Brown's Chemistry Advanced A Level Notes - Theoretical–Physical
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 8
Part 8.5 The Theory and Practice of Steam Distillation
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What is steam distillation? Why is steam
distillation useful for thermally unstable compounds?
What apparatus do
you need? How do you do a steam distillation?
A steam distillation
calculation to theoretically predict the composition of the distillate
are fully explained.
8.5 Extraction using steam distillation of immiscible liquids
distillation of miscible liquids involving water e.g. fractionally
distilling fermented sugar solution to extract ethanol)
- The technique of steam distillation
is a very useful method for extracting molecules with a high boiling
point, which under normal distillation conditions might thermally
- i.e. the kinetic energies of the
molecules at the boiling point may be sufficient to overcome the
activation energies of possible reactions such as decomposition
into smaller molecules or transformation into another molecule
of similar size.
- As has been stated earlier, when a
liquid is heated, the vapour pressure rises, and when it equals the
ambient pressure the liquid boils.
- If a mixture of two immiscible
liquids (or solutions) is heated, BOTH molecules can contribute to
the vapour pressure and BOTH will express their full saturated
vapour pressure because being immiscible, the two liquids act
- so at a given temperature
Ptot = pA
+ pB, and is irrespective of the actual ratio of
the volumes of liquids.
Ptot = pH2O
+ product, where Pproduct is the vapour
pressure of the material extracted
- If the larger proportion of Ptot
is from water vapour, you can then distil over substances at
~100oC which might normally only distil over at much higher
temperatures of say 150–300oC.
- This means you can predict two
things from vapour pressure tables data:
- (1) the temperature at which
the mixture will distil over i.e. at normal pressure in the
- i.e. when Ptot
= pH2O + pproduct = 760 mmHg
- In the calculation
procedure below to theoretically calculate the
percentage by mass of water and desired product by mass,
p = partial pressure, n = mol, m =
mass in g, M = relative molecular mass
- (2) the composition of the
distilled mixture if you know the distillation temperature
- you do this calculation
from the vapour pressure ratio as follows
= nproduct/nH2O where
n = number
of moles of each component
- this assumes mole
ratio and vapour pressure ratio are identical, which
is strictly speaking only true for ideal gases, but
accurate enough in this context.
- So, bringing in relative
molecular masses and mols n = mass m/Mr
= (mproduct/Mproduct) / (mH2O/18)
= (Pproduct x Mproduct) / (PH2O
- to give the mass ratio
in the distillate
- using the mass ratio
- % mext
x 100) / (mproduct
- In practice the vapour pressure due
to water often far exceeds that of the extracted material BUT the
important thing to realise is that a thermally unstable material can
then be distilled over at a temperature much lower than its
decomposition or transformation temperature.
Below is shown the
experimental technique for
extracting phenylamine from a
nitrobenzene reduction in section 8.5 Organic Redox reactions
- The technique is widely used in the
extraction of molecules from natural products e.g.
- In the perfumery industry, perfume molecules like those
from lavender can be extracted from natural plant materials
using steam distillation – an
unforgettable 'odour' experienced from the rural distilleries of
Provence whilst driving in France! – not that the 'kids' where
interested back in those days!
- Vacuum or reduced pressure
distillation is a superior method BUT is not always as convenient?
or practical? or cheap? in the context of a rural industry!
Part 8 sub–index:
8.1 Vapour pressure origin and examples * 8.2.1
Introduction to Intermolecular Forces * 8.2.2
Detailed comparative discussion of boiling points of 8 organic molecules * 8.3
Boiling point plots for six
homologous series * 8.4 Other case studies of
boiling points related to intermolecular forces * 8.5 Steam
distillation – theory and practice * 8.6 Evidence and theory
for hydrogen bonding in simple covalent hydrides *
Solubility of covalent compounds, miscible and
Advanced Equilibrium Chemistry Notes Part 1. Equilibrium,
Le Chatelier's Principle–rules * Part 2. Kc and Kp equilibrium expressions and
calculations * Part 3.
Equilibrium and industrial processes * Part 4.
solubility product and ion–exchange * Part 5.
pH, weak–strong acid–base theory and
calculations * Part 6. Salt hydrolysis,
Acid–base titrations–indicators, pH curves and buffers *
Redox equilibria, half–cell electrode potentials,
electrolysis and electrochemical series
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