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Francois Marie Raoult
(1830 - 1901) |
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If you have studied Raoult’s Law you will be familiar
with the graph of vapour pressure vs composition at constant temperature. You will probably also be familiar with the boiling
point/composition diagram (at constant pressure) that is used to explain fractional distillation. What is often absent from
the books is the link between the two diagrams.
Here it is.
- As temperature increases, so does the vapour pressure of a liquid.
- The vapour pressure/ composition diagram is plotted at constant temperature
- a series of these plots at different temperatures ( fig 1 ) is needed in order to obtain a boiling point/composition diagram.
- The graph shown is for a mixture of hexane and heptane, which forms an ideal liquid
mixture.
- Since vapour pressure does not increase linearly with an increase in temperature, the
vapour pressure lines at the different temperatures are not parallel.
- Boiling-point composition diagrams are usually plotted at atmospheric pressure, though
for an ideal liquid mixture they’d look more or less the same whatever the chosen pressure.
- The horizontal line is at 1 atm (760 mmHg), and gives the composition of the liquid
that gives rise to a vapour pressure of 1 atm at each temperature.
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- Boiling-point composition diagrams are usually plotted
at atmospheric pressure
- For an ideal liquid mixture they’d look more
or less the same whatever the chosen pressure.
- The horizontal line is at 1 atm (760 mmHg), and gives
the composition of the liquid that gives rise to a vapour pressure of 1 atm at each temperature.
- The points generated are then transferred to
axes having the same composition scale but whose vertical axis is the temperature (fig 2).
- The boiling point/composition diagram is now completed
by putting in the line that represents the vapour composition (shown in red).
- The vapour in equilibrium with liquid at a given temperature
is always richer in the more volatile component.
- The vapour composition is shown by a point at the same
temperature,
- Its composition is nearer to the more volatile
(lower boiling temperature) constituent.
- The link between the vapour pressure/composition and
boiling temperature/composition diagrams for non-ideal (azeotropic) mixtures is the same,
- Such systems give a more complex diagram because of
the vapour pressure maximum or minimum.
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