Chemistry - C3.4 - Analysis and Synthesis

C3.4.1 - Test for Positive Ions

  • Positive ions identified by flame test or using sodium hydroxide solution
  • Flame test: Li+ = crimson/red, Na+ = yellow, K+ = lilac, Ca2+ = red, Ba2+ = green
  • Hydroxides of most metals that have ions with 2+ or 3+ charges are insoluable in water - when sodium hydroxide is added to solutions of these ions, precipitate of the metal hydroxide forms
  • Al, Ca and Mg ions form white precipitates - when excess sodium hydroxide solution is added the aluminium hydroxide precipitate dissolves - Cu(II) hydroxide is blue, Fe(II) hydroxide is green, Fe(III) hydroxide is brown
  • Show reactions of positive ions with sodium hydroxide solution by balanced ionic equations - Fe3+(aq) + 3OH-(aq) -> Fe(OH)3(s)
1 of 9

C3.4.2 - Test for Negative Ions

  • Caronate ions: add dilute hydrochloric acid to substance and test gas given off in limewater - limewater goes milky it contains carbonate ions
  • 2HCl(aq) + CaCO3(s) -> CaCl2(aq) + H2O(l) + CO2(g)
  • Halide ions: add dilute nitric acid then silver nitrate solution - chloride ions give white precipitate, bromide ions give cream precipitate, iodide ions give yellow precipitate
  • AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)
  • Sulfate ions: add dilute hydrochloric acid then barium chloride solution - white precipitate forms if sulfate ions are present
  • BaCl2(aq) + MgSO4(aq) -> BaSO4(s) + MgCl2(aq)
2 of 9

C3.4.3 - Titrations

  • Acid + alkali -> salt + water - neutralisation - exact volumes used found through titration
  • Pipette used to accurately measure volume of alkali put into conical flask with indicator - burette filled with acid and added gradually to flask
  • When indicator changes colour - end point has been reached - colume of acid found from initial and final burette reading
  • Titration should be done several times to improve repeatability of results
3 of 9

C3.4.4 - Titration Calculations

  • Concentration of solution meaured in g/dm³ or mol/dm³
  • No of moles(mol) = C (mol/dm³) X V (cm³) / 1000
  • No of moles = mass / molar mass
  • Titrations used to find volume of solutions that react exactly - if concentration of one solution and volume of both is known, concentration of other solution can be calculated - concentrations calculated using balanced symbol equations and moles:
  • Write equation, find ratio, find no. of moles in known solution, find no of moles in unknown solution (using ratio), find concentration/volume
4 of 9

C3.4.5 - Chemical Analysis

  • Scientists use a various methods to analyse substances fr purposes including environmental, medical and forensic investigations
  • Can use 'wet chemistry' methods or GC-MS
  • Qualitative - find out what substance sample is, quantative - how much is in a substance
  • Large databses with analysis results have been built - used to identify substances in samples, identify individuals or monitor changes in amounts of substances over time
5 of 9

C3.4.6 - Chemical Equalibrium

  • Some reactions are reversible: products react together to make reactants
  • In closed system no reactants/products can escape - for reversible reactions equilibrium is reached when rate of forward reaction = rate of reverse reaction - both reactions happen but amount of reactant and product are equal
  • Amount of reactant/product from reversible reaction can be changed by changing conditions - important in industry in controlling reactions - eg. increasing concentration of reactant increases amount of product formed or if product is removed, more products form to try and reach equilibrium
  • ICl + Cl2 Equilibrium symbol ( ICl3
6 of 9

C3.4.7 - Altering Conditions

  • Change conditions of system at equilibrium: position of equilibrium shifts to try and cancel out change
  • Changing pressure affects reversible reactions with different no of gas molecules on one side - eg. increase pressure: position of equilibrium shifts to reduce pressure, favoring reaction producting less gas) - forward reaction produces more gas: increased pressure = decreased amount of product: decreased pressure = increased amount of product
  • Reversible reactions: exothermic Equilibrium symbol (  endothermic - increasing temperature favors endothermic reaction: equilibrium shifts to lower temperature by taking in energy - forward reaction is exothermic: increased temperature = decreased amount of product: decreased temperature = increased amount of product
7 of 9

C3.4.8 - Making Ammonia - the Haber Process

  • Haber process used to manufacture ammonia: used to make chemicals eg. fertilisers
  • Nitrogen (from air) and hydrogen (from natural gas) purified and mixed
  • Gases passed over iron catalysts at ~450°C and at pressure of ~200 atmospheres - give fast rate of reaction and reasonable yield of ammonia
  • N2(g) + 3H2(g) Equilibrium symbol ( 2NH3(g)
  • Some ammonia produced breaks down into N and H; yield ~15%
  • Gases coming out of reactor cooled so ammonia condenses and separated from unreacted gases that are recycled so aren't wasted
8 of 9

C3.4.9 - The Economics of the Haber Process

  • Haber process: H and N react to make NH3(ammonia)
  • products have fewer gas molecules than reactants: higher pressure = greater yield - more energy needed to compress gas and stronger reaction vessels: increased cost - pressure ~200 atmospheres: compromise between cost and yield
  • Forward reaction is exothermic: lower temoerature = greater yield - but reaction rate decreaces with temperature and iron catalyst becomes ineffective: take long time for reaction to happen - temperature ~450°C: gives reasonable yield in short time
9 of 9


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Analysing substances resources »