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A-LEVEL NOTES

A-LEVEL OCR ChEMISTRY NOTES 
Chromatography and qualitative analysis


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Chromatography
  • Chromatography is a separation technique used for separating and identifying the species present in a mixture.
  • The mobile phase is the substance in chromatography that carries the soluble components of the mixture
  • The stationary phase is the substance in chromatography that holds back the components that are attracted to it.
  • Each component in the mixture has a different level of solubility in the mobile phase and retention by the stationary phase, resulting in the separation of these components
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Thin Layer Chromatography
  • In thin layer chromatography a plate is coated with a solid (stationary phase) and a solvent (mobile phase) moves up the plate
    • An organic solvent is used as the mobile phase
    • A sheet coated with a thin layer of silica gel or alumina acts as the stationary phase
  • Thin layer chromatography can be used to separate and identify components of a mixture by their Rf values.
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  • Different components have different attractions to the solvent and to the stationary phase. If a component is strongly attracted to the stationary phase it will not travel very far up the chromatogram
  • TLC can be used to check the purity of a sample, or to determine the extent of a chemical reaction
  • The Rf values can be compared to database values in order to identify components
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 Gas Chromatography
  •  In gas chromatography (GC) a column is packed with a solid or with a solid coated by a liquid, and a gas is passed through the column under pressure at high temperature.
    • ​Stationary phase is a solid or liquid coating inside a long-coiled tube
    • An inert gas such as N2 acts as the mobile phase
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  • The time taken for a component to leave the coil is called the retention time. This can be compared with standards to identify different substances.
  • A chromatograph shows these retention times as a series of peaks with the area under each peak being proportional to the amount of that component present
  • Mass spectrometry can be used to analyse the components separated by GC
  • The mass spectrum of each component can be compared to spectra in a database, allowing the components to be identified with greater certainty.

​Tests for Ions

  • Qualitative tests give information on the identity of ions present in a sample
  • To test for the presence of carbonate ions in a sample:
    • Add a dilute strong acid to the sample. If carbonate ions are present CO2 (g) will evolve
    • Test the gas evolved by bubbling it through limewater, if it is CO2 the limewater turns cloudy due to the formation of CaCO3
  • To test for the presence of halide ions in solution, add dilute nitric acid and an aqueous solution of AgNO3
Silver Halide
Colour of Precipitate
Addition of Dilute Ammonia
Addition of Concentrated Ammonia
AgCl
White
Dissolves to give a colourless solution
Dissolves to give a colourless solution
AgBr
Cream
Remains
Dissolves to give a colourless solution
AgI
Yellow
Remains
Remains
  • To test for the presence of sulfate ions in a sample, add barium ions. Ba2+ (aq) + SO42- (aq) → BaSO4 (s). BaSO4 is a white precipitate
  • To test for ammonium ions add sodium hydroxide solution and warm gently
    • NH4+ (aq) + OH- (aq)→NH3 (g) + H2O (l)
      • Test the gas evolved with damp red litmus paper. If the sample contains ammonium ions, the red litmus paper will turn blue
  • To test for transition metal ions, add aqueous ammonia or sodium hydroxide dropwise
    • Cu(II) → Blue precipitate
    • Fe(II) or Cr (III) → Green precipitate
    • Mn(II) → Brown precipitate
  • To distinguish Fe(II) or Cr(III) add excess ammonia
    • Cr(III) → Purple solution
    • Fe(II) → No change

​Tests for Functional Group
Functional Group
Test
Result
Alkene
Br2 (aq)
Decolourises
Haloalkane
AgNO3 (aq) and ethanol
White ppt – Cl-
Cream ppt – Br-
Yellow ppt – I-
Carbonyl
Brady’s reagent
Yellow/orange ppt
Aldehyde
Tollen’s reagent
Silver mirror
Carboxylic acid
Universal indicator or metal carbonate
pH of a weak acid. Effervescence with a carbonyl
Phenol
Universal indicator, no reaction with carbonates
pH of a weak acid. No effervescence with a carbonyl
Alcohol
H+ / Cr2O72-
Colour change from orange to green if it is a 1o or 2o alcohol

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