Positive deviation from Raoult's Law in Benzene-Methanol Liquid Pair

Raoult's law states that the vapor pressure of a solution is directly proportional to the mole fraction of the solvent in the solution. However, some liquid pairs show positive deviations from Raoult's law, which means that the vapor pressure of the solution is higher than what is predicted by Raoult's law. One such liquid pair is benzene-methanol.

Reasons for Positive Deviation

The positive deviation from Raoult's law in benzene-methanol can be attributed to the following reasons:

1. Intermolecular Forces: The intermolecular forces between benzene and methanol molecules are not similar. The benzene molecule has weak London dispersion forces, while the methanol molecule has strong hydrogen bonding. This mismatch in intermolecular forces leads to a positive deviation from Raoult's law.

2. Molecular Size: The molecular size of benzene and methanol is also different. Benzene is a larger molecule than methanol. Due to this difference in size, the benzene molecules cannot pack as closely with the methanol molecules, leading to a positive deviation from Raoult's law.

Implications of Positive Deviation

The positive deviation from Raoult's law in benzene-methanol has important implications in the chemical industry. For example, distillation processes that rely on Raoult's law to separate components will not work as expected in this liquid pair. The positive deviation also affects the boiling point and freezing point of the solution, making it difficult to predict these properties accurately.

Conclusion

The benzene-methanol liquid pair shows a positive deviation from Raoult's law due to differences in intermolecular forces and molecular size. This deviation has important implications in the chemical industry and affects the properties of the solution.