AECHE Semester 2 Exam Cheatsheet

01/09/2026

Chemistry Semester 2 COMPLETE STUDY GUIDE (Units 1&2 – A-Grade Sheet)

Read this once. It is everything that appears in every WACE/WATP/WAEP paper (2016–2023).

Exam Structure (same every year)

Section 1: 25 MC, 25 marks, 50 min – separate answer sheet.
Section 2: Short answer, 9–10 Q, ~35%, 60 min.
Section 3: Extended answer, 5 Q, ~40%, 70 min.
Total 100%, 3 hours. Show all working – each line = a mark.

The Golden Equations

n = m / M
n = cV (V in L)
Vgas = n × 22.71 (STP)
%mass = (mass of element / molar mass) × 100
N = n × 6.022 × 10²³

Universal recipe: 1. balance eqn → 2. given → moles → 3. mole ratio → 4. moles → required unit.

1. Collision Theory & Rate (every paper, 10–15 marks)

4 levers that increase rate:

Temperature ↑: increases the average kinetic energy of the particles, increasing both the frequency of collisions and the proportion of successful collisions (more particles have energy > activation energy).
Catalyst: provides an alternative reaction pathway with a lower activation energy, so a greater proportion of particles have sufficient energy to react.
Concentration / pressure ↑: more particles per unit volume, so a higher frequency of collisions.
Surface area ↑ (powder): larger exposed surface area, higher frequency of collisions.

Energy profile diagrams (draw every time): axes = Enthalpy vs Progress of reaction. Exothermic: products lower, ΔH negative. Label Ea (peak) and ΔH. Catalyst = second, lower-peaked curve. Never say catalyst “increases energy” – it lowers activation energy.

2. Intermolecular Forces & Properties (every paper)

Dispersion forces: all molecules; increase with molecular mass / electron count (H₂Te strongest of group 16 hydrides).
Dipole-dipole: polar molecules.
Hydrogen bonding: H bonded to N, O, or F.
Boiling point ↑ / vapour pressure ↓ = stronger IMFs = more energy required to break the intermolecular forces.
Hexane (non-polar, dispersion only) → low bp, high vapour pressure.
Water/ethanol (H-bonding) → high bp, low vapour pressure.
N₂ vs H₂O: N₂ is non-polar, only dispersion forces; H₂O is polar with hydrogen bonding; the sum of IMFs in H₂O is greater, so more energy is needed to break them.

Always name the specific IMFs of both substances, then compare total strength.

3. Structure & Bonding (every paper)

Metallic (iron): lattice of positive metal ions surrounded by a sea of delocalised electrons; bonding is non-directional, so layers can slide without breaking → malleable/ductile.
Ionic (Fe₂O₃, K₂S, NaCl): cations and anions in a rigid 3D lattice; strong electrostatic attraction → hard; when a force is applied, like charges align and repel, causing it to shatter → brittle.
Covalent network (diamond, SiO₂/sand): strong 3D network of covalent bonds, very difficult to break → high melting point, does not dissolve.
Diamond vs graphite: diamond = each C bonded to 4, all electrons localised → no conduction. Graphite = each C bonded to 3, 4th electron delocalised → conducts.
Conductivity in solution: ions are free to move and act as charge carriers. Never say “electrons” carry current in solution!
Dissolving K₂S: ionic bonds in the lattice are broken; ion–dipole forces form between the ions and water; ions dissociate and are hydrated (surrounded by polar water molecules).
Endothermic dissolving: more energy is required to break the ionic bonds (lattice energy) than is released when ion–dipole forces form (hydration energy), so energy is absorbed and temperature decreases.

4. VSEPR Shapes + Polarity (every paper)

Domains	Lone pairs	Shape	Examples
2	0	linear	CO₂, BeCl₂
3	0	trigonal planar	SO₃, H₂CO
4	0	tetrahedral	CH₄, SiH₄, CCl₄
3	1	pyramidal	NH₃, PH₃
2	2	bent / v-shaped	H₂O, H₂S, SO₂

Group 16 hydrides → v-shaped/bent
Group 17 hydrides → linear

Polarity: bond dipoles cancel in symmetrical molecules (CO₂, CCl₄, SiCl₄, SiH₄, CH₄ → non-polar). Most polar of SiH₄/PH₃/H₂S = H₂S: greatest electronegativity difference between H and S; H₂S has 2 lone pairs so dipoles don’t cancel. Most polar group-17 hydride = HF (largest EN difference).

5. Ionic Equations (every paper)

Write: correct species, balance, state symbols. Cancel spectator ions.

Al₂(CO₃)₃ + 6H⁺ → 2Al³⁺ + 3H₂O + 3CO₂
NH₄Cl + OH⁻ → H₂O + NH₃ + Cl⁻
MgCO₃ + 2H⁺ → H₂O + CO₂ + Mg²⁺
3Cd²⁺ + 2PO₄³⁻ → Cd₃(PO₄)₂
NH₄NO₃ + OH⁻ → NO₃⁻ + NH₃ + H₂O
I₂ + C₈H₁₆ → C₈H₁₆I₂ (brown → colourless)

For ammonium salts + base, the product is ammonia gas, not nitrate!

6. Stoichiometry (guaranteed 15 marks)

Types + methods (see full cheatsheet for worked numbers):

Ar from isotopes: Σ(mass × abundance)/100. Then N = n × 6.022 × 10²³.
Mass → moles → mass: given mass →(n=m/M)→ moles A →(ratio)→ moles B →(m=nM)→ mass B.
Concentration: n = cV, V in litres (mL ÷ 1000!).
% mass: element mass ÷ M × 100. Multiply by the subscript! (Fe₂O₃: 2 × 55.85/159.7 = 69.94%).
Gas volume: V = n × 22.71 L at STP.
Limiting reