(O) Topic 33 - Chromatography

Question 1

What is a mobile phase?

Answer 1

a phase where molecules can move, ad is always either a liquid or gas.

Question 2

What is a stationary phase?

Answer 2

a phase where molecules cannot move. This must be a solid, or a liquid on a solid support.

Question 3

What are the basic principles of chromatography?

Answer 3

1) the mobile phase moves through or over the stationary phase
2) the distance each substance moves up the plate depends on its solubility in the mobile phase and its retention by the stationary phase
3) Components that are more soluble in the mobile phase will move further up the plate

Question 4

What are the 4 types of chromatography

Answer 4

1. Paper Chromatography
2. Column Chromatography
3. Gas Chromatography
4. Thin Layer Chromatography (TLC)

Question 5

Describe how thin layer chromatography would be carried out

Answer 5

1. In TLC, the stationary phse is a thin layer of silica or alumina, fixed to a glas or metal plate.
2. Draw a pencil line near the bottom of the TLC plate, this is called the baseline.
3. Put a small drop of the mixtures being tested/ seperated on the line and allow to dry
4. Place the plate in a beaker with a small volume of solvent in, which must sit below the baseline.
5. Allow the solvent to move up the plate. It will bring the substances with it, seperating them out.
6. Stop the experiment by removing the plate from the solvent just before the solvent front reaches the top, and draw on a line to mark the solvent front
7. Leave the plate to dry and identify the positions of the substances and calculate the Rf values

Question 6

Why must the solvent sit below the baseline?

Answer 6

So the sample does not dissolve away

Question 7

What is the final sample plate produced by chromatography called?

Answer 7

A chromatogram

Question 8

How can colourless chemicals be identified in chromatography?

Answer 8

1. Many TLC plates have a fluorescent dye which will glow when UV light shines on it. Draw around the dark patches to mark the position of the substances.
2. Expose the chromatogram to iodine vapour, which will cause the chemicals on the plate to show up as brown or purple spots.
3. Spray the chromatogram with ninhydrin to detect amino acids. Causes them to appear as a purple colour.

Question 9

What are Rf values used for?

Answer 9

Identifying substances using a table of standard Rf values.

Note: if you use a temperature other then standard, 298K, you will get different Rf values

Question 10

What is the main use of column chromatography?

Answer 10

Purifying organic products

Question 11

Describe how column chromatography works

Answer 11

1. Agluc column is packed with an absorbent material such as aluminium oxide. This is the stationary phase.
2. The mixture to be seperated is added to the top of the column, and a solvent is then run slowly and continually throught the column (this is the mobile phase)
3. As the mixture is washed through the column, its components seperate out according to how soluble they are in the mobile phase, and how strongly they are adsorbed onto the stationary phase
4. Therefore each substance will pass out the bottom of the column chromatography at a different time

Question 12

Describe how gas chromatography works

Answer 12

1. The stationary phase is a solid, or a solid coated by a viscous liquid, packed into a long tube and built into an oven.
2. The mobile phase is an unreactive carrier gas such as nitrogen
3. Each component takes a different amount of time from being injected into the tube to being recorded at the other end. This is retention time.

Question 13

Why do different substances have different retention times and why is this useful

Answer 13

Retention time depends on how much time the component spends with the carrier gas, and how long it spends stuck to the viscous liquid. It can be used to id entify the components of a mixture, as each seperate substnace will have a unique retention time.