3rd Lecture on Elements of groups 16, 17 & 18 Chemistry Part I12th Standard by Rizwana Mohammed

1. The Malegaon High School & Jr. College
Malegaon, (Nashik), 423203
3rd Lecture on Elements of
groups 16, 17 & 18
Chemistry Part I, 12th Science
By
Rizwana Mohammad

2. Allotropy:
Elements of the group 16 exhibit allotropy.
Oxygen has two allotropes O2 and O3.
Sulphur exists in a number of allotropic forms, rhombic and monoclinic
sulphur are the important allotropes of sulphur.
Se exists in two allotropic forms red (non metallic) and grey (metallic)
Te exists in two allotropic forms crystalline and amorphous,
Po exhibits two allotropic forms α & β (both metallic).
Allotropes of sulphur

4. Oxoacids:
Oxoacids of sulphur:
i. Sulphurous acid, H2SO3
ii. Sulphuric acid, H2SO4
O
S O
HO
HO
iii. Di or pyrosulphuric acid, H2S2O7 (Oleum)
S O S OHHO
O
O
O
O
:
HO
S
HO
O

5. iv. Peroxy monosulphuric acid, H2SO5
v. Peroxy disulphuric acid, H2S2O8 (Marshall’s acid)
HO O S OH
O
O
HO S OHOO S
O
O
O
O
vi. Thiosulphuric acid, H2S2O3
HO S OH
S
O

6. Oxoacids of halogens:

7. Structures of oxoacids of chlorine
i. Hypochlorous acid HOCl
O
H Cl
ii. Chlorous acid, HClO2
OO
H Cl
iii. Chloric acid, HClO3
OO
H Cl
O
iv. Perchloric acid, HClO4
OO
H Cl
O
O

8. Oxygen and compounds of oxygen:
Dioxygen
a. Preparation
i. Laboratory methods:
2KClO3(s) 2KCl(s) + 3O2(g)
2Ag2O(s) ՜
Δ
4Ag(s) + O2(g)
2HgO(s) ՜
Δ
2Hg(l) + O2(g)
2PbO2(s) ՜
Δ
2PbO(s) + O2(g)
2H2O2(aq) 2H2O(l) + O2(g)
ii. Electrolysis:
During electrolysis of water H2 is liberated at cathode and oxygen at
anode
2H2O(l)
𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠
2H2(g) + O2(g)
iii. Industrial method:
On fractional distillation of liquid air dioxygen and dinitrogen are
obtained.
Heat
MnO2
Heat
MnO2

9. b. Physical properties:
i. Dioxygen is colourless and odourless gas.
ii. It is sparingly soluble in water.
iii. It liquefies at 90K and freezes at 55K.
iv. Oxygen has three stable isotopes 16O, 17O and 18O.
v. O2 exhibits paramagnetism.
c. Chemical properties:
i. Reaction with metals:
2Ca + O2 → 2CaO
4Al + 3O2 → 2Al2O3
ii. Reaction with nonmetals:
C + O2 → CO2
P4 + 5O2 → P4O10
iii. Reaction with some compound:
2ZnS +3O2 ՜
Δ
2ZnO + 2SO2
CH4 +2O2 → CO2 + 2H2O
2SO2 + O2
V2O5
2SO3
4HCl + O2 2Cl2 + 2H2OCuCl2
Δ

10. Uses:
i. Dioxygen is important for respiration to sustain animal and aquatic
life.
ii. It is used in the manufacturing of steel.
iii. It is used in oxyacetylene flame for welding and cutting of metals.
iv. Oxygen cylinders are widely used in hospitals, high altitude flying
and mountaineering.
v. It is used in combustion of fuels; e.g. hydrazine in liquid oxygen
provides tremendous thrust (energy) in rockets.
Simple oxides:
Oxides can be classified into
a. Acidic oxides
b. Basic oxides
c. Amphoteric oxides
d. Neutral oxides

