Distillation (Ch.4)

What is distillation?

A separation process that isolates liquids based on differences in boiling points.

What is the boiling point (BP)?

The temperature at which a liquid's vapor pressure equals atmospheric pressure.

Why is boiling point not a perfect measure of purity?

BP depends on pressure, unlike melting point.

How does atmospheric pressure affect boiling point?

Higher pressure → higher boiling point; lower pressure → lower boiling point.

What is vapor pressure?

Pressure exerted by molecules escaping from the liquid phase into vapor.

When does a liquid boil?

When its vapor pressure equals atmospheric pressure.

What is the purpose of this distillation experiment?

1. Measure boiling points

2. Separate cyclohexane + toluene mixture

3. compare simple vs fractional distillation

What are the boiling points of the liquids used?

Cyclohexane: 81°C

Toluene: 111°C

How does a mixture of two miscible liquids boil?

At a temperature between the boiling points of the pure liquids.

What determines the boiling point of a liquid mixture?

The mole fraction of each liquid and their vapor pressures.

Raoult's Law equation?

Ptotal​= (X1)​ (P1^0)​+ (X2)​(P2^0)​

What does each symbol in Raoult's Law mean?

1. X1​,X2​: mole fractions

2. P10,P20P_1^0, P_2^0P10​,P20​: vapor pressures of pure liquids

3.Ptotal​: total vapor pressure

What is an azeotrope?

A mixture that boils at a constant temperature and cannot be separated by simple fractional distillation.

Example of an azeotrope?

95% ethanol + 5% water (bp 78°C).

Why can't azeotropes be fully separated by distillation?

Components vaporize together at a fixed ratio.

Positive azeotrope?

Boiling point lower than either pure component.

Negative azeotrope?

Boiling point higher than either pure component.

What is a theoretical plate?

One vaporization-condensation cycle.

How does efficiency relate to theoretical plates?

More theoretical plates → higher purification efficiency.

Which is more efficient: simple or fractional distillation?

Fractional, because it has more theoretical plates.

What is simple distillation used for?

Separating liquids with boiling points differing by ≥ 50-75°C.

Why is simple distillation less efficient?

Short distance from flask to condenser → fewer vapor cycles → poorer separation.

Example condition where simple distillation fails?

When boiling points are close (e.g., cyclohexane 81°C and toluene 111°C).

What makes fractional distillation more efficient?

A fractionating column that provides multiple condensation-vaporization cycles.

What is the purpose of packing material (steel wool, copper sponge)?

Provides large surface area for vapor-liquid contact → increases theoretical plates.

When is fractional distillation used?

When liquids have close boiling points.

What is the key feature of the fractional distillation setup?

Long vertical column between flask and condenser.

Why does fractional distillation give better separation?

Multiple vaporization-condensation steps create a gradient where the lower BP component enriches at the top.

What does a temperature vs. volume plot tell you?

Shows how effectively each component is separated during distillation.

What happens if you distill too fast?

Poor separation; components distill together.

Q: What is the typical setup angle for a distillation condenser?

Slight downward slant into receiving flask to allow proper liquid flow.