CHEM 2122 / Techniques 1: Distillation

What is distillation? How does it basically work?

Purification of liquid (liq) with low boiling point (bp).

1. Heat solution (sol).

2. Vapours rise to heat up the thermometer and vapours go into condensor.

3. Condensor condenses the vapours to give a liquid.

4. Liquid runs down condensor and drips into collecting vessel.

What does contamination of solids and liquids lead to?

• Contamination leads to decreasing and broadening of mp of solids.

• Contamination leads to increasing or decreasing of bp of liquids.

What are the three common types of distillation? Differentiate each.

• Simple distillation: Separates liquid from dissolved solid or two liquids that differ in bps > 100ºC.

• Fractional distillation: Separates liquids differing in bps < 100ºC. Used for separating components in crude oil.

• Steam distillation: Separating water insoluble compound using steam. Used for separating natural products.

Explain the principle of distillation.

> When the liquid in the distillation flask boils, vapour rises to the top of the flask, through the three-way adapter (aka distillation head), past the thermometer, out the side arm (the whole piece to the right of the and into the condenser. Some vapour will condense on the walls of the flask, three-way adapter, and on the thermometer.

> As long as vapour is flowing out the side arm, the condensed liquid on the tip of the thermometer is in equilibrium with said vapour, and an accurate boiling point can be determined.

> Generally, when thermal equilibrium has been reached, the boiling point remains relatively constant (not changing by more than 1-2°C) during the distillation. When a significant change in temperature (decrease or increase by 5°C) is observed, the vapour composition has changed and therefore it's likely that another compound will come out of the take-off/tail adapter. When the temperature changes significantly, collect the distillate with another vessel since this component will not be pure.

Explain how distillation is set up.

> Place the distillation flask on hot plate, and clamp the distillation flask. Liquid must be half of the round-bottom flask to ensure accurate temp reading.

> Insert three-way adapter.

> Insert condenser into three way adapter. Use elastic to bind the three way adapter and the water out inlet.

> Clamp the condenser.

> Insert the take-off adapter. Use another elastic to bind the water out inlet and the take off adapter.

> Insert the thermometer into three-way adapter. Tip of thermometer right below the intersection of three-way adapter.

> Place distillate flask below take-off adapter and in an ice bath.

> Heat up. General distill rate would be 1 drop/second, controlled by heating temp.

Why should the round-bottom flask be no more than half full during a distillation process?

> Round-bottom flask more than half full.

> Bumping/splashing into condenser.

> Contamination.

What causes bumping during boiling, and how can it be prevented for smooth, even distillation?

> Superheating the liquids + Pressure build-up.

> Bumping =Increasing energy required for bubble formation.

> Bubble forming with explosive power leading to splashing of solvent.

> Stir bead/boiling chip = Provides good nucleation sites.

> Good nucleation sites = Even boiling.

+ Smooth + consistent bubbles = good!

Why is it important not to make the clamps too tight in a distillation setup?

> Clamp tight.

> Distort/misalign glassware.

> Small gaps between connections.

> Vapour leaks = Nothing collected.

What could happen if it is positioned too low or too high?

> Thermometer too low/high.

> Bulb not in vapour path.

> Inaccurate temp reading.

How do you deconstruct the distillation setup?

In the reverse order of setting the distillation setup.

What are the two main purposes of distillation in the context of separating and purifying liquids?

> Removing hazardous compounds from solvent or liquid into the distillation flask as a non-volatile solid or concentrated solution.

> Purifying volatile liquids.

What happens if you have a mixture of two or more liquids in regards to their boiling bps when heated up together?

What is an azeotrope?

> Components will interact.

> Mixture made.

> New bp made.

> Bp of either component not collected.

What is an azeotrope, and how does its boiling point compare to its components?

An azeotrope is a liquid mixture that boils at a constant temperature, and its boiling point can be either higher or lower than that of its individual components.

What should you do when the temp of distillation head changes?

When the temp of the distillation head changes, collect distillate w/ a different flask for purity.

What should you record during distillation when droplets start collecting in the receiving vessel, and why?

Record the boiling point or the range of boiling points at which the droplets start to collect, as this indicates the temperature at which the component is distilling over.

Why should you never distill the distillation flask to dryness?

Presence of liquid prevents flask from overheating, breaking, and pyrolytic tars.

Why should the distillation flask never be too large during distillation?

> Large flask.

> Condensation and collection inside.

> No reaching the condenser.

Why should the distillation flask never be too small during distillation?

> Small flask.

> Loss of mixture from foaming, splashing, or boiling over into condenser.

> Poor yield and contamination.

For a liquid to boil, what does the temp need to be?

For a liquid to boil, the setup needs to be slightly above bp before initiation of bubble formation.

If bubbles of vapours form within this liquid, what are the two things that could happen?

> Bubbles will collapse if its below a minimum size because of the surface tension of the liquid.

> Bubbles will grow to the surface of the liquid.

Why is it problematic if bubbles collapse or grow unevenly during boiling?

> If bubbles collapse below a minimum size due to surface tension, it can cause uneven heating because not all bubbles are properly made.

> If bubbles grow too large, they can increase the risk of splashing or bumping.

If a liquid that is free of solid impurities or free of dissolved gases is heated slowly, what does the temp tend to be?

A temp well beyond bp of the liquid can be reached without any boiling taking place.

What are the two things that can be done to prevent overheating and bubbling?

> Swirl flask.

> Add boiling add to swirl flask.

Why should boiling aids never be added to a hot liquid?

Adding boiling aids, like boiling chips, to a hot liquid can cause sudden, violent bubbling or splashing.

What does the IR spectrophotometer measure?

IR spectrophotometer will measure amount of light trasmitted through the sample.

 Why is intimate contact of the crushed analyte sample with the crystal important in IR spectroscopy?

> Maximum exposure of sample to the IR light.

> Accurate IR spectrum.

What causes poor quality in an IR spectrum, and how can it be identified?

Poor quality in an IR spectrum can result from the sample not fully covering the crystal surface. It’s often identified by a low signal-to-baseline ratio and a “hairy” appearance in the spectrum.

How does the concentration of a sample affect the quality of results in spectrophotometry?

> If the concentration is too large, it can swamp the detector, causing inaccurate readings.

> If the concentration is too small or dilute, the peaks may get lost in the noise, making it harder to detect the signal.

In regards to intensity of peaks, what will more functional groups bring about vs. less functional groups?

More functional groups in sample will absorb more and lead to higher intensity than other groups with less of their functional group.