• Live Webinars (online lectures) with

recordings.

• Online Query Solving

• Online MCQ tests with detailed

solutions

• Online Notes and Solved Exercises

• Career Counseling

3. Thermal stability of hydrides of group 16

This trend can be explained on the basis of bond

energy Δdiss ( M – H ) kJ mol – 1 .

1. The thermal stability decreases in the order

H2O > H2S > H2Se > H2Te.

2. Water dissociates at 2073 – 2273 K while H2S

dissociates only at 873 K. The extra stability of

water is due to hydrogen bonding.

4. Acidic character of hydrides of group 16

M – H bonds present in these hydrides are polar

and dissociates in aqueous solution to produce H+

ions. It combines with

H2O molecule to form

H3O+ ions.

Hence these hydrides

act as acids.

1. The hydrides of group 16 elements are weakly

acidic and behave as weak diprotic acids. In

aqueous medium, they dissociate as

2. The hydrides react with bases to form two types

of salts. For example, H2S reacts with NaOH to

form

2 2 3

2

2 3

H S H O H O HS

HS H O H O S

€

€

3. Due to the decrease in dissociation enthalpy of

bond M – H down the group acidic character

increases. The acid strength of these hydrides

increases in the order

H2O < H2S < H2Se < H2Te

5. Reducing character of hydrides of group 16

1. Except H2O, all other hydrides of elements of

group 16 acts as reducing agents.

2. The reducing character depends upon the

thermal stability. Lesser the thermal stability,

greater is the reducing power of the hydride.

Since H2O is very stable. It does not act as

reducing agent.

3. The other hydrides are relatively less stable

and acts as reducing agents. Since the thermal

stability of hydride decreases

on moving down the

group, the reducing power

of hydride increases from

H2S to H2Te.

Halides Group 16 Elements

Elements of Group 16 form a large number of

halides of the type, EX6, EX4 and EX2 where E is

an element of the group and X is a halogen. The

stability of the halides decreases in the order

F − > Cl − > Br − > I − . Amongst hexahalides,

hexafluorides are the only stable halides. All

hexafluorides are gaseous in nature. They have

octahedral structure.

1. The compounds of oxygen with fluorine are called

fluorides because fluorine is more electronegative

than oxygen. For example, OF2 is named oxygen

difluoride. Since oxygen is more electronegative

than other halogens ( Cl, Br, I ) the compounds of

chlorine, bromine and iodine with oxygen are

called as halogen oxides. For example, CIO2 is

referred as chlorine dioxide and Cl2O7 as chlorine

heptoxide.

2. Sulphur form monohalides, dihalides,

tetrahalides and hexahalides. Sulphur

hexafluoride SF6 is exceptionally stable for

steric reasons. The central atom S in SF6 has

sp3 d2 hybridization.

3. SF4 is gas, SeF4 is liquid while TeF4 is solid.

These fluorides have sp3 d hybridization and

have trigonal bipyramidal structure in which

one of the equatorial positions is occupied by a

lone pair of electrons. This geometry is also

called see – saw geometry.

4. All elements form dichlorides and dibromides.

These dihalides are formed by sp3

hybridization and have tetrahedral structure.

5. The well known monohalides are dimeric in

nature, for example, S2F2, S2Cl2, S2Br2, Se2Cl2

and Se2Br2. These diametric halides undergo

disproportionation as follows

2 2 42Se Cl SeCl 3Se

Reactivity with oxygen

1. All these elements form oxides of type MO2 and

MO3 ( where M = S, Se, Te or Po ).

2. O3 and SO2 are in gases state where as SeO2

(selenium dioxide) is solid.

3. Reducing Property of dioxide decreases from SO2

to TeO2, SO2 is reducing agent while TeO2 is

oxidizing agent.

4. Besides MO2 type MO3 type oxides are also

formed by sulphur (SO3), selenium (SeO3) and

tellurium (TeO3).

