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AQA A Level Chemistry > 3.16 Chromatography > Flashcards

3.16 Chromatography Flashcards

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1
Q

What is the mobile phase?

A

Where molecules can move

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2
Q

What is the mobile phase always?

A

Always liquid or gas

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3
Q

What is the stationary phase?

A

Where molecules can’t move

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4
Q

What is the stationary phase always?

A

Solid, or a liquid on solid support

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5
Q

What does the distance that each substance moves up the plate by depend on?

A

Substance’s solubility in mobile phase and its retention by stationary phase

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6
Q

State 3 types of chromatography

A
  • Thin-Layer Chromatography
  • Column Chromatography
  • Gas Chromatography
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7
Q

State what the stationary phase in thin-layer chromatography is

A

thin layer of silica (silicon dioxide) or alumina (aluminium oxide) fixed to glass or metal plate

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8
Q

When handling the plate in TLC, what should you do and why?

A

Wear gloves to avoid contamination by substances on your hand

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9
Q

Describe a method for thin-layer chromatography (TLC)

A
  1. Draw line in pencil near bottom of TLC plate (the baseline) & put small drop of each mixture on line
  2. Allow spots to dry
  3. Place plate in beaker with small volume of solvent (mobile phase)
    • Solvent level must below baseline
  4. As solvent moves up plate, it carrier substances in mixture with it
    • Some chemicals will be carried faster than other and and travel further up plate
  5. Remove plate from beaker when solvent has nearly reached top of plate
  6. Before it evaporates, use a pencil to make how far solvent travelled up plate (solvent front)
  7. Place plate in fume cupboard to dry
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10
Q

Why must the solvent level be below the baseline?

A

So solvent doesn’t dissolve samples away

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11
Q

Why should you place a TLC plate in fume cupboard to dry?

A

Fume cupboard will prevent any toxic or flammable fumes from escaping into room

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12
Q

What is the result produced from TLC called?

A

Chromatograph

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13
Q

Name 2 substances you can use to reveal colourless chemicals

A
  • UV Light
  • Iodine
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14
Q

Describe how you can use UV light to reveal colourless chemicals

A
  1. Many TLC plates have special fluorescent dye added to silica or alumina layer that glows when UV light shines on it
  2. Can put plate under UV lamp and draw around dark patches to show where spots of chemical are
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15
Q

Describe how you can use iodine to reveal colourless chemicals

A
  1. Expose chromatogram to iodine vapour
    • i.e. leave plate in sealed jar with some iodine crystals
  2. Iodine vapour is a location agent - sticks to chemicals on plate and they’ll show up as brown/purple spots
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16
Q

How can you find the number of chemicals present in a TLC chromatograph?

A

Count no. of spots formed on plate

17
Q

How can you find out what each chemical is from TLC?

A

By calculating a Rf value

18
Q

State the formula to calculate a Rf value

A
19
Q

Once you’ve calculated a Rf value, what do you do next?

A

Look up Rf values in table of standard Rf values to identify what the substance is

20
Q

Name 3 factors that can change Rf values

A

If composition of TLC plate, solvent or temperature changes even slightly = get different Rf values

21
Q

As it’s hard to keep conditions ideal in TLC, what can you do to combat the differences in Rf values?

A

∴ if you suspect mixture contains e.g. chlorophyll best to put spot of chlorophyll on baseline of same plate as mixture and run them both at same time

22
Q

What is column chromatography used for?

A

Used for purifying an organic product

(Done to separate product from unreacted chemicals and by-products)

23
Q

Describe column chromatography (method)

A
  1. Involves packing glass columns with slurry of absorbent material e.g. aluminium oxide, coated with water
  2. Mixture to be separated is added to top of column and allowed to drain down into slurry
  3. Solvent is then run slowly and continually through column
24
Q

What is the stationary phase in column chromatography?

A

Water-coated particles of Al2O3

25
Q

What is the moblie phase in column chromatography?

A

Solvent

26
Q

Describe how a mixture is separated in column chromatography

A
  1. As mixture is washed through column = its components separate out according to how soluble they are in mobile phase and how strongly they are adsorbed onto stationary phase (retention)
  2. More soluble each component is in mobile phase, quicker it’ll pass through column
27
Q

What is gas chromatography (GC) used for?

A

Used to separate mixture of volatile liquids so you can identify them

28
Q

What is the stationary phase in gas chromatography?

A

Solid or solid coated by viscous liquid e.g. oil packed into long tube

29
Q

How and where is the tube arranged in GC?

A

Tube is coiled to save space and built into an oven

30
Q

What is the mobile phase in gas chromatography?

A

Unreactive carrier gas e.g. nitrogen

31
Q

What is meant by rentention time in GC?

A

The time taken for a component to travel between the injection point and the detector

32
Q

Describe how you identify components of mixture in GC

A

Each separate substance will have unique retention time = so use retention time to identify components of mixture

(Have to run known sample under same conditions for comparison)

33
Q

What does retention time depend on?

A

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

34
Q

What does the area under each peak show?

A

Relative amount of each component that’s present in mixture

35
Q

Name two uses of GC

A
  • To find level of alcohol in blood or urine
    • Very accurate
  • To find proportions of various esters in oils used in paints
    • Allows picture restorers know exactly what paint was originally used
36
Q

Describe how gas chromatography-mass spectrometry (GC-MS) works

A
  1. Sample is separated using gas chromatography, but instead of going to detector, separated components are fed into mass spectrometer
  2. Spectrometer produces mass spectrum for each component = used to identify each one and show what original sample consisted of
37
Q

Describe the advantage of using GC-MS over GC

A

Components separated out by chromatography can be positively identified which can be impossible from chromatogram alone

38
Q

Describe how the GC-MS process can be automated

A

Computers can be used to match up mass spectrum for each component of mixture against database

39
Q

Why does two-way chromatography makes identification of the components of the mixture more certain?

A

Each component has 2 Rf values both of which must match with a known component

AQA A Level Chemistry (36 decks)

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