Exp 2 - Distillation

OCHEM LAB LECTURE FALL 25

What is distillation?

A purification technique that separates liquids based on differences in boiling points.

What is a boiling point?

The temperature at which a liquid becomes a gas due to overcoming intermolecular forces

Do boiling points depend on intermolecular forces?

Yes — stronger forces → higher boiling point

Why is symmetry less important for boiling point than melting point?

Because boiling involves molecules escaping entirely — no crystal “stacking.”

Which molecule has the higher BP: large or small?

Larger molecules—greater surface area → stronger London dispersion forces

Why does CH₃–I have a higher BP than CH₃–F?

Iodine is bigger → more polarizable → stronger London forces

What is polarizability?

How easily an atom’s electron cloud shifts in response to its environment. Larger atoms are more polarizable.

Rank intermolecular forces from strongest to weakest

H-bonding > dipole–dipole > London dispersion (LDF)

Higher polarizability causes what change in boiling point?

Higher BP

What is a phase composition diagram?

A graph showing how liquid and vapor compositions change with temperature and mole fraction during boiling

In a benzene/toluene diagram, what does Xₜ = 1 mean?

Pure toluene

In a benzene/toluene diagram, what does Xₜ = 0 mean?

Pure benzene

Where does the mixture start boiling on the diagram?

At the liquid line

Where do you find the vapor composition at boiling?

Go horizontally from the boiling point to the vapor line

A 50/50 benzene:toluene mixture begins boiling at approximately what temperature?

~91°C

At 91°C, what is the vapor composition?

~20% toluene / ~80% benzene

Why is the vapor richer in benzene?

Benzene has a lower BP → vaporizes more easily

What happens when vapor at 20% toluene condenses?

The new liquid boils at ~84°C

Why does redistillation further enrich the lowest BP component?

Lower BP components preferentially enter the vapor phase

What is an azeotrope?

A liquid mixture with a constant boiling point whose composition cannot be changed by simple distillation. (Ex: 95% ethanol/water.)

When do you use simple distillation?

When the BP difference ≥ 40°C

When do you use fractional distillation?

When the BP difference < 40°C

What does a fractionating column do?

Performs multiple mini-distillations, improving separation

Why does vapor rise in a distillation apparatus?

Heating increases kinetic energy → molecules enter gas phase

What does cold water in the condenser achieve?

Cools vapor → condenses it into liquid

In a simple distillation, which compound comes out first?

The lower boiling point component. (Cyclohexane in your experiment.)

Why does the high-BP component come out later?

Requires more heat to enter vapor phase

What is the purpose of collecting three 10-mL fractions?

To track composition changes throughout the distillation

What data do you record during distillation?

Temperature at first drop and at each 1-mL increment

What type of graph do you generate from distillation data?

Temperature vs. volume graph

Why doesn’t BP change immediately during distillation?

Composition of liquid and vapor remains stable until one component decreases significantly

What is retention time in GC?

The time it takes a compound to travel through the GC column. Lower BP → lower retention time

In GC, which compound appears first?

Lower boiling point component (cyclohexane)

What did GC Fractional Fraction 1 show?

~80% cyclohexane, ~20% toluene

What did GC Fractional Fraction 3 show?

~4.5% cyclohexane, ~95.4% toluene

Why does Fraction 3 contain mostly toluene?

Most cyclohexane has already distilled off earlier

What mixture is distilled in your experiment?

50/50 cyclohexane and toluene

Why are simple and fractional distillations compared?

To evaluate purification effectiveness.

Which type of distillation gives better separation?

Fractional distillation

Why does fractional distillation give better separation?

Multiple vapor–condensation cycles occur inside the column

Why can’t simple distillation fully separate close-boiling liquids?

Only a single vaporization/condensation step occurs

What is the role of the hot plate?

Provides thermal energy to initiate boiling

What happens if heat is applied too quickly?

Poor separation due to rapid vapor mixing

Why do we analyze Fractions 1 and 3 specifically?

They represent “early” and “late” distillate, showing separation effectiveness