5. Both MO2 and MO3 type of oxides are acidic in

nature.

Anomalous behavior of oxygen

Oxygen differs considerably from other elements

of group 16 in several properties. The anomalous

behaviour is due to A ] its small size B ] its

high electronegativity C ] the absence of vacant

d – orbitals in valence shell.

Some important points of difference between

oxygen and other elements of group 16 are as

follows.

1. Physical state :

Oxygen is a gas at ordinary temperature while

other members are solids.

2. Atomicity :

Oxygen is diatomic ( O2 ). While molecules of

other elements of group 16 are polyatomic, for

example, sulphur and selenium form octa-atomic

molecules ( S8 and Se8 ) and have puckered ring

structure.

3. Magnetic behaviour :

Molecular oxygen ( O2 ) is paramagnetic while

other elements are diamagnetic

4. Oxidation states :

Oxygen shows an oxidation state of – 2 in most

of its compounds. Due to absence of vacant d

orbitals it cannot exhibit higher oxidation state.

The other elements of the group can exhibit – 2,

+ 2, + 4 and + 6 oxidation states.

5. Hydrogen bonding :

Due to higher value of electronegativity, oxygen is

capable of forming hydrogen bonding in its

compounds like water, alcohols, carboxylic acids etc.

there elements of group being much less

electronegative do not form hydrogen bonds.

6. Nature of compounds :

The compound of oxygen are more ionic than those of

the other elements of the group. Thus O2 – is very

common but S2 –, Se2 – and Te2 – are less common.

7. Multiple bonds :

Oxygen is capable of forming pπ – pπ multiple

bonds with elements of comparable size like carbon,

nitrogen. The other elements of the group do not

show much tendency to form such multiple bonds.

8. Hydrides :

The hydrides of oxygen, ie H2O is a liquid at room

temperature while the hydrides of all other elements

are gases.

Dioxygen

Oxygen is the most abundant element on the

earth. Oxygen constitutes about 50% by

weight of the earth’s crust. It occurs both in

the free state as well as in the combined state.

In free state, oxygen occurs to an extent of

23.2% by weight ( or 21% by volume ) in

atmospheric air.

In the combined state it is present in water, earth’s

crust and in the tissues of all plants and animals.

Water consists of 88.8% oxygen by weigh.

Almost all the dioxygen present in the

atmosphere is probably due to photosynthesis by

green plants. In a simple form, it can be represented

as

General methods of preparation of oxygen

A. By the thermal decomposition of certain

oxygen rich salts

Certain oxygen rich salts such as chlorates,

nitrates, permagnates, dichromates etc.

2

heat

3 2MnO

heat

3 2 2

heat

4 2 4 2 2

heat

2 2 7 2 2 3 2

heat

4 2

i) 2KClO 2KCl 3O

ii) 2KNO 2KNO O

iii)2KMnO K MnO MnO O

iv)2K Cr O 2K O 2Cr O 3O

v) 2KClO 2KCl 4O

B. By the thermal decomposition of certain

metallic oxides

The oxides of metals like Hg, Ag, Au, Pb etc.

decompose on heating and give dioxygen.

C. By the action of water

Dioxygen can be prepared by treating sodium

peroxide or acidified potassium permanganate

with water.

2 2 2 2

4 2 2 2 4 2 4 4 2 2

2Na O 2H O 4NaOH O

2KMnO 5Na O 8H So K SO 2MnSo 8H O 5O

D. Laboratory method

In the laboratory dioxygen can be prepared

very conveniently by heating a mixture of

KCIO3 (4 parts) and MnO2 ( 1 part ) in a

hard glass tube to about 420 K. Oxygen gas

is be collected by downward displacement

of water.

MnO2 acts as catalyst. In the absence of

manganese dioxide, the thermal

decomposition of KClO3 requires

temperature of 670 K – 720 K.MnO2 lowers

it to about 420K.