Untitled - Kelly Walsh High School

178 CHEMISTRY FOR THE UTTERLY CONFUSED
lower than expected vapor pressure, are showing a negative deviation from
Raoult’s law, while those with a higher than expected vapor pressure are showing
a positive deviation from Raoult’s law.
In general, ideal solutions result when the intermolecular forces between the
particles are similar to those in the solvent or solute alone. When the intermolecular
forces in the solution are weaker, the molecules tend to escape more
readily and produce a positive deviation. If the intermolecular forces in the
solution are greater than those in the individual constituents then the particles
stay together instead of vaporizing. These solutions show a negative deviation.
If a pure liquid is the solvent and you add a nonvolatile solute, the vapor pressure
of the resulting solution is always less than the pure liquid. The addition of
the solute lowers the vapor pressure and the amount of lowering is proportional
to the number of solute particles added.
There is an even distribution of solvent particles throughout the solution, even at
the surface. There are fewer solvent particles at the gas-liquid interface.
Evaporation takes place at this interface. Fewer solvent particles escape into the
gas phase and thus the vapor pressure is lower. The higher the concentration of
solute particles, the less solvent is at the interface and the lower the vapor pressure.
Freezing-point Depression and Boiling-Point Elevation
The freezing point of a solution of a nonvolatile solute is always lower than the
pure solvent and the boiling point is always higher. It is the number of solute
particles that determines the amount of the lowering of the freezing point and
raising of the boiling point.
The amount of lowering of the freezing point is proportional to the molality of
the solute and is given by the equation:
T f iK f m
T f is the number of degrees that the freezing point has been lowered (the difference
in the freezing point of the pure solvent and the solution). K f is the
freezing-point depression constant (a constant of the individual solvent). The
molality (m) is the molality of the solute, and i is the van’t Hoff factor, which is
the ratio of the number of moles of particles released into solution per mole of
solute dissolved. For a nonelectrolyte such as sucrose, the van’t Hoff factor
would be 1. For an electrolyte such as sodium sulfate, you must take into consideration
that if 1 mol of Na 2SO 4 dissolves, 3 mol of particles would result
(2 mol Na , 1 mol SO 4 2 ). Therefore, the van’t Hoff factor should be 3. However,
because sometimes there is a pairing of ions in solution the observed van’t Hoff
factor is slightly less. The more dilute the solution, the closer the observed van’t
Hoff factor should be to the expected one.