Question

What is meant by positive and negative deviations from Raoult's law and how is the sign of Δ_solH related to positive and negative deviations from Raoult's law?

Answer 1

According to Raoult’s law, the partial vapour pressure of each volatile component in any solution is directly proportional to its mole fraction. The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions. The solutions that do not obey Raoult’s law (non-ideal solutions) have vapour pressures either higher or lower than that predicted by Raoult’s law. If the vapour pressure is higher, then the solution is said to exhibit positive deviation, and if it is lower, then the solution is said to exhibit negative deviation from Raoult’s law.

Vapour pressure of a two-component solution showing positive deviation from Raoult’s law

Vapour pressure of a two-component solution showing negative deviation from Raoult’s law

In the case of an ideal solution, the enthalpy of the mixing of the pure components for forming the solution is zero.

Δ_solH = 0

In the case of solutions showing positive deviations, absorption of heat takes place.

∴Δ_solH = Positive

In the case of solutions showing negative deviations, evolution of heat takes place.

∴Δ_solH = Negative