 g H

at STP

10 L of O

at STP is produced from

≈

68 10

g = 29.9 g 30 g H O

22.7

Therefore, strength of H

in 10 volume

solution = 30 g/L = 3% H

solution

Melting point/K 272.4 Density (liquid at 298 K)/g cm

–3

1.44

Boiling point(exrapolated)/K 423 Viscosity (290K)/centipoise 1.25

Vapour pressure(298K)/mmHg 1.9 Dielectric constant (298K)/C

/N m

70.7

Density (solid at 268.5K)/g cm

–3

1.64 Electrical conductivity (298K)/Ω

–1

5.1×10

–8

Table 9.4 Physical Properties of Hydrogen Peroxide

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294 CHEMISTRY

9.7.3 Structure

Hydrogen peroxide has a non-planar

structure. The molecular dimensions in the gas

phase and solid phase are shown in Fig 9.3

Fig. 9.3 (a) H

structure in gas phase, dihedral

angle is 111.5°. (b) H

structure in solid

phase at 110K, dihedral angle is 90.2°.

9.7.4 Chemical Properties

It acts as an oxidising as well as reducing agent

in both acidic and alkaline media. Simple

reactions are described below.

(i) Oxidising action in acidic medium

(

)

(

)

(

)

( ) ()

( ) ( ) ( ) ()

2 +

22 4 2

2Fe aq 2H aq H O aq

Fe aq 2H O l

PbS s 4H O aq PbSO s 4H O l

++ →

+ →+

(ii) Reducing action in acidic medium

– 2

4 22 2 2

22 3 2

2MnO 6H 5H O 2Mn 8H O 5O

HOCl H O H O Cl O

+ +

+−

++ → + +

+ → ++

(iii) Oxidising action in basic medium

2 3

2 4

2Fe H O 2Fe 2OH

Mn H O Mn 2OH

+ + −

+→+

(iv) Reducing action in basic medium

2 22 22

–

4 22 22

–

I H O 2OH 2I 2H O O

2MnO 3H O 2MnO 3O

2H O 2OH

−−

+ + →+ +

+ → ++

9.7.5 Storage

decomposes slowly on exposure to light.

(

)

(

)

(

)

22 2 2

2H O l 2H O l O g

→+

In the presence of metal surfaces or traces of

alkali (present in glass containers), the above

reaction is catalysed. It is, therefore, stored in

wax-lined glass or plastic vessels in dark. Urea

can be added as a stabiliser. It is kept away

from dust because dust can induce explosive

decomposition of the compound.

9.7.6 Uses

Its wide scale use has led to tremendous

increase in the industrial production of H

Some of the uses are listed below:

(i) In daily life it is used as a hair bleach and

as a mild disinfectant. As an antiseptic it is

sold in the market as perhydrol.

(ii) It is used to manufacture chemicals like

sodium perborate and per-carbonate,

which are used in high quality detergents.

(iii) It is used in the synthesis of hydroquinone,

tartaric acid and certain food products and

pharmaceuticals (cephalosporin) etc.

(iv) It is employed in the industries as a

bleaching agent for textiles, paper pulp,

leather, oils, fats, etc.

(v) Nowadays it is also used in Environmental

(Green) Chemistry. For example, in

pollution control treatment of domestic and

industrial effluents, oxidation of cyanides,

restoration of aerobic conditions to sewage

wastes, etc.

9.8 HEAVY WATER, D

It is extensively used as a moderator in nuclear

reactors and in exchange reactions for the

study of reaction mechanisms. It can be

prepared by exhaustive electrolysis of water or

as a by-product in some fertilizer industries.

Its physical properties are given in Table 9.3.

It is used for the preparation of other deuterium

compounds, for example:

(

)

()

22 22

32 24

43 2 4

CaC 2D O C D Ca OD

SO D O D SO

Al C 12D O 3CD 4Al OD

+ →+

+→

+ →+

9.9 DIHYDROGEN AS A FUEL

Dihydrogen releases large quantities of heat on

combustion. The data on energy released by

combustion of fuels like dihydrogen, methane,

LPG etc. are compared in terms of the same

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295HYDROGEN

amounts in mole, mass and volume, are shown

in Table 9.5.

From this table it is clear that on a mass

for mass basis dihydrogen can release more

energy than petrol (about three times).

