Chapter 10 Worked Example 1
Explain the trend in the boiling points of the hydrogen halides:
HCl, -85 oC; HBr, -67 oC; HI, -35 oC.
- Electronegativity differences: HCl > HBr > HI
- Number of electrons and London forces: HCl < HBr < HI
→ not by dipole-dipole forces, but by London forces
Chapter 10 Worked Example 2
Account for the trend in boiling points of the noble gases,
which increase from helium to xenon.
Solution
In the noble gases, only London forces need be considered:
These increase as the number of electrons increases (size
of the atom increases):
He (2)
B.Pt
Ne(10) Ar(18)
Kr(36)
Xe(54)
Chapter 10 Worked Example 3
Which of the following molecules can take part in hydrogen
bonding with other molecules of the same compound:
(a) CH3OH; (b) PH3; (c) H-O-Cl?
Solution
(a) and (c), since these both have a H-O covalent bond:
CH3
H
O
H 3C
H
Cl
H
O
O
Cl
H
O
(most likely)
Chapter 10 Worked Example 4
Identify the kinds of intermolecular forces that might arise between
molecules of each of the following substances:
(a) NH2OH; (b) CBr4; (c) H2SeO4; (d) SO2
Solution
(a)
(b)
(c)
(d)
London forces, dipole-dipole forces, hydrogen bonding
London forces
London forces, dipole-dipole forces, hydrogen bonding
London forces, dipole-dipole forces
Chapter 10 Worked Example 5
Suggest, giving reasons, which substance in each of the following
pairs is likely to have the higher normal melting point (Lewis
structures may help your arguments):
(a) HCl or NaCl; (b) C2H5OC2H5 (diethyl ether) or C4H9OH (butanol);
(c) CHI3 or CHF3; (d) C2H4 or CH3OH.
Solution
Chapter 10 Worked Example 6
Which will have the higher boiling point, 1,1-dichloroethene
or t rans-1,2-dichloroethene?
Solution
1,1-dichloroethene is polar, whereas tr ans-1,2-dichloroethene
is nonpolar:
Cl
H
C
Cl
Cl
H
C
C
H
H
C
Cl
Hence, dipole-dipole (as well as London) forces exist
in 1,1-dichloroethene, giving it the higher boiling point.
Chapter 10 Worked Example 7
Predict how each of the following properties of a liquid varies as
the strength of intermolecular forces increases and explain your
reasoning: (a) boiling point; (b) viscosity; (c) surface tension.
Solution