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WCH13 Practical Skills in Chemistry I

Edexcel IAL Chemistry, Unit 3 (AS unit, usually Year 12). 19 lessons so far, in the suggested order.

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Lessons in WCH13

  1. How the Unit 3 practical paper works
    Unit 3: 1 h 20 min, 50 marks, skills from Units 1 and 2. Answer exactly what is asked, give one answer, always say why, show working and revise the core practic
  2. Choosing and using measuring apparatus
    Pipette for fixed accurate volumes, burette for variable accurate volumes, volumetric flask for standard solutions, measuring cylinder only where precision does
  3. Drawing and labelling apparatus
    Labelled, connected, no gaps, never sealed when heated, water in at the bottom. Reflux: vertical open condenser. Distillation: bulb at the side arm. Buchner: pe
  4. Measurement uncertainty and percentage uncertainty
    Uncertainty from two readings is doubled. % uncertainty = uncertainty / value x 100. Add % uncertainties to combine. Bigger measurements and better resolution r
  5. Accuracy, precision, errors and percentage error
    Accurate = close to the true value; precise = close together. Repeats reduce random error but not systematic error. If % error is far bigger than % uncertainty,
  6. Recording data, significant figures and anomalies
    Headings: quantity / unit. Consistent decimal places matching the apparatus. Answer to the s.f. of the least precise data. Mean only concordant titres; leave ou
  7. Plotting and reading graphs
    x: what you change; y: what you measure. Fill more than half the grid, label quantity / unit, crosses, one best line. Gradient with a big triangle and units. Ex
  8. Hazards, risks and safety precautions
    Hazard = could harm; risk = chance it will. Match each precaution to a named hazard. Fume cupboard for toxic gases, no flames for flammables, acid into water wh
  9. Tests for cations: flame tests and ammonium ions
    Clean the wire in conc HCl, use a blue flame. Li red, Na yellow, K lilac, Ca orange-red, Sr crimson, Ba green. NH4+: warm with NaOH, damp red litmus turns blue.
  10. Tests for anions and gases, and the order of testing
    Test carbonate first. Sulfate: HCl then BaCl2, white ppt. Halide: HNO3 then AgNO3, then ammonia. Learn the exact gas test wording. Heat to constant mass by repe
  11. Tests for organic functional groups (AS)
    Bromine water for C=C; PCl5 (steamy HCl fumes) for -OH; acidified dichromate orange to green for primary and secondary alcohols; Fehling's or Tollens' for aldeh
  12. Gas volume experiments (Core Practical 1)
    Molar volume = V(gas) / n(gas). Acid in excess. Stop gas escaping (do not open the flask; use a syringe for CO2). pV = nRT in Pa, m3, K. Check the answer is se
  13. Enthalpy experiments: analysis, errors and improvements
    q = mcΔT with the mass of solution only. Divide by moles of the limiting reactant; give the sign. Per mole of water for neutralisation. Main error is heat loss;
  14. Titration technique and standard solutions (Core Practicals 3 and 4)
    Weigh by difference, dissolve, transfer with washings, make up to the mark, invert. Rinse pipette and burette with their solutions; flask with water. White tile
  15. Titration calculations and their uncertainty
    n = cV, use the ratio, scale up for the aliquot, then concentration, M, purity or x. c1V1 = c2V2 only for 1 : 1. Show every step.
  16. Rates experiments: methods, variables and data (Core Practical 5)
    Rate is proportional to 1/time. Control moles, volumes and temperature. CP5: C-I fastest, tertiary fastest; ethanol as solvent. Rate-temperature graphs curve up
  17. Preparing and purifying an organic liquid (Core Practicals 6 and 7)
    React, separate (by density), wash with NaHCO3, dry until clear, distil over a narrow range. Distil propanal out as it forms; reflux for propanoic acid. Mass of
  18. Preparing inorganic compounds and finding water of crystallisation
    Excess insoluble base, filter, evaporate to the crystallisation point, cool, filter, dry. x = n(water) / n(anhydrous salt) after heating to constant mass. Dry g
  19. Evaluating methods and suggesting improvements
    Identify, state the effect, give a specific improvement with its reason. Work out which way each error moves the result. Do not write 'human error' or 'more acc

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