calculated from the difference between the enthalpy (heat) of formation of the product and reactant.
ΔH
0
reaction = Σ ΔH
f
0
(products)
– Σ ΔH
f
0
(reactant)
Question 7
7) Which one of the following molecules is held together by dative bond?
a) AlBr
3
b) NaCl
c) Al
2
Cl
6
d) C
2
H
6
e) H
2
O2
Correct Answer:
c)Al
2
Cl
6
Explanation:
In Al
2
Cl
6
lone pairs of electron from chlorine are donated to electron deficient aluminium in such a
way that it is held together by dative bond, where aluminium acts as an electron acceptor and chlorine
acts as an electron donor.
AlBr
3
is a neutral ionic molecule. NaCl- ionic bond, C
2
H
6
- covalent bond,H
2
O
2
– Hydrogen bond.
Question 8
Arrange the following atoms and ions in the increasing order of atomic size
Mg, Mg
2+
, Al, Al
3+
a) Al
3+
> Al > Mg
2+
> Mg
b) Mg
2+
Mg > Al
3+
> Al
c) Mg > Mg
2+
> Al > Al
3+
d) Mg > Al > Mg
2+
> Al
3+
e) Al
3+
> Mg
2+
> Al > Mg
Correct Answer:
d) Mg > Al > Mg
2+
> Al
3+
Explanation:
Atomic radii decrease across the period. Cations are smaller than their parent atoms. Among these
isoelectronic ions, the one with larger positive nuclear charge will have the smaller radius. So, here
the largest atom is Mg and the smallest one is Al
3+
.
Question 9
Which of the following groups has
–I effect?
a) -CH
3
b) -C
2
H
5
c) -C(CH
3
)
3
d)
–C
6
H
5
e) Both a and c
Correct Answer:
d)
–C
6
H
5
