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A Level Chemistry > Chromatography > Flashcards

Chromatography Flashcards

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1
Q

What is chromatography used for?

A

To separate and identify the components in a mixture.

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2
Q

What is the mobile phase in chromatography?

A

Where the molecules can move: always a gas or liquid.

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3
Q

What is the stationary phase in chromatography?

A

Where the molecules can’t move: either a solid or a liquid on a solid support.

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4
Q

How do the mobile and stationary phase interact with eachother?

A

The mobile phase moves through or over the stationary phase

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5
Q

What does the distance each substance moves (during chromatography) depend on?

A

Its solubility in the mobile phase and its retention by the stationary phase

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6
Q

Why will some components travel further than others?

A

Components that are more soluble in the mobile phase will travel further than components that are more strongly adsorbed to the stationary phase

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7
Q

What separates the components of a mixture?

A

The differences in retention (to the stationary phase) and solubility (in the mobile phase)

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8
Q

What are the three types of chromatography you need to know?

A

Thin-layer chromatography
Gas Chromatography
Column chromatography

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9
Q

What is the mobile phase in TLC?

A

A liquid solvent e.g. ethanol

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10
Q

What is the stationary phase in TLC?

A

A thin layer of silica (silicon dioxide) or alumina (aluminium oxide) FIXED to a glass/metal plate

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11
Q

How do you separate a mixture using TLC?

A

Draw pencil line near bottom of TLC plate
Add small drop of mixtures on line
Allow spots on plate to dry
Place plate in beaker w/ small volume of solvent (solvent level must be below baseline so that it doesn’t dissolve your samples away
Cover the top of the beaker with a watch glass
Solvent moves up the plate, as solvent moves up it carries the substances in the mixture
Leave beacker until solvent has reached top of plate
Use pencil to mark how far the solvent has travelled up the plate
Place plate in fume cupboard and leave to dry
Use the result (called a chromatogram) to use the positions of the chemicals on the chromatogram to identify the chemicals.

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12
Q

Why do we need to use a fume cupboard during TLC?

A

The fume cupboard will prevent any toxic or flammable fumes from escaping into the room

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13
Q

Why must the solvent be below the baseline?

A

So it doesn’t dissolve your samples away

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14
Q

What is the baseline?

A

The pencil line near the bottom of the TLC plate.

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15
Q

What is the result called at the end of thr TLC experiment?

A

A chromatogram

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16
Q

How do you calculate the Rf value?

A

distance travelled by spot / distance travelled by solvent

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17
Q

How do you make colourless spots on the chromatogram visible? Give one way

A

Add a special fluorescent dye to the silica/alumina layer that glows when UV light is shone on it. Put plate under UV lamp and draw around the dark patches to show where the spots of chemical are

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18
Q

How do you make colourless spots on the chromatogram visible? Give the second way

A

Expose the chromatogram to iodine vapour. Iodine vapour is a locating agent, it sticks to the chemicals on the plate and they will show up as purple spots.

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19
Q

How do you ‘expose’ the chromatogram to iodine vapour?

A

Leave the plate in a sealed jar with a couple of iodine crystals

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20
Q

When measuring for the Rf value, where do you measure from?

A

The baseline to the vertical centre of the spot

21
Q

If you suspect that a mixture contains a certain substance, what should you do to ensure that?

A

Say you believed that the mixture contained chlorophyll, then you would add a drop of chlorophyll to the baseline as well as a drop of the mixture to the baseline. Run them both at the same time.

22
Q

What does ‘CC’ stand for?

A

Column chromatography

23
Q

What is column chromatography used for?

A

Purifying an organic product
This is done to separate it from unreacted chemicals or side products

24
Q

What is the stationary phase of column chromatography?

A

A glass column that is packed with a solid, absorbent material such as aluminium oxide coated with water ( this is called a ‘slurry’)

25
Q

What is the mobile phase of column chromatography?

A

The solvent that is run slowly and continually through the column

26
Q

Describe the column chromatography apparatus:

A

It involves packing a glass column with a solid absorbent material e.g. aluminium oxide coated with water – this is called a slurry. The mixture to be separated is added to the top of the column and allowed to drain down into the slurry. A solvent is then run slowly and continually through the column - this is the mobile phase.

27
Q

How does column chromatography work?

A

As the mixture is washed through the column, its components separate out according to how soluble they are in the mobile phase and how strongly they are adsorbed onto the stationary phase (retention).
The longer a component spends dissolved in the mobile phase, the quicker it travels down the column.
If a component spends a long time adsorbed onto the stationary phase - it will take a long time to travel down the column. The more soluble a component is in the mobile phase, the quicker it will pass through the column.

28
Q

What happens when a component of the mixture reaches the end of the column in ‘CC’?

A

As it reaches the end, it is collected, it can then be identified using the time taken to pass through the column or you could use mass spectrometry to identify the component.

29
Q

What is meant by ‘retention time’?

A

the time taken for a component of a mixture to pass through the column (in CC)

30
Q

What does ‘GC’ stand for?

A

Gas chromatograpghy

31
Q

When do we use GC?

A

If you’ve got a mixture of volatile liquids then we used ‘GC’ to separate them out and identify them

32
Q

What are volatile liquids?

A

Ones that turn into gases easily - evaporate easily.

33
Q

What is the stationary phase in GC?

A

A solid or a solid coated by a viscous liquid such as oil, packed into a long column.

34
Q

What is the mobile phase in GC?

A

An unreactive carrier gas

35
Q

What is the apparatus used for gas chromatography?

A

A solid or a solide coated by a viscous liquid such as oil, packed into a long column. The column is coiled, to save space, and is built into an oven. The mobile phase is an unreactive carrier gas e.g. nitrogen. The sample is vapourised and passed through the oven as a gas.

36
Q

How does gas chromatography work?

A

Each component takes a different amount of time from being injected into the column to being recorded at the other end (a retention time). These times are shown on a chromatogram.

37
Q

Why is the column coiled?

A

To save space

38
Q

What does the retention time depend on in gas chromatography?

A

How much time the component spends moving along with the carrier gas and how much time it spends adsorbed to the liquid

39
Q

How can you identify the different components of a mixture in GC?

A

Using each of their retention times

40
Q

What does the area under each peak tell us on the gas chromatogram?

A

the relative amount of each component in the mixture

41
Q

What conditions does GC have to be in?

A

Under standard conditions, this will give each component a unique retention time.

42
Q

In everyday, what can GC be used for?

A

to find the level of alcohol in blood or urine.

43
Q

What is GC-MS?

A

Gas chromatograpghy - mass spectrometry

44
Q

What are the advantages of GC that make GC-MS good together?

A

It is very good at separating a mixture into its individual components but isn’t very good at identifying those components.

45
Q

What are the advantages of MS that make GC-MS good together?

A

It is a technique that is used to identify substances from their mass/charge ratio. It is very good at identifying unknown compounds HOWEVER it would give confusing results from a mixture of substances

46
Q

What is the benefit of using GC-MS?

A

Combines the benefits of mass spec and gas chromatography to make a useful analytical tool

47
Q

How does GC-MS work?

A

The sample is separated by gas chromatography but instead of going to a detector, the separated components are fed into a mass spectrometer. The spectrometer produces a mass spectrum for each component of the mixture against a database so that the whole process can be automated.

48
Q

Why is GC-MS better than GC on its own?

A

the components separated out can be positively identified - which is normally impossible using a chromatogram alone as similar compounds often have very similar retention times.

A Level Chemistry (17 decks)

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