Engineering Chemistry S Datta

40 ENGINEERING CHEMISTRY
Q. 27. What is hybridisation of atomic orbitals?
Ans. The intermixing the atomic orbitals of slightly different energies so as to redistribute
their energies and to give a new set of orbitals (known as hybrid orbitals) of equivalent
energies and shape is known as hybridisation.
Q. 28. Mention the types of hybridisation.
Ans. (i) sp 3 , (ii) sp 2 , (iii) sp, (iv) dsp 2 , (v) sp 3 d, (vi) d 2 sp 3 .
Q. 29. What do you mean by hybrid orbitals?
Ans. Hybrid orbitals are formed from the mixing of atomic orbitals of different types
and are oriented in space in a characteristic manner determined by the type of hybridisation.
Q. 30. Calculate the bond orders of: (i) H 2
O, (ii) H + 2
, (iii) He + 2
, (iv) He 2
and interpret the
result.
Ans. (i) B.O. = N b − N a
= 2 − 0 = 1
2 2
H-atoms are held together by one covalent bond.
(ii) B.O. = 1 − 0 = 1 2 2
H
+
2
has a transient existence. Here the number of bonding electrons exceeds the number
of electrons in antibonding orbital.
(iii) B.O. = 2 − 1 = 1 2 2
He
+
2
also has transient existence. Here, the number of electrons in s-orbital is greater
than the number of electrons in s*-orbital.
(iv) B.O. = 2 − 2 = 0
2
Here the number of electrons in σ-orbital = the number of electrons in σ*-orbital. The
molecule does not exist. So He is monoatomic.
Q. 31. O 2
molecule is paramagnetic. Explain.
Ans. The electronic configuration of:
O-atom ⇒ 1s 2 2s 2 2p 4
The electronic configuration of:
O 2
-molecule ⇒ KK(σ2s 2 )(σ*2s 2 )(σ2p 2 )(π y
2p 2 )(π z
2p 2 )(π y
*2p 1 )(π z
*2p 1 )
B.O. ⇒ N b − N a
⇒ 8 − 4 = 2
2
2
So, O 2
molecule contains double bond (O = O), one is σ and the other is π-bond. According
to Hund’s rule two unpaired electrons fill the degenerate (orbitals of same energy) π y
*2p and
π z
*2p orbitals (see Figs. 2.31 and 2.32). That is why, O 2
molecule is paramagnetic.
Q. 32. Explain why HF is liquid, HCl is gas and HF is less acidic than HCl.
+ δ
+ δ
+ δ
+ δ
−
Ans. H−
F
δ −
... H−
F
δ −
... H−
F
δ −δ
... H−
F
Due to hydrogen bonding, there is molecular association in HF and effective molecular
weight of HF is high and that is why its volatility is less and it is liquid whereas in HCl
molecule there is no hydrogen bonding because Cl is less electronegative than F and atomic
radius of Cl is greater than that of F. Hydrogen bond in HF inhibits release of H + partially in
aqueous solution, so it is less acidic.