Page 221 - Physical chemistry understanding our chemical world

188 PHASE EQUILIBRIA

How does a rotary evaporator work?

Thermodynamics of phase changes

Rotary evaporators are a common feature in most undergraduate laboratories. Their
primary purpose is to remove solvent following a reflux, perhaps before crystallization
of a reaction product.
To operate the evaporator, we place the reaction solution in a round-bottomed flask
O
while the pressure inside the evaporator is decreased to about 1 × p . The flask
30
O
is then rotated. The solvent evaporates more easily at this low pressure than at p .
The solvent removed under vacuum is trapped by a condenser and collected for easy
re-use, or disposal in an environmentally sensitive way.
But molecules need energy if they are to leave the solution during boiling. The
energy comes from the solution. The temperature of the solution would decrease
rapidly if no external supply of energy was available, as a reflection of its depleted
energy content (see p. 33). In fact, the solution would freeze during evaporation, so
the rotating bulb is typically immersed in a bath of warm water.
An atmosphere of vapour always resides above a liquid, whether
Strictly, the term s.v.p. the liquid is pure, part of a mixture, or has solute dissolved within
applies to pure liq- it. We saw on p. 180 how the pressure of this gaseous phase is
uids. By using the term called its saturation vapour pressure, s.v.p. The s.v.p. increases with
s.v.p., we are implying increased temperature until, at the boiling point T (boil) , it equals the
that all other com- external pressure above the liquid. Evaporation occurs at tempera-
ponents are wholly tures below T (boil) , and only above this temperature will the s.v.p.
involatile, and the exceed p . The applied pressure in a rotary evaporator is less than
O
s.v.p. relates only to
O
p , so the s.v.p. of the solvent can exceed the applied pressure
the solvent.
O
(and allow the liquid to boil) at pressures lower than p .
We see this phenomenon in a different way when we look back at
Normal in the con- the phase diagram in Figure 5.6. The stable phase is liquid before
text of phase equilibria applying a vacuum. After turning on the water pump, to decrease
means ‘performed at a the applied pressure, the s.v.p. exceeds p (applied) , and the solvent
pressure of 1 bar, p ’. boils at a lower pressure. The bold arrow again indicates how a
O
phase change occurs during a depression of the external pressure.
We see how decreasing the pressure causes boiling of the solvent
at a lower temperature than at its normal boiling temperature, i.e.
The rotary evaporator
O
is a simple example of if the external pressure were p . Such a vacuum distillation is
a vacuum distillation. desirable for a preparative organic chemist, because a lower boiling
temperature decreases the extent to which the compounds degrade.
Coffee, for example, itself does not evaporate even at low pressure, since it is
a solid. Solids are generally much less volatile than liquids, owing to the stronger
interactions between the particles. In consequence, the vapour pressure of a solid is
several orders of magnitude smaller than that above a liquid.