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, C2H5OHStrong covalent bonds between the hydrogen and oxygen atoms (intramolecular) and hydrogen bonds between molecules (intermolecular forces).WaterH2OPolarHydrogen bond between moleculesCovalent bonds within molecule

Iodine, I2blue-black solid Non PolarShape - LinearThe iodine molecules are held together by weak intermolecular van der Waals forces.

Covalent bonds / shared electrons between iodine atoms in the moleculeWeak intermolecular van der waals forces between individual molecules

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 MoleculesPhysical properties are governed by the intermolecularforcesa)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 waterPolar 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 Harbins 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.

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 = 1500C (ionic) Mpt (F2) = - 220C (non-polar)Mpt K = 760 C (metallic) Mpt (HF) = - 83 C (polar)Melting Point and Boiling Point

Electrical ConductivityCovalent 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)

VolatilityVolatility 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.

ViscosityHow easily a fluid (liquid or gas) flowsGlycerol (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 temperatureSimple 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