11. a. Acidic oxides:
An oxide which dissolves in water to give an acid or reacts with a base to give a salt is
called acidic oxide.
e.g. SO2, SO3, CO2, N2O5, Cl2O7 etc.
SO2 + H2O → H2SO3
SO3 + 2NaOH → Na2SO4 + H2O
b. Basic oxides:
An oxide which dissolves in water to give a base or reacts with an acid to give salt is
called basic oxide.
e.g. Na2O, CaO, BaO etc
CaO + H2O → Ca(OH)2
BaO + 2HCl → BaCl2 + H2O
c. Amphoteric oxides:
The oxide which reacts with a base as well as with an acid to give salt is called n
amphoteric oxide.
e.g. Al2O3, ZnO
Al2O3(s) + 6NaOH(aq) + 3H2O(l) → 2Na3[Al(OH)6]
acidic
Al2O3(s) + 6HCl(aq) + 9H2O(l) → 2[Al(H2O)6]3+ + 6Cl-
(aq)
Basic

12. d. Neutral oxides:
The oxides which are neither acidic nor basic, are called as neutral
oxides e.g. CO, NO, N2O etc.
Ozone:
Ozone is an allotrope of oxygen.
Oxygen in the upper atmosphere absorbs energy in the form of ultra
violet light and changes to atomic oxygen, which combines with
molecular oxygen to form O3.
O2
U.V. light
O + O
O2 + O → O3
a. Preparation of ozone:
Ozone is prepared by passing silent electric discharge through pure
and dry oxygen in an apparatus called ozoniser. It is an endothermic
process.
3O2(g) ⇌ 2O3; ΔH = +142 kJ/mol

13. b. Physical properties of ozone:
i. Pure ozone is a pale-blue gas, dark blue liquid and violet-black solid.
ii. Ozone has a characteristic smell. When inhaled in concentration
above 100 ppm, it causes nausea and headache.
iii. It is diamagnetic in nature.
c. Chemical properties:
i. Oxidising properties:
Ozone is a powerful oxidising agents as it easily decomposes to liberated
nascent oxygen
O3 → O2 + O
PbS(s) + 4O3(g) → PbSO4(s) + 4O2(g)
2KI(aq) + H2O(l) + O3(g) → 2KOH(aq) + I2(g) + O2(g)
NO(g) + O3(g) → NO2(g) + O2(g)
ii. Bleaching property:
O3 → O + O2
coloured matter +O → colourless matter.
Ozone bleaches in absence of air so it is also known as dry bleach.

14. iii. Reducing property:
H2O2 + O3 → H2O + 2O2
BaO2 + O3 → BaO + 2O2
iv. Ozone depletion:
Thinning of ozone layer in upper atmosphere is called ozone depletion.
• The ozone (O3) layer in the upper atmosphere, absorbs harmful UV
radiations from the sun, thus protecting people on the earth.
• Depletion of ozone layer in the upper atmosphere is caused by nitrogen
oxide released from exhausts system of car or supersonic jet aeroplanes.
NO(g) + O3(g) → NO2(g) + O2(g).
• Depletion (thinning) of ozone layer can also be caused by chlorofluoro
carbons (freons) used in aerosol and refrigerators and their subsequent
escape into the atmosphere.
• The depletion of ozone layer has been most pronounced in polar regions,
especially over Antarctica.
• Ozone depletion is a major environmental problem because it increases
the amount of ultraviolet (UV) radiation that reaches earth's surface, thus
causing an increase in rate of skin cancer, eye cataracts and genetic as well
as immune system damage among people.

15. d. Structure of ozone:
Ozone is an angular molecule.
The two O-O bond lengths in the ozone molecule are identical, 128 pm and
the O-O-O bond angle of about 117°.
It is resonance hybrid of two forms.
e. Uses of ozone:
Ozone is used for air purification at crowded places like cinema halls, tunnels,
railways, etc.
In sterilizing drinking water by oxidising all germs and bacteria.
For bleaching ivory, oils, starch, wax and delicate fabrics such as silk.
In the manufacture of synthetic camphor, potassium permanganate, etc.