Question

18. Explain on a particle basis how the addition of a solute affects the boiling point, the freezing point, and the vapor pressure of the solvent. (6 points)

Answer 1

Start with the additional IMFs created by the solute-solvent interactions

Answer 2

Boiling point elevation, freezing point lowering, and vapor pressure lowering are colligative properties. They depend on the number of particles in solution and not on the chemical nature of the solute. Boling point elevation; additional attractive forces exist between solute and solvent; they must be overcome for the solution to boil. Kinetic energy must be added to overcome these forces. Freezing point lowering: more kinetic energy must be withdrawn from a solution because the solute particles are surrounded by shells of solvent molecules. These shells interfere with the formation of the orderly pattern that the particles assume as the solvent changes from liquid to solid. Vapor pressure lowering: the formation of solvent shells around the solute particles reduces the number of solvent particles that have sufficient kinetic energy to vaporize.