What is meant by positive deviation from Raoult’s law? Give an example. What is the sign of for positive deviation?

OR

Define azeotropes. What type of azeotrope is formed by positive deviation from Raoult’s law? Give an example.

Raoult’s law states that partial vapour pressure of each volatile component in a solution is directly proportional to its mole fraction and the graph obtained is linear for ideal solution. Ideal solution implies the interactions between particles of solvent-solvent, solute- solute and solvent –solute are all same.

For a positive deviation the vapour pressure is higher than the predicted pressure from Raoult’s law. This occurs when solute-solvent interactions are weaker than solute-solute or solvent-solvent interactions.

Eg: mixture of ethanol and acetone, carbondisulphide and acetone. In case of ethanol and acetone mixture acetone interferes with the hydrogen bonding of ethanol molecules decreasing their interaction leading to more vapour pressure and hence the positive deviation.

for positive deviation is positive i.e., energy is absorbed to weaken the solvent-solvent or solute-solute interactions.

OR

Azeotrope is a mixture of two or more liquids, of a particular composition which have constant boiling point and composition throughout distillation i.e., they cannot be separated as the composition of vapour phase is same as that of liquid phase.

A minimum boiling azeotrope is formed by positive deviation from Raoult’s law. Minimum boiling azeotrope means that which boils below the normal boiling points of the individual components.

Eg: water (4.5%) and ethanol (95.5%) of this consistency act as one component. Boiling point of water is 100⁰C and ethanol is 78.4^oC, but this azeotrope boils at 78.2^oC.