Answer to Question #141008 in Physical Chemistry for Fletcher Madden

Question

1a) State Raoult’s law. b) Name the components of a solution that deviates positively from Raoult’s law. c) Why does the solution in 1b deviate from ideal behavior?

Answer 1

Ans

a) Raoult’s law states that a solvent’s partial vapour pressure in a solution (or mixture) is equal or identical to the vapour pressure of the pure solvent multiplied by its mole fraction in the solution.

Mathematically, Raoult’s law equation is written as;

P_solution = Χ_solventP⁰_solvent

Where,

P_solution = vapour pressure of the solution

Χ_solvent = mole fraction of the solvent

P⁰_solvent = vapour pressure of the pure solvent

b) Positive Deviation from Raoult’s Law occurs when the vapour pressure of the component is greater than what is expected in Raoult’s Law. For Example, consider two components A and B to form non-ideal solutions. Let the vapour pressure, pure vapour pressure and mole fraction of component A be P_A , P_A⁰ and x_A respectively and that of component B be P_B , P_B⁰ and x_B respectively.

These liquids will show positive deviation when Raoult’s Law when:

P_A > P_A⁰ x_A and P_B > P⁰_B x_B, as the total vapour pressure (P_A⁰ x_A + P⁰_B x_B) is greater than what it should be according to Raoult’s Law.

c) i) The solute-solvent forces of attraction is weaker than solute-solute and solvent-solvent interaction that is, A – B < A – A or B – B

ii) The enthalpy of mixing is positive that is, Δ_mix H > 0 because the heat absorbed to form new molecular interaction is less than the heat released on breaking of original molecular interaction

iii) The volume of mixing is positive that is, Δ_mix V > 0 as the volume expands on the dissolution of components A and B