Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Chromatography > Chromatography (A level Chemistry) > Flashcards

Chromatography (A level Chemistry) Flashcards

1
Q

What are the advantages of TLC ?

A

simple process
low cost
Quick process of separation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Why do amino acids have different Rf values ?

A

different polarities
therefore different solubilities in the developing solvent.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is a stationary phase ?

A

a solid or a liquid on a solid support

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is a mobile phase ?

A

what the mixture is added to- either a gas or a liquid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what does separation depend upon ?

A

the balance between solubility in the mobile phase and retention in the stationary phase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What do polar substances dissolve well in ?

A

polar solvents/mobile phases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What do non polar substances dissolve well in ?

A

non polar solvents/ mobile phases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the stationary phase in thin-layer chromatography ?

A

A glass, aluminium or plastic plate coated with a powdered solid using silica (SO2) or alumina(Al203)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the mobile phase in TLC ?

A

a solvent which moves up the plate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Describe the procedure of TLC

A

a pencil line is drawn on the plate, usually about 1cm from the bottom
the mixture to be analysed is spotted onto this ‘origin’ line
The plate is placed in a developing tank containing a little of the mobile phase such that the liquid level is below the line.
a lid is placed on the developing tank
The solvent will move up the plate and the components in the mixture will also move, but at different rates so will separate out.
the plate is then removed form the beaker before the solvent reaches the top
Before the solvent dries, the solvent front is marked with a pencil while it is still visible
the plate is then left to dry in a fume hood- because some solvents are toxic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Explain why gloves should be worn when preparing a TLC plate

A

to avoid contaminating the plate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Explain why a lid is placed on the developing tank

A

to ensure the atmosphere is saturated with solvent vapour. This stops the solvent evaporating as it rises up the plate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What are the three ways the spots can be visualised ?

A

spray the plate with ninhydrin solution which will turn the spots purple

expose the plates to iodine vapour- it sticks to the chemicals on the plate turning them brown/purple

Use a plate that has a fluorescent dye added to it. This dye glows under a UV lamp. The spots will mask this dye and will appear dark under the lamp and can be circled with a pencil.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

How is Rf or Retention Factor calculated ?

A

distance travelled by the spot / distance of the solvent front

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What does it mean if a component travels further up the plate ?

A

it has a greater solubility in the mobile phase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What does it mean if the components in the mixture remain on the line.

A

the component in the mixture is insoluble in the solvent chosen and the experiment will need to be repeated with a different mobile phase.

17
Q

What are the limitations of TLC ?

A

similar compounds will have similar Rf values and hence might not fully separate

Unknown compounds have no Rf value available for comparison.

18
Q

What is Column Chromatography used for ?

A

to separate mixtures of soluble compounds

19
Q

What is the stationary phase of Column Chromatography ?

A

is a powder such as silica or alumina or an ion exchange resin
it is packed into a narrow column similar to a burette

20
Q

What is the mobile phase in Column Chromatography ?

A

is a splvent
it is allowed to run in from the top of the column

21
Q

Explain how Column Chromatography works

A

The sample mixture to be separated is dissolved in a little of the mobile phase and added to the top column

As the mobile phase moves down the column the components of the sample mixture will separate out depending on the balance between how well they adsorb onto the surface of the stationary phase and how well they dissolve in the mobile phase.

As a result different components move at different speeds

Each portion can be collected separately as they leave the column at the bottom.

22
Q

What could be done to achieve better and or faster separation when using Column Chromatography ?

A

different solvents

23
Q

What is column chromatography often used for ?

A

to separate amino acids
to purify an organic product

24
Q

A mixture of ethanol and cyclohexane using column chromatography. The stationary phase is polar. State which compound will elute first and why.

A

ethanol is polar where as cyclohexane is not
This means ethanol will be more attracted to the stationary phase than the mobile phase so will be more retained.
Therefore cyclohexane elutes first as it is non-polar so more attracted to the mobile phase.

25
Q

Why is the origin line drawn in pencil ?

A

it is insoluble so will not be carried up the plate.

26
Q

Why must the solvent depth be below the origin line ?

A

to prevent washing the samples away

27
Q

why is a lid placed over the developing tank ?

A

to stop solvent evaporating as it rises up the plate

28
Q

What is Gas Chromatography used to separate ?

A

volatile compounds

29
Q

What is the stationary phase in Gas chromatography ?

A

is a solid or a thin layer of liquid
The capillary column

30
Q

What is the mobile phase ?

A

the mobile phase is usually inert ?

31
Q

Explain the process of Gas Chromatography

A

the sample is injected into instrument, vaporised and flushed through the column by the mobile phase.

Separation usually carried out under pressure at high temperatures

Compounds are separated according to their relative solubilities in the mobile phase, the greater the compounds attraction to the stationary phase the more it is slowed down/retained.

Each component of the mixture is detected as it leaves the column.

Data is presented a graph called a chromatogram,

32
Q

In a chromatogram what is the area under the peak proportional to ?

A

the amount of the component present in the sample

33
Q
A

Chromatography (1 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions