Question

(i) What is meant by the positive deviations from Raoult's law? Give an example
(ii) What is the sign of ${{\Delta }_{mix}}H$ for positive deviation?

Answer 1

In ideal mixtures or the mixtures which obey Raoult's law, the forces between the particles in the mixture are exactly equal to those in the pure liquids and the tendency for the particles to escape is the same in the pure liquids and in the mixture. But in the case of non-ideal mixtures there would be positive and negative deviations which we will discuss further.

Complete step by step answer:
(i) Let's start with the concept of Raoult's law. The law states that for an ideal solvent, the partial pressure of a solvent in a solution will be equal to the vapour pressure of pure solvent multiplied by the mole fraction of the component in the solution.
- Those solutions which obey Raoult's law are known as Ideal solutions and they behave in a way in which the Raoult's law is followed as it is. The solutions which show some deviation from Raoult's law are known as non-ideal solutions.
-While this law is obeyed by many solutions, some of the solutions show deviations from the expected behaviour of ideal solutions and these deviations can be either negative or positive.
- A negative deviation from Raoult's law is observed when the vapour pressure is less than that of the expected value and a positive deviation is observed when the vapour pressure of the mixture is more than the predicted or expected vapour pressure.
- The reason for deviation from ideal behaviour is due to a flaw in consideration that there is no solute-solvent interaction. But this is not always true. A positive deviation occurs when the solute and solvent are not as tightly bound to each other, then the solvent molecules will find it a lot easier to escape from the solution into the gas phase.
- A mixture formed between ethanol and acetone exhibits positive deviation from Raoult's law. As we know pure ethanol has hydrogen bonding and when acetone is added some of these hydrogen bonds are being broken and due to this weakened interaction, the solution shows positive deviation from Raoult's law.

(ii) The solutions which exhibits positive deviation, endothermic process or absorption of heat is taking place and ${{\Delta }_{mix}}H$ will be greater than zero , which means that the ${{\Delta }_{mix}}H$ will have a positive sign.

Note: It should be noted that if a mixture has higher vapour pressure than ideal mixture or in other words if the mixture exhibits positive deviation, it indicates that the mixture will have a low boiling point. The reason is that the molecules are escaping more easily and we won't have to heat the mixture to overcome the intermolecular attractions.