Grade 12 Physical Properties | Factors Affecting Physical Properties of Organic Compounds

The lesson introduces physical properties like melting point, boiling point, vapor pressure, and viscosity, and their relationship with intermolecular forces in organic compounds. It defines key terms and categorizes homologous series based on the types of intermolecular forces present (London forces, dipole-dipole forces, and hydrogen bonds). Carboxylic acids have stronger hydrogen bonds due to two bonding sites compared to alcohols with one. London forces are the weakest, followed by dipole-dipole, and hydrogen bonds are the strongest.

Three main factors affect the strength of intermolecular forces: chain length, branched chains, and the number and type of intermolecular forces. Longer carbon chains increase surface area, leading to stronger London forces. More branched chains decrease the surface area, resulting in weaker intermolecular forces. Hydrogen bonds are stronger than dipole-dipole, which are stronger than London forces. The first two factors are used when comparing compounds within the same homologous series, while the third is for comparing different homologous series.

Boiling point, melting point, and viscosity are directly proportional to the strength of intermolecular forces. Stronger intermolecular forces lead to higher boiling points, melting points, and viscosity. Vapor pressure, however, has an inverse relationship; stronger intermolecular forces result in lower vapor pressure. Therefore, vapor pressure is inversely proportional to boiling point and melting point.

To compare compounds within the same homologous series, the ISSE method is used: I (Identify intermolecular forces), S (Compare structure/surface area), S (Compare strength of intermolecular forces), E (Explain energy required). An example with methane, ethane, and propane shows that as chain length increases, surface area and London forces strengthen, requiring more energy to overcome, thus increasing the boiling point.

For comparing compounds in different homologous series, the ISE method is used: I (Identify intermolecular forces), S (Compare strength of intermolecular forces), E (Explain energy required). An example comparing butane (alkane), butanol (alcohol), and butanoic acid (carboxylic acid) demonstrates that butanoic acid has the strongest hydrogen bonds (two sites), followed by butanol (one site), and butane with only London forces. This leads to a trend of increasing boiling points from butane to butanoic acid due to the increasing energy required to overcome the respective intermolecular forces.