4.5
Physical Properties of Covalent Molecules
Summary of Bonding Types
Small Covalent Molecules (polar & non-polar)
e.g. H2O, I2, CO2, NH3, HF, CH4, C2H5OH
Water
Hydrogen bond
between
molecules
H 2O
Polar
Covalent
bonds within
molecule
Strong covalent bonds between the hydrogen and oxygen atoms
(intramolecular) and hydrogen bonds between molecules
(intermolecular forces).
Iodine, I2
•
•
•
•
Weak
intermolecular
van der waals
forces between
individual
molecules
blue-black solid
Non Polar
Shape - Linear
The iodine molecules are held
together by weak
intermolecular van der Waals
forces.
Covalent bonds /
shared electrons
between iodine atoms
in the molecule
Large Molecules (macromolecules)
•
•
Polymers are
macromolecules made up
of long repeating units of
groups of atoms.
Many biological
molecules are polymers.
E.g. proteins, lipids,
nucleic acids,
polysaccharides
DNA
Amino acids are the
repeating units of
protein molecules
Physical Properties of Covalent
Molecules
Physical properties are governed by the intermolecular
forces
a) van der Waals (non-polar covalent
molecules)
b) permanent dipole-permanent dipole (the
strongest IMF found in polar covalent
molecules)
c) hydrogen bonds (found in polar covalent
molecules with O-H, F-H and N-H bonds)
All molecules contain van der waals forces
Solubility in water
•
•
•
•
Polar covalent molecules dissolve in polar solvents like
water. Remember the general rule ‘Like dissolves
like’
As a polar molecule gets larger its solubility in water
decreases. E.g. Individual amino acids are soluble in
water but large protein polymers are generally insoluble.
As the non-polar hydrocarbon chain of a polar molecule
increases, the solubility of the molecule decreases
because the non-polar carbon chain outweighs the polar
part. E.g. ethanol (CH3CH2OH) is more soluble in water
than butanol (CH3CH2CH2CH2OH)
Non-polar covalent molecules are generally insoluble in
water but are soluble in non-polar solvents.
Lining up for fresh water after the city of Harbin’s water supply, the Songhua
river was contaminated by a chemical explosion in a benzene (C6H6) factory.
Being a very stable, unreactive non-polar molecule, its insolubility in water
posed challenges for those involved in cleaning up the spill.
Melting Point and Boiling Point
•
As the molecular mass of a covalent molecule increases,
the number of electrons increases, increasing the strength
of the van der Waals attractions. Therefore more energy to
break them, increasing the mpt and bpt.
• Covalent molecules have low melting points and boiling
points compared to giant ionic, giant covalent and metallic
solids.
• Impurities lower the mpt of solid covalent molecules.
Mpt KF = 1500°C (ionic)
Mpt K = 760 ° C (metallic)
Mpt (F2) = - 220°C (non-polar)
Mpt (HF) = - 83 °C (polar)
Electrical Conductivity
•
Covalent molecules are not charged because they
are overall neutral and therefore do not conduct
electricity.
• Some covalent molecules can react with water
and produce free ions which can carry an
electrical current.
• E.g. ammonia, NH3
NH3 (l) + H2O (l) ↔ NH4+ (aq) + OH- (aq)
Volatility

Volatility is the tendency of a substance to pass
from a solid or liquid into a gas state at low
temperatures.
 Covalent molecules are more volatile than giant
ionic, giant covalent and metallic substances
because it is the weak intermolecular forces that
are broken.
 The most volatile covalent molecules are those
that are non-polar because they have the weakest
intermolecular forces (van der Waals) between
their molecules.
Viscosity

How easily a fluid (liquid or gas) flows
 Glycerol (HOCH2CH(OH)CH2OH) has high and
water (H2O) has low viscosity.
 The more viscous fluid flows more slowly
because the intermolecular forces between the
molecules are stronger increasing the attraction
the molecules have for one another.
 Viscosity increases with temperature.
State at room temperature

Simple covalent molecules tend to be gases,
liquids or low melting point solids.
 A covalent molecule has an increasing tendency
to become a solid as its molecular mass
increases. This is because the strength of the
van der waals forces increases, decreasing the
distance between the molecules.
 E.g. pentan-3-one, (C2H5)2CO liquid
phenolethanone (CH3COC6H5 solid