The States of Matter
Chemical Thermodynamics and the Equilibrium State
|For a particular solvent, the amount of freezing-point lowering and boiling-point elevation depends on the concentration of dissolved particles (molality). Remember that for the MCAT. Freezing-point lowering and boiling point elevation depend on the number of dissolved particles (moles per kg of solvent). It does not depend on the identity of the dissolved particles. |
Will a one molal solution of sodium chloride have the same freezing point depression as a one molal solution of glucose? No! That was a trick question. Each mole of sodium chloride will give rise to two moles of dissolved ions.
Those are the two tricks with freezing point depression/boiling point elevation questions. First, the question writer will give you a wrong answer that tries to get to you say that one type of particle will cause more or less freezing point depression/boiling point elevation than another. Second, the question writer will try to get you to forget that some substances (electrolytes) disassociate and give rise to more moles of particles than moles of solute.
A related principle is Raoult's Law, which states that the partial pressure of solvent vapor in equilibrium with a dilute solution is directly proportional to the mole fraction of solvent in the solution. Raoult's Law, like freezing point depression/boiling point elevation, does not depend on the identity of the solute, just the concentration of particles. This is fertile ground for MCAT question writers.
By the way, why does having something dissolved lower the freezing point, raise the boiling point, and reduce the vapor pressure. The solute decreases the thermodynamic activity of the solvent, which decreases its free energy relative to the both the gaseous and solid phases. If the free energy of a side of the equilibrium is lowered, that means that forming it is more likely to correspond to a free energy decrease. By lowering the free energy of the liquid phase, the temperature range over which liquid is favored is broadened.