WCH16 Practical Skills in Chemistry II

Lessons in WCH16

1. How the Unit 6 practical paper works
   Unit 6: 1 h 20 min, 50 marks, A2 techniques plus all the AS tests. Use the data given, do not round early, know the technical terms and how each method is actua
2. Combining uncertainties and judging validity
   Double single-reading uncertainties for differences; add percentages; exact factors add nothing. The biggest % uncertainty limits the result. Compare % error wi
3. Advanced graph work: tangents, log graphs and Arrhenius plots
   Rate = gradient of a tangent. log(rate) vs log[A]: gradient = order. ln k vs 1/T: gradient = -Ea/R, intercept ln A. Use the best-fit line; give units.
4. Tests for transition metal ions
   Add dropwise, then excess, then stand. Cu2+ deep blue in excess NH3; Cr3+ and Zn2+ dissolve in excess NaOH; Fe2+ green turns brown; Fe3+ orange-brown; Mn2+ off-
5. Identifying an inorganic unknown (Core Practical 15)
   Look, test cations, test anions, use extra clues, then balance charges. Observations are what you see; inferences name ions. Double salts have two cations.
6. Tests for carbonyls, acids, phenols and arenes
   2,4-DNPH: any C=O; Tollens' or Fehling's: aldehyde only; iodoform: CH3CO-; NaHCO3: COOH; bromine water with white precipitate: phenol; smoky flame: arene. Warm
7. Identifying an organic unknown from data
   Percentages to moles to simplest ratio, then scale to M+. Tests and IR give the groups; NMR and fragments give the structure. Fragments carry a + charge.
8. Kinetics experiments: choosing and analysing a method (Core Practical 9)
   Colorimetry (with calibration), gas, quench and titrate, or pH. Clock reactions use a small fixed amount so 1/t reflects the initial rate. Calculate concentrati
9. Activation energy experiments (Core Practical 10)
   Warm reactants separately, then mix and time. Plot ln(1/t) against 1/T in K; Ea = -gradient x R. Main errors: temperature drift and judging the end point.
10. pH measurements and finding Ka (Core Practical 11)
    Calibrate with buffers, rinse with deionised water. pH = pKa at half-neutralisation; make it by adding an equal portion of acid to a neutralised portion. Ka = [
11. Electrochemical cell experiments (Core Practical 12)
    Clean electrodes, 1.00 mol dm^-3 solutions, KNO3 salt bridge, Pt for ion/ion half-cells, high-resistance voltmeter. Readings differ because of concentration, te
12. Redox titration technique (Core Practical 13)
    MnO4-: in the burette, read the top of the meniscus, excess dilute H2SO4, self-indicating, end point first permanent pink; autocatalysis by Mn2+ with ethanedioa
13. Redox titration calculations
    Use the half-equation ratio (1 : 5 for MnO4- : Fe2+; 1 : 2 for I2 : S2O3²-; 1 : 1 for Cu2+ : S2O3²-). Scale for the aliquot. Do not round until the end.
14. Preparing a transition metal complex (Core Practical 14)
    Dissolve, add ammonia (fume cupboard) to a deep blue solution, add ethanol to crystallise, filter under reduced pressure, wash with cold ethanol, dry. Wet cryst
15. Organic preparations at A2 and recrystallisation (Core Practical 16)
    Diazotise at 0 to 5 °C (diazonium decomposes to phenol above). Recrystallise: minimum hot solvent, hot filter, cool, filter under reduced pressure, cold wash, d
16. Checking purity: melting temperature and chromatography
    Dry, powdered sample in a capillary, oil not water above 100 °C, heat slowly near the melting point, record the range. Pure: sharp, at the data value; impure: l
17. Planning experiments and assessing risks at A2
    Plan: aim, variables, sized apparatus, calculated quantities, numbered method, processing, specific safety. Mass = c x V x M. Work back from a target titre to c
18. Evaluating A2 experiments and explaining unexpected results
    Explain unexpected results with chemistry (autocatalysis, air oxidation, water uptake, failing assumptions). For each problem: what, which way, effect on result

Next exams for WCH16

• January 2027: Fri 22 Jan 2027, afternoon, 1h 20m
• Summer 2027: Thu 10 Jun 2027, afternoon, 1h 20m

From Pearson's final timetables. All exam dates. Always confirm your entries with your exams officer.