These are Online Notes on "p-Block elements" (Chapter=>7 ) Part 2 for practice of CBSE BOARD, CBSE NEET, CSIR NET Chemical Sciences etc.
19. Dinitrogen: Preparation=>
NH4Cl(aq) + NaNO4 (aq) ==heat==> N (g) + 2H2O(l) + NaCl(aq)
(NH4)2 Cr2O7 ==heat==> N2 + H2O + Cr2O3
Ba(N3)2 ==heat==> Ba + 3N2
Properties:
It is a colouless, odourless, tasteless and non – toxic gas. It is chemically un-reactive at ordinary temperature due to triple bond in N ≡ N which has high bond dissociation energy.
20. Ammonia: Ammonia molecule is trigonal pyramidal with nitrogen atom at the apex. It has 3 bond pairs and 1 lone pair. N is sp3 hybridised.
Preparation:
Haber’s process:
N2(g) + 3H2(g) ==> 2NH3(g) ΔHo = -46.1 kJmol-1
Pressure = 200 x 105 Pa
Temperature 773 K
Catalyst is FeO with small amounts of K2O and Al2O3
21. Nitric Acid:
a. Ostwald Process:
4 NH3 + 5O2 ==Pt/Rhgauge==> 4NO + 6H2O ...(i)
2NO + O2 ==> 2NO2 ...(ii)
3NO2(g) + H2O(l) ==> 2HNO3(aq) + NO(g) ...(iii)
NO thus formed is recycled and the aqueous HNO3 can be concentrated by distillation upto ~ 68% by mass. Further concentration to 98% can be achieved by dehydration with concentrated H2SO4.
Nitric acid is strong oxidizing agent in the concentrated as well as in the dilute state.
22. Phosphorus:
a. It shows the property of catenation to maximum extent due to most stable P – P bond.
b. It has many allotropes, the important ones are :
i. White phosphorus
ii. Red phosphorus
iii. Black phosphorus
White phosphorus
|
Red phosphorus
|
Black phosphorus
|
Discrete tetrahedral P4 molecules
|
Polymeric structure
consisting of chains of P4 units
linked together
|
Exists in two forms - a
black P and b black P
|
Very reactive
|
Less reactive than white P
|
Very less reactive
|
Glows in dark
|
Does not glow in dark
|
-
|
Translucent waxy
solid
|
Has an iron grey luster
|
Has an opaque
monoclinic or rhombohedral crystals
|
Soluble in CS2 but
insoluble in water
|
Insoluble in water as well as CS2
|
-
|
It has low ignition temperature, therefore, kept
under water
|
-
|
-
|
23. Phosphine:
Preparation:
Ca2P2 + 6H2O ==> 3Ca(OH)2 + 2PH3
Calcium water Calcium Phosphine
Phosphide Hydroxide
Ca2P2 + 6HCl ==> 3CaCl2 + 2PH3
Phosphine
ii. P4 + 3NaOH + H2O ==> 3NaH2PO2 + PH3 Sodium Phosphine
Hypophosphite
Phosphine is highly poisonous, colourless gas and has a smell of rotten
fish.
24. Chlorides of Phosphorous:
Calcium water Calcium Phosphine
Phosphide Hydroxide
Ca2P2 + 6HCl ==> 3CaCl2 + 2PH3
Phosphine
ii. P4 + 3NaOH + H2O ==> 3NaH2PO2 + PH3 Sodium Phosphine
Hypophosphite
Phosphine is highly poisonous, colourless gas and has a smell of rotten
fish.
24. Chlorides of Phosphorous:
PCl3
|
PCl5
|
Colourless oily liquid
|
Yellowish white
powder
|
P4 + 6
Cl2 ® 4PCl3
P4 + 8 SOCl2 ® 4PCl3 + 4 SO2 + 2 S2Cl2
|
P4 + 10 Cl2
® 4PCl5
P4 + 10 SO2Cl2 ® 4PCl5 + 10 SO2
|
Is hydrolysed in the presence of
moisture
PCl3 + 3H2O ¾¾¾®H3PO3 + 3HCl
|
PCl5 + H2O ¾¾¾® POCl3
+ 2HCl POCl3 + 3 H2O ¾¾¾® H3PO4
+ 3HCl
|
3 CH3COOH + PCl3 ® 3
CH3COCl + H3PO3
|
3 CH3COOH + PCl5 ® CH3COCl + POCl3
+ HCl
|
3 C2H5OH + PCl3 ® 3
C2H5Cl
+ H3PO3
|
C2H5OH + PCl5 ® C2H5Cl +
POCl3
+
HCl
|
Pyramidal shape, sp3 hybridisation
|
TBP geometry, sp3d hybridisation
|
-
|
2Ag
+ PCl5 ® 2
AgCl + PCl3 Sn + 2 PCl5 ® SnCl4 + 2 PCl3
|
GROUP 16 ELEMENTS
25. Oxidation states: They show -2, +2, +4,
+6 oxidation states. Oxygen does not show +6 oxidation state due to absence of
d – orbitals. Po does not show +6 oxidation state due to inert pair effect.
The stability of -2 oxidation state decreases down the group due to
increase in atomic size and decrease in electronegativity.
Oxygen shows -2 oxidation state
in general except in OF2 and O2F2
The stability of +6 oxidation state decreases and +4 oxidation state
increases due to inert pair effect.
26.
Ionisation
enthalpy: Ionisation enthalpy of elements of group 16 is lower than group
15 due to half filled p-orbitals in group 15 which are more stable. However,
ionization enthalpy decreases down the group.
27.
Electron
gain enthalpy: Oxygen has less negative electron gain enthalpy than S
because of small size of O.
From S to Po electron gain enthalpy becomes less negative to Po
because of increase in atomic size.
28. Melting and boiling point: It increases with increase in atomic
number. Oxygen has much lower melting and boiling points than
sulphur because oxygen is diatomic (O2) and sulphur is octatomic (S8).
29. Reactivity with hydrogen:
All group 16 elements form hydrides.
Bent shape
Acidic
This is because the H-E bond
length increases down the group. Therefore, the bond dissociation enthalpy
decreases down the group.
Thermal
stability: H2O < H2S < H2Se < H2Te
< H2Po
This is because the H-E bond length increases down the group.
Therefore, the bond dissociation enthalpy decreases down the group.
Reducing
character: H2O < H2S < H2Se < H2Te < H2Po
This is because the H-E bond
length increases down the group. Therefore, the bond dissociation enthalpy
decreases down the group.
30.
Reactivity
with oxygen: EO2 and EO3
Reducing character of dioxides decreases down the group because oxygen has
a strong positive field which attracts the hydroxyl group and removal of H+ becomes
easy.
Acidity also
decreases down the group.
SO2 is a gas whereas SeO2 is solid. This is because SeO2 has a chain polymeric structure whereas SO2 forms discrete units.
31.
Reactivity
with halogens: EX2 EX4 and EX6
The stability
of halides decreases in the order F- > Cl- > Br- > I-. This
is because E-X bond length increases with increase in size.
Among hexa halides, fluorides are the most stable because of steric
reasons.
Dihalides are
sp3 hybridised,
are tetrahedral in shape.
Hexafluorides are
only stable halides
which are gaseous
and have
sp3d2 hybridisation
and octahedral structure.
H2O is a liquid while
H2S is a gas. This is
because strong hydrogen bonding is present in water. This is due to small size
and high electronegativity of O.
Click HERE for Online Notes on "p-Block elements" (Chapter=>7 )
Part 3
Click HERE for Online Notes on "p-Block elements" (Chapter=>7 )
Part 3
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