KEY
Polarity
Name - ________________
1.) Which of the following molecules will be polar and which will be nonpolar?
a.)
S
b.)
Cl
Cl
Cl
c.)
B
Cl
H
H
C
Cl
e.)
H
H
f.)
C
O
H
H
Mg
H
O
H
Cl
H
H
H
d.)
g.)
C
H
C
C
H
h.)
O
O
S
H
O
O
H
2.) Examine the following melting temperatures. All the molecules are polar.
NH3 = -78oC
PH3 = -133oC
AsH3 = -116oC
SbH3 = -88oC
a.) Why do the melting temperatures of SbH3, AsH3 and PH3 steadily decrease?
Answer – Sb, As and P in this order have decreasing electron numbers. The more electrons you have
the greater your London forces are (temporary dipole). The greater your London forces
are the higher your melting point is.
b.) Why does the melting temperature of NH3 suddenly increase going from PH3 up to NH3?
Answer – NH3 has a nitrogen bonded to hydrogen. This means there is hydrogen bonding occurring
along with the London forces, whereas PH3 only has London forces, which leads to a big
jump up in the melting temperature.
3.) Which of the following substances would you expect to involve hydrogen bonds between like
molecules?
a.) CH4
c.) H2O
e.) CH3-NH2
g.) CH3-CH2-OH
b.) HCl
d.) H2S
f.) CH3-SH
h.) HF
1
KEY
4.) Suggest a reason why liquid propane has a very low viscosity, whereas liquid glycerine has a very high
viscosity.
Propane = CH3-CH2-CH3
glycerine = CH2
CH
CH2
OH
OH
OH
Answer – Propane's viscosity is much lower because there are less forces and they are weaker than
the forces in glycerine. The glycerine has both hydrogen and London forces while propane
is just weak London forces. High forces leads to high viscosity.
5.) Classify each of the following with respect to the most important type(s) of bonding or forces(s)
existing between the particles.
a.) 2 molecules of O2 in O2 (s)
London Forces
b.) 2 atoms of Xe in Xe (s)
London Forces
c.) An atom of C and an atom of Cl in CCl4
Covalent Bond
d.) 2 molecules of CH3F in CH3F (l)
Dipole-dipole and London Forces
e.) F and Cs in CsF (s)
Ionic bond
f.) atoms of He and Kr
London Forces
6.) Which should melt at a higher temperature and why?
a.) He or Xe
Answer – Xe larger London forces
b.) HBr and Kr
Answer – HBr has dipole-dipole
c.) CH3-CH3 or HO-CH2-CH2-OH
Answer – HO-CH2-CH2-OH has hydrogen bonding
d.) F2 or Br2
Answer – Br2 larger London forces
2