Moreover, pollutants in combustion of

dihydrogen will be less than petrol. The only

pollutants will be the oxides of dinitrogen (due

to the presence of dinitrogen as impurity with

dihydrogen). This, of course, can be minimised

by injecting a small amount of water into the

cylinder to lower the temperature so that the

reaction between dinitrogen and dioxygen may

not take place. However, the mass of the

containers in which dihydrogen will be kept

must be taken into consideration. A cylinder

of compressed dihydrogen weighs about 30

times as much as a tank of petrol containing

the same amount of energy. Also, dihydrogen

gas is converted into liquid state by cooling to

20K. This would require expensive insulated

tanks. Tanks of metal alloy like NaNi

, T i–TiH

Mg–MgH

etc. are in use for storage of

dihydrogen in small quantities. These

Table 9.5 The Energy Released by Combustion of Various Fuels in Moles, Mass and Volume

Energy released on Dihydrogen Dihydrogen LPG CH

gas Octane

combustion in kJ (in gaseous (in liquid) (in liquid

state) state) state)

per mole 286 285 2220 880 5511

per gram 143 142 50 53 47

per litre 12 9968 25590 35 34005

limitations have prompted researchers to

search for alternative techniques to use

dihydrogen in an efficient way.

In this view Hydrogen Economy is an

alternative. The basic principle of hydrogen

economy is the transportation and storage of

energy in the form of liquid or gaseous

dihydrogen. Advantage of hydrogen economy

is that energy is transmitted in the form of

dihydrogen and not as electric power. It is for

the first time in the history of India that a pilot

project using dihydrogen as fuel was launched

in October 2005 for running automobiles.

Initially 5% dihydrogen has been mixed in

CNG for use in four-wheeler vehicles. The

percentage of dihydrogen would be gradually

increased to reach the optimum level.

Nowadays, it is also used in fuel cells for

generation of electric power. It is expected that

economically viable and safe sources of

dihydrogen will be identified in the years to

come, for its usage as a common source of

energy.

SUMMARY

Hydrogen is the lightest atom with only one electron. Loss of this electron results in an

elementary particle, the proton. Thus, it is unique in character. It has three isotopes,

namely : protium (

H), deuterium (D or

H) and tritium (T or

H). Amongst these three,

only tritium is radioactive. Inspite of its resemblance both with alkali metals and halogens,

it occupies a separate position in the periodic table because of its unique properties.

Hydrogen is the most abundant element in the universe. In the free state it is almost

not found in the earth’s atmosphere. However, in the combined state, it is the third most

abundant element on the earth’s surface.

Dihydrogen on the industrial scale is prepared by the water-gas shift reaction from

petrochemicals. It is obtained as a byproduct by the electrolysis of brine.

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296 CHEMISTRY

The H–H bond dissociation enthalpy of dihydrogen (435.88 kJ mol

–1

) is the highest

for a single bond between two atoms of any elements. This property is made use of in the

atomic hydrogen torch which generates a temperature of ~4000K and is ideal for welding

of high melting metals.

Though dihydrogen is rather inactive at room temperature because of very high

negative dissociation enthalpy, it combines with almost all the elements under appropriate

conditions to form hydrides. All the type of hydrides can be classified into three categories:

ionic or saline hydrides, covalent or molecular hydrides and metallic or non-stoichiometric

hydrides. Alkali metal hydrides are good reagents for preparing other hydride compounds.

Molecular hydrides (e.g., B

, CH

, NH

, H

O) are of great importance in day-to-day life.

Metallic hydrides are useful for ultrapurification of dihydrogen and as dihydrogen storage

media.

Among the other chemical reactions of dihydrogen, reducing reactions leading to

the formation hydrogen halides, water, ammonia, methanol, vanaspati ghee, etc. are of

great importance. In metallurgical process, it is used to reduce metal oxides. In space

programmes, it is used as a rocket fuel. In fact, it has promising potential for use as a

non-polluting fuel of the near future (Hydrogen Economy).

Water is the most common and abundantly available substance. It is of a great

chemical and biological significance. The ease with which water is transformed from

liquid to solid and to gaseous state allows it to play a vital role in the biosphere. The

water molecule is highly polar in nature due to its bent structure. This property leads to

hydrogen bonding which is the maximum in ice and least in water vapour. The polar

nature of water makes it: (a) a very good solvent for ionic and partially ionic compounds;

(b) to act as an amphoteric (acid as well as base) substance; and (c) to form hydrates of

different types. Its property to dissolve many salts, particularly in large quantity, makes

it hard and hazardous for industrial use. Both temporary and permanent hardness can

be removed by the use of zeolites, and synthetic ion-exchangers.

Heavy water, D

O is another important compound which is manufactured by the

electrolytic enrichment of normal water. It is essentially used as a moderator in nuclear

reactors.

Hydrogen peroxide, H

has an interesting non-polar structure and is widely used

as an industrial bleach and in pharmaceutical and pollution control treatment of

industrial and domestic effluents.

