Equilibria

Equilibria

1. Raoult's Law

   • The partial vapour pressure of a substance in a mixture is proportional to its mole fraction in the solution and its vapour pressure when pure.
   • PJ = XJ PJ*
   • Solute can block solvent molecules from escaping but not returning. Higher entropy of solution than pure solvent, system gains less entropy on vapourisation of the solvent.
   • Henry's Law
     • In dilute solutions the vapour pressure of the solute is proportional to its mole fraction, but the constant of proportionality is not the vapour pressure of the pure solute. 
     • PB = XB KB
       • KB (Henry's Law constant) is the tangent to the curve at XB = 0
     • Applies to dilute solutions called ideal-dilute solutions, to predict the vapour pressure of solute.

2. Colligative Properties

   • Chemical potential of a solvent in a solution is reduced with respect to the pure liquid, so the boiling point is elevated and the freezing point depressed. 
   • Entropic effects which depend on number of molecules not identity of solute. All non-electrolyte solutions with the same molality have the same boiling and freezing points. 
   • uA = uA$ + RT ln(xA)