WCH12 Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Lessons in WCH12

1. Enthalpy changes and Hess's law
   Exothermic: ΔH negative. Hess's law: ΔH is route-independent. ΔH = sum ΔfH(products) - sum ΔfH(reactants).
2. Standard enthalpy changes and level diagrams
   Standard: 100 kPa, 298 K, 1 mol/dm³, standard states. Formation, combustion, neutralisation and atomisation each refer to 1 mol of a stated substance.
3. Measuring enthalpy changes by calorimetry
   Q = mcΔT (m is the mass of what is heated); ΔH = -Q/n in kJ/mol; extrapolate to correct for heat loss.
4. Hess cycles from combustion data and Core Practical 2
   From combustion data: ΔH = sum ΔcH(reactants) - sum ΔcH(products). Core Practical 2: measure two reactions with acid and combine them using Hess's law.
5. Bond enthalpies
   ΔH = bonds broken - bonds made. Mean bond enthalpies are averages for gases, so answers are estimates.
6. Intermolecular forces and physical properties
   London forces: all molecules, rise with electrons and contact. Dipole-dipole: polar molecules. Hydrogen bonds: H on N, O or F to a lone pair on N, O or F.
7. Oxidation numbers and redox equations
   OIL RIG: oxidation is loss, reduction is gain of electrons. Oxidation numbers sum to the charge. Balance half-equations with H2O, H+ and e-; cancel electrons to
8. Groups 1 and 2: reactions and solubility
   Reactivity increases down Groups 1 and 2 as IE falls. Group 2 hydroxides more soluble down the group; sulfates less soluble.
9. Thermal stability and flame tests
   Thermal stability increases down Groups 1 and 2: larger cations polarise the anion less. Group 1 nitrates give nitrite + O2; Group 2 (and Li) nitrates give oxid
10. Group 7: trends and displacement
    Down Group 7: higher boiling points, lower electronegativity, less reactive. More reactive halogens displace less reactive halides. AgCl white, AgBr cream, AgI
11. Chlorine, halides and sulfuric acid
    Chlorine disproportionates in water (HCl + HClO), cold alkali (NaCl + NaClO) and hot alkali (NaCl + NaClO3). Halide reducing power: Cl- < Br- < I-.
12. Tests for ions, gases and organic groups (Core Practical 8)
    NH4+: NaOH, warm, red litmus blue. CO3²-: acid, limewater milky. SO4²-: HCl then BaCl2, white ppt. Halides: HNO3 then AgNO3. C=C: bromine water. OH: PCl5.
13. Titrations and standard solutions (Core Practicals 3 and 4)
    n = cV; use the equation ratio; average concordant titres only. Standard solution: weigh, dissolve, transfer with washings, make up in a volumetric flask, inver
14. Reaction rates, collision theory and catalysts
    Successful collisions need energy of at least Ea. Temperature increases the fraction of molecules with energy above Ea; catalysts lower Ea.
15. Dynamic equilibrium and Le Chatelier's principle
    Dynamic equilibrium: equal rates, constant concentrations, closed system. The position shifts to oppose any change; catalysts do not move it.
16. Halogenoalkanes: substitution and elimination
    KOH(aq): alcohol (substitution). KOH in ethanol: alkene (elimination). NH3 in ethanol, sealed tube: amine. KCN in ethanol: nitrile. Nucleophiles attack the δ+ c
17. Rates of hydrolysis of halogenoalkanes (Core Practical 5)
    Hydrolysis rate: C-I > C-Br > C-Cl because bond enthalpy decreases; tertiary > secondary > primary. Use AgNO3 in ethanol in a water bath and time the precipitat
18. Alcohols: combustion, halogenation and dehydration (Core Practical 6)
    Alcohols: burn; PCl5, KBr/H2SO4 or P/I2 give halogenoalkanes; concentrated H3PO4 dehydrates to alkenes. CP6: tertiary alcohol + conc HCl, wash, dry, distil.
19. Oxidation of alcohols (Core Practical 7)
    Primary: aldehyde (distil) or carboxylic acid (reflux). Secondary: ketone. Tertiary: no reaction. Orange Cr2O7²- turns green Cr3+.
20. Preparing and purifying organic liquids
    Reflux: long heating, no loss of vapour. Separate and wash in a separating funnel. Dry with an anhydrous salt. Distil at the boiling point; a sharp boiling poin
21. Mass spectra of organic compounds
    M+ (highest m/z, ignoring M+1) gives Mr. Fragments: 15 CH3+, 29 C2H5+ or CHO+, 43 C3H7+ or CH3CO+. Isomers differ in fragmentation.
22. Infrared spectroscopy
    Broad O-H about 3200 to 3600 (alcohol) or very broad 2500 to 3300 (acid); C=O about 1700. Absorption needs a change in dipole.

Next exams for WCH12

• January 2027: Tue 12 Jan 2027, afternoon, 1h 30m
• Summer 2027: Mon 10 May 2027, afternoon, 1h 30m

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