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Chemistry A2 > Required Practical 12: Chromatography > Flashcards

Required Practical 12: Chromatography Flashcards

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1
Q

Chromatography

A

Analytical technique by which a chemical mixture is separated into it components

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2
Q

Mobile Phase

A

Moves and carries soluble components of mixture, i.e. solvent in TLC or eluent in column chromatography

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3
Q

Stationary Phase

A

Attracts components of mixture, i.e. filter paper or silica plate in TLC

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4
Q

Thin Layer Chromatography

A

A silica or aluminium oxide plate is coated with solid and solvent moves up plate

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5
Q

Explain how components of a mixture are separated by TLC

A
  • mixture is adsorbed onto plate which is polar
  • plate placed in solvent which moves up plate due to capillary action
  • balance between affinity of mixture to mobile and stationary phase in order to provide great enough separation
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6
Q

Rf

A

Retardation Factors = distance moved by spot/distance moved by solvent

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7
Q

Compare paper and thin layer chromatography

A
  • both have liquid mobile phase
  • both separate mixture in liquid state
  • TLC is less time consuming
  • separation is more sharp for TLC
  • stationary phase is more robust for TLC (silica plate vs paper)
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8
Q

Suggest applications of TLC

A
  • test purity of a substance
  • check composition of a mixture
  • determine end point of a reaction
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9
Q

Column Chromatography

A

A column is packed with powdered silica and eluent moves down column

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10
Q

Suggest why column in column chromatography has a mineral wool plug

A

Allows passage of solvent but not powder

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11
Q

Suggest how a mixture is separated by column chromatography

A

Components of mixture run down column at different rates so have different retention times based on their relative affinity for the mobile and stationary phase

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12
Q

Gas Chromatography(-Mass Spectrometer)

A

A column is packed with solid or solid coated by liquid and a gas is passed through column under pressure at high temperature

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13
Q

Suggest how components of a mixture are identified by gas chromatography

A

Components have different retention times so leave column at different times to be detected and identified by mass spectrometer

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14
Q

Suggest applications of gas chromatography-mass spectrometer / high pressure liquid chromatography

A
  • environmental monitoring of pollution
  • blood/urine drug tests for athletes
  • food/beverage safety and quality analysis
  • forensics
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15
Q

Suggest how components with similar Rf values can be accurately identified using TLC

A
  • run chromatography a second time using a different solvent
  • rotate plate 90 degrees
  • check if any component is masked by another
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16
Q

Suggest ways organic molecules which are colourless can be identified on a chromatogram

A
  • ninhydrin spray for amino acids

- ultraviolet light for other organic molecules

17
Q

Describe a method for thin layer chromatography

A
  • Hold TLC plate with plastic gloves
  • Draw a thin 1.5cm pencil line from the bottom of the plate
  • Use a capillary tube to apply a small drop of solution onto mid point of pencil line
  • Allow spot to dry completely
  • Place TLC plate in tank with line above solvent level and cover lid
  • When solvent nears top remove plate from solvent
  • Mark solvent line immediately (volatile solvent)
  • Allow to dry in fume cupboard (toxic solvent)
  • Locate spots using UV light or developer spray
  • Calculate Rf values
18
Q

Suggest why it is essential to wear gloves when handling TLC plate

A

Avoid contamination of plate from amino acids transferred from fingerprints

19
Q

Suggest why it is not essential for solvent front to reach near the top of plate but recommended

A
  • Rf value is ratio of lengths

- the longer the distance measured the smaller the percentage uncertainty

20
Q

Suggest why different amino acids have different Rf values

A
  • different R groups

- different solubility/ affinity for solvent

21
Q

State what determines the distance travelled by a spot in TLC

A

Relative affinity to stationary AND mobile phase (not solubility in solvent alone)

22
Q

Explain why component has a higher Rf value with a solvent mixture of hexane and ethyl ethanoate compared to hexane alone

A
  • solvent is more polar
  • component has higher affinity for mobile phase
  • travels further along TLC plate
23
Q

Explain why component has a different retention time to another in column chromatography

A
  • polarity of each component is different based on the type of bonding present
  • more polar components have a greater affinity for the stationary phase so higher retention time

Chemistry A2 (12 decks)

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