Unit 4 - Choromatography

Question 1

What is chromatography

Answer 1

• a method used to seperate soluble substances by their partition between two different phases
  (A method to separate mixture of substances )

Question 2

What phases to all types of chromatography have

Answer 2

Mobile phase

Stationary phase

Question 3

What is the stationary and mobile phase

Answer 3

Stationary phase - substance stays fixed inside the column

Mobile phase - solvent moving through column

Question 4

What does TLC use

Answer 4

Instead of chromatography paper (paper chromatography ) TLC uses a thin layer of silica gel or alumina coated on glass/ metal or plastic

Question 5

Silica gel is a _____ formed from silicic acid that has many _______ groups
What does this do ?

Answer 5

Polymer
Hydroxyl
Makes the surface of the gel very polar

Question 6

So.. ______ substances tend to be held by the gel , making their movement much slower than that of ___ ______ substances

Answer 6

Polar

Non polar

Question 7

Remember : chromatography separates substances by making use of differences in their _____ or _______ ______

Answer 7

Polarity

Molecular size

Question 8

What makes up the stationary phase / mobile phase in TLC

Answer 8

The water held on silica gel makes up stationary phase

The mobile phase is a suitable solvent or mixture of solvents

Question 9

Different components in the mixture travel across the _________ _____ at different rates
What does this allow

Answer 9

Stationary phase (the silica gel)
This allows them to separate as the mobile phase moves along the stationary phase

Question 10

How is a TLC plate prepared and explain the chromatography happening

Answer 10

• to prepare a TLC plate , a fine pencil line called the base line is drawn about 1.5cm from one end. A pencil cross is put where a sample is to be added.
• the sample solution is spotted on several times to get a concentrated spot in the plays
• the TLC plate is then placed in a suitable solvent (the mobile phase ) with the solvent is below the spot
• the solvent travels up the plate, and the components of the sample separate out according to their relative attractions to the stationary phase on the plate
• once finished a pencil line is drawn to show where the solvent has reached on the chromatogram - this is called the solvent front.

Question 11

The distance the solvent has moved is measured in _______

Answer 11

Centimetres

Question 12

How is the Rf value (retardation value) calculated

Answer 12

Rf = distance travelled by the sample (to centre of spot) / distance travelled by solvent (where the solvent front is)

Rf = spot / solvent front

Question 13

Remember
: molecules that have more polarity will move _____ through the plate than molecules with less polarity
Molecules with a large molecular mass will move more _____ than molecules with a low molecular mass

Answer 13

• slower

- slowly

Question 14

So… larger and more polar molecules will have _____ rf values

Answer 14

Lower

Question 15

The uses of TLC

Answer 15

• to identify a compound /substance

- to assess the purity of a compound

Question 16

Identifying a compound / substance with TLC

Answer 16

The identity of a compound can be confirmed by comparing Rf values with a literature or known value determined under the same conditions. Pure samples can be run alongside the sample being analysed to see whether the same Rf values are obtained

Question 17

Assessing the purity of a compound using TLC

Answer 17

• another way to determine whether a sample is pure is to use a co spot.
• this is a 50:50 mixture of the sample and a known pure sample and it can be spotted on the same cross. If the sample is pure then only one spot will be present on the chromatogram. If the sample is impure, then more than one spot will be observed

Question 18

What has an effect on Rf value for an individual component

Answer 18

• type of stationary phase
• composition of mobile phase
• polarity of component
• the solvent used
• Distance solvent front moves has no effect
• distance solvent moves has no effect
• concentration of sample has no effect
• length of TLC plate has no effect