4.5 Physical Properties of Covalent Molecules

Summary of Bonding Types

Small Covalent Molecules (polar & non-polar) e.g. H 2 O, I 2, CO 2, NH 3, HF, CH 4, C 2 H 5 OH Strong covalent bonds between the hydrogen and oxygen atoms (intramolecular) and hydrogen bonds between molecules (intermolecular forces). Water H 2 O Polar Hydrogen bond between molecules Covalent bonds within molecule

Iodine, I 2 blue-black solid blue-black solid Non Polar Non Polar Shape - Linear Shape - Linear The iodine molecules are held together by weak intermolecular van der Waals forces. The iodine molecules are held together by weak intermolecular 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 (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’ 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 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 (CH 3 CH 2 OH) is more soluble in water than butanol (CH 3 CH 2 CH 2 CH 2 OH) 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 (CH 3 CH 2 OH) is more soluble in water than butanol (CH 3 CH 2 CH 2 CH 2 OH) Non-polar covalent molecules are generally insoluble in water but are soluble in non-polar solvents. 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 (C 6 H 6 ) 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. 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. 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. Impurities lower the mpt of solid covalent molecules. Mpt KF = 1500°C (ionic) Mpt (F 2 ) = - 220°C (non-polar) Mpt K = 760 ° C (metallic) Mpt (HF) = - 83 °C (polar) Melting Point and Boiling Point

Electrical Conductivity Covalent molecules are not charged because they are overall neutral and therefore do not conduct electricity. 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. Some covalent molecules can react with water and produce free ions which can carry an electrical current. E.g. ammonia, NH 3 E.g. ammonia, NH 3 NH 3 (l) + H 2 O (l) ↔ NH 4 + (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 (HOCH 2 CH(OH)CH 2 OH) has high and water (H 2 O) 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, (C 2 H 5 ) 2 CO liquid phenolethanone (CH 3 COC 6 H 5 solid phenolethanone (CH 3 COC 6 H 5 solid