EXERCISES

9.1 Justify the position of hydrogen in the periodic table on the basis of its electronic

configuration.

9.2 Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes?

9.3 Why does hydrogen occur in a diatomic form rather than in a monoatomic form

under normal conditions?

9.4 How can the production of dihydrogen, obtained from ‘coal gasification’, be

increased?

9.5 Describe the bulk preparation of dihydrogen by electrolytic method. What is the

role of an electrolyte in this process ?

9.6 Complete the following reactions:

(i)

() ()

2 mo

H g MO s

∆

  →

(ii)

() ()

catalyst

CO g + H g

∆

   →

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297HYDROGEN

(iii)

() ()

38 2

catalyst

C H g 3H O g

∆

+ →

(iv)

() ( )

thea

Zn s NaOH aq

+    →

9.7 Discuss the consequences of high enthalpy of H–H bond in terms of chemical

reactivity of dihydrogen.

9.8 What do you understand by (i) electron-deficient, (ii) electron-precise, and (iii)

electron-rich compounds of hydrogen? Provide justification with suitable examples.

9.9 What characteristics do you expect from an electron-deficient hydride with respect

to its structure and chemical reactions?

9.10 Do you expect the carbon hydrides of the type (C

2n + 2

) to act as ‘Lewis’ acid or

base? Justify your answer.

9.11 What do you understand by the term “non-stoichiometric hydrides”? Do you

expect this type of the hydrides to be formed by alkali metals? Justify your answer.

9.12 How do you expect the metallic hydrides to be useful for hydrogen storage?

Explain.

9.13 How does the atomic hydrogen or oxy-hydrogen torch function for cutting and

welding purposes ? Explain.

9.14 Among NH

, H

O and HF, which would you expect to have highest magnitude of

hydrogen bonding and why?

9.15 Saline hydrides are known to react with water violently producing fire. Can CO

a well known fire extinguisher, be used in this case? Explain.

9.16 Arrange the following

(i) CaH

, BeH

and TiH

in order of increasing electrical conductance.

(ii) LiH, NaH and CsH in order of increasing ionic character.

(iii) H–H, D–D and F–F in order of increasing bond dissociation enthalpy.

(iv) NaH, MgH

and H

O in order of increasing reducing property.

9.17 Compare the structures of H

O and H

9.18 What do you understand by the term ’auto-protolysis’ of water? What is its

significance?

9.19 Consider the reaction of water with F

and suggest, in terms of oxidation and

reduction, which species are oxidised/reduced.

9.20 Complete the following chemical reactions.

(i)

(

)

(

)

Pb S s H O aq

+ →

(ii)

(

)

(

)

–

4 22

MnO aq H O aq

+ →

(iii)

(

)

(

)

CaO s H O g

+→

(v)

(

)

(

)

3 2

AlCl g H O l

+→

(vi)

(

)

(

)

32 2

Ca N s H O l

+→

Classify the above into (a) hydrolysis, (b) redox and (c) hydration reactions.

9.21 Describe the structure of the common form of ice.

9.22 What causes the temporary and permanent hardness of water ?

9.23 Discuss the principle and method of softening of hard water by synthetic ion-

exchange resins.

9.24 Write chemical reactions to show the amphoteric nature of water.

9.25 Write chemical reactions to justify that hydrogen peroxide can function as an

oxidising as well as reducing agent.

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298 CHEMISTRY

9.26 What is meant by ‘demineralised’ water and how can it be obtained ?

9.27 Is demineralised or distilled water useful for drinking purposes? If not, how can

it be made useful?

9.28 Describe the usefulness of water in biosphere and biological systems.

9.29 What properties of water make it useful as a solvent? What types of compound

can it (i) dissolve, and (ii) hydrolyse ?

9.30 Knowing the properties of H

O and D

O, do you think that D

O can be used for

drinking purposes?

9.31 What is the difference between the terms ‘hydrolysis’ and ‘hydration’ ?

9.32 How can saline hydrides remove traces of water from organic compounds?

9.33 What do you expect the nature of hydrides is, if formed by elements of atomic

numbers 15, 19, 23 and 44 with dihydrogen? Compare their behaviour towards

water.

9.34 Do you expect different products in solution when aluminium(III) chloride and

potassium chloride treated separately with (i) normal water (ii) acidified water,

and (iii) alkaline water? Write equations wherever necessary.

9.35 How does H

behave as a bleaching agent?

9.36 What do you understand by the terms:

(i) hydrogen economy (ii) hydrogenation (iii) ‘syngas’ (iv) water-gas shift reaction

(v) fuel-cell ?

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