GCSE C3 Chapter 4

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Tests for positive ions

Flame test

  • Lithium (Li+) = Crimson
  • Sodium (Na+) = Yellow
  • Potassium (K+) = Lilac
  • Calcium (Ca2+) = Red
  • Barium (2+) = Green

Sodium hydroxide test

  • Aluminium, calcium and magnesium ions form a white precipitate. Aluminium hydroxide dissolves in excess sodium hydroxide solution
  • Copper ions form blue precipitate
  • Iron (Fe2+) forms green precipitate
  • Iron (Fe3+) forms brown precipitate

Hydroxides of most metals with 2+ or 3+ charges are insoluble in water

Fe3+(aq) + 3OH-(aq) = Fe(OH)3(s)

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Tests for negative ions

Carbonate ions:

  • Add dilute hydrochloric acid
  • Substance fizzes and gas produced turns limewater cloudy / milky (CO2)
  • 2HCl(aq) + CaCO3(s) = CaCl2(aq) + CO2(g) + H2O(l)

Halide ions:

  • Add dilute nitric acid then silver nitrate solution
  • White precipitate = chloride ions Cream = bromide ions  Pale yellow = Iodide ions
  • AgNO3(aq) + NaCl(aq) = AgCl(s) + NaNO3(aq)

Sulphate ions:

  • Add dilute hydrochloric acid then barium chloride solution
  • If white precipitate forms sulphate ions are present
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  • A titration is the method used to find out the volumes of two solutions that react exactly



  • Use pipette to measure known volume of alkali
  • Pour into conical flask
  • Add indicator - neutralisation reactions cannot be seen
  • Pour acid into burette
  • Open tap to allow acid to slowly pour into alkali
  • When indicator changes colour end point is reached, alkali has been neutralised
  • Turn off tap
  • Exact volume of acid that reacted can be found by taking the final burette reading from the initial reading
  • Should be repeated several times to increase repeatability
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Titration calculations

Volumes must be converted into dm (divide cm3 by 1000)

Moles = concentration X volume

Concentration = moles / volume

Moles = Mass / relative formula mass

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Chemical analysis

  • Chemical analysis is used by many scientists
  • Variety of methods:
    • Quantative methods: how much of substances react - titrations, gas chromotography and mass spectrometry
    • Qualitative methods: what substances are in a sample - flame test, sodium hydroxide test
  • Traditional "wet" chemistry - similar to those studied in this chapter and mass spectrometry and gas chromotography
  • Large databases are built up of results of analysis with aid of computers
  • Used to identify substances in samples, identify individuals or to monitor changes in amounts of substances over time
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Chemical equilibrium

  • Reversible reactions - A + B = C + D
  • Closed system - no reactants or products can escape
  • For a reversible reaction in a closed system equilibrium is achieved when the rate of the forward reaction is equal to the rate of the reverse reaction. The reactions continue to happen at the same rate but there is no change in the amount of reactants or products
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Altering conditions

If the conditions in a reversible reaction in a closed system are changed the position of equilibrium will shift to try to cancel out the change

If the forward reaction produces more molecules of gas or it is exothermic:

  • Increasing the pressure/temperature decreases the amount of product formed
  • Decreasing the pressure/temperature increases the amount of product formed

If the forward reaction produces fewer molecules of gas or it is endothermic:

  • Increasing the pressure/temperature increases the amount of product formed
  • Decreasing the pressure/temperature deceases the amount of product formed
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The haber process

  • Haber process is used to manufacture ammonia 
  • Ammonia is used as a fertiliser and in other chemicals
  • Nitrogen is obtained from air and hydrogen is obtained from natural gas
  • Mixed in correct proportions 
  • Passed over iron catalyst
  • Temperature of 450C
  • Pressure of 200 atmospheres
  • N2(g) + 3H2(g) = 2NH3(g)
  • Some of the ammonia produced breaks back down into nitrogen and hydrogen therefore yield of ammonia is only about 15%
  • Gases which come out of the reactor are cooled causing ammonia to condense into a liquid which can then be seperated from the unreacted gases. The unreacted gases can be recycled
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Economics of the haber process

Optimum pressure of 200 atmospheres:

  • Forward reaction produces fewer gas molecules so increasing pressure increases amount of product formed
  • Larger pressure means stronger reaction vessels needed which increases cost
  • Pressure comprimise between yield and cost

Optimum temperature of 450C:

  • Forward reaction is exothermic so decreasing temperature increases yield
  • Lower temperatures mean reaction happens at much slower rate
  • Temperature comprimise betwen yield and time
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