4.5 4.5

Physical Properties of Covalent Physical Properties of Covalent MoleculesMolecules

Summary of Bonding TypesSummary of Bonding Types

Small Covalent Molecules (polar & non-polar)Small Covalent Molecules (polar & non-polar) e.g. H e.g. H22O, IO, I22, CO, CO22, NH, NH33, HF, CH, HF, CH44, C, C22HH55OHOH

Strong covalent bonds between the hydrogen and oxygen atoms (intramolecular) and hydrogen bonds between molecules (intermolecular forces).

Water

H2O

Polar

Hydrogen bond between molecules

Covalent bonds within molecule

Iodine, IIodine, I22

• blue-black solid blue-black solid • Non PolarNon Polar• Shape - LinearShape - Linear• The iodine molecules are held The iodine molecules are held

together by weak intermolecular together by weak intermolecular van der Waals forces. van der Waals forces.

Covalent bonds / shared electrons between iodine atoms in the molecule

Weak intermolecular van der waals forces between individual molecules

Large Molecules Large Molecules (macromolecules)(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 Physical Properties of Covalent MoleculesMolecules

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 waterSolubility in water

• Polar covalent molecules dissolve in polar solvents like Polar covalent molecules dissolve in polar solvents like water. Remember the general rule water. Remember the general rule ‘Like dissolves ‘Like dissolves like’ like’

• As a polar molecule gets larger its solubility in water As a polar molecule gets larger its solubility in water decreases. E.g. Individual amino acids are soluble in decreases. E.g. Individual amino acids are soluble in water but large protein polymers are generally insoluble.water but large protein polymers are generally insoluble.

• As the non-polar hydrocarbon chain of a polar molecule As the non-polar hydrocarbon chain of a polar molecule increases, the solubility of the molecule decreases increases, the solubility of the molecule decreases because the non-polar carbon chain outweighs the polar because the non-polar carbon chain outweighs the polar part. E.g. ethanol (CHpart. E.g. ethanol (CH33CHCH22OH) is more soluble in water OH) is more soluble in water than butanol (CHthan butanol (CH33CHCH22CHCH22CHCH22OH) OH)

• Non-polar covalent molecules are generally insoluble in Non-polar covalent molecules are generally insoluble in water but are soluble in non-polar solvents.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.

• As the molecular mass of a covalent molecule increases, As the molecular mass of a covalent molecule increases, the number of electrons increases, increasing the strength the number of electrons increases, increasing the strength of the van der Waals attractions. Therefore more energy to of the van der Waals attractions. Therefore more energy to break them, increasing the mpt and bpt.break them, increasing the mpt and bpt.

• Covalent molecules have low melting points and boiling Covalent molecules have low melting points and boiling points compared to giant ionic, giant covalent and metallic points compared to giant ionic, giant covalent and metallic solids. solids.

• Impurities lower the mpt of solid covalent molecules.Impurities lower the mpt of solid covalent molecules.

Mpt KF = 1500°C (ionic)Mpt KF = 1500°C (ionic) Mpt (F Mpt (F22) = - 220°C (non-) = - 220°C (non-polar)polar)

Mpt K = 760 ° C (metallic) Mpt (HF) = - 83 °C (polar)Mpt K = 760 ° C (metallic) Mpt (HF) = - 83 °C (polar)

Melting Point and Boiling Point

Electrical ConductivityElectrical Conductivity

• Covalent molecules are not charged because they Covalent molecules are not charged because they are overall neutral and therefore do not conduct are overall neutral and therefore do not conduct electricity. electricity.

• Some covalent molecules can react with water Some covalent molecules can react with water and produce free ions which can carry an and produce free ions which can carry an electrical current. electrical current.

• E.g. ammonia, NHE.g. ammonia, NH33

NHNH33 (l)(l) + H + H22O O (l)(l) ↔ NH ↔ NH44++ (aq)(aq) + OH + OH- - (aq)(aq)

VolatilityVolatility

Volatility is the tendency of a substance to pass Volatility is the tendency of a substance to pass from a solid or liquid into a gas state at low from a solid or liquid into a gas state at low temperatures. temperatures.

Covalent molecules are more volatile than giant Covalent molecules are more volatile than giant ionic, giant covalent and metallic substances ionic, giant covalent and metallic substances because it is the weak intermolecular forces that because it is the weak intermolecular forces that are broken.are broken.

The most volatile covalent molecules are those The most volatile covalent molecules are those that are non-polar because they have the weakest that are non-polar because they have the weakest intermolecular forces (van der Waals) between intermolecular forces (van der Waals) between their molecules.their molecules.

ViscosityViscosity

How easily a fluid (liquid or gas) flowsHow easily a fluid (liquid or gas) flows Glycerol (HOCHGlycerol (HOCH22CH(OH)CHCH(OH)CH22OH) has high and OH) has high and

water (Hwater (H22O) has low viscosity.O) has low viscosity.

The more viscous fluid flows more slowly The more viscous fluid flows more slowly because the intermolecular forces between the because the intermolecular forces between the molecules are stronger increasing the attraction molecules are stronger increasing the attraction the molecules have for one another.the molecules have for one another.

Viscosity increases with temperature.Viscosity increases with temperature.

State at room temperatureState at room temperature

Simple covalent molecules tend to be gases, Simple covalent molecules tend to be gases, liquids or low melting point solids.liquids or low melting point solids.

A covalent molecule has an increasing tendency A covalent molecule has an increasing tendency to become a solid as its molecular mass to become a solid as its molecular mass increases. This is because the strength of the increases. This is because the strength of the van der waals forces increases, decreasing the van der waals forces increases, decreasing the distance between the molecules.distance between the molecules.

E.g. pentan-3-one, (CE.g. pentan-3-one, (C22HH55))22CO CO liquidliquid

phenolethanone (CHphenolethanone (CH33COCCOC66HH55 solidsolid