Vapour Pressure of Solutions

Question 1

Intro

Answer 1

> a study to understand that properties of liquid solution is different from a pure solvent

Question 2

In a pure volatile liquid

Answer 2

> P0 = vapour pressure above the liquid
to evaporate:
-molecules reach the surface of a liquid
-overcomes intermolecular forces with the other liquid molecules

Question 3

In a solution of solvent and nonvolatile solute (water + NaCl)

Answer 3

> nonvolatile = doesn’t evaporate
solvent molecule’s access to the surface is blocked, so less evaporation, also, nothing prevents existing solvent vapour from condensing and rejoining liquid
usually has a lower solvent vapour pressure (Psltn < P0, at any temperature) (rate of vaporization < rate of condensation)
new equilibrium is reached when [vapour] drops low enough where rate of vaporization = rate of condensation (condensation rate decreases b/c less gas molecules hit the liquid surface)

Question 4

Raoult’s Law

Answer 4

> equation relating solvent vapour pressure in presence and absence of a nonvolatile solute

Psltn = Xsolvent * P0solvent
>Psltn = solution's vapour pressure
>Xsolvent = mole fraction of solvent in solution
>P0 = vapour pressure of pure solvent

> increase in [solute] = decrease in Xsolvent
decrease of Xsolvent = decrease in solvent vapour pressure for a solution
increase in Psltn = increase in Xsolvent

Question 5

Total number of particles in Raoult’s Law

Answer 5

> dissociation of solute particles can increase the number of ions in a solution
multiply moles by number of ions in the solution

Question 6

If there is more than one volatile compound

Answer 6

Psltn = (XaP0a)+(XbP0b)

> ideal solution: a solution where >1 solution component contributes to total vapour pressure above the solution that still follows Raoult’s Law (act as if they only had one volatile component)

Question 7

Positive deviation from Raoult’s Law

Answer 7

> component A-A or B-B > A-B
interaction between the two compounds is weaker than interactions between themselves
enthalpy of solution is positive, solution doesn’t form, non-ideal situation

Question 8

Negative deviation from Raoult’s Law

Answer 8

> component A-A or B-B < component A-B
interactions between the two compounds are stronger (such as H+ bonding)than interacting between themselves
a lot of energy is released, enthalpy of the solution is negative, favouring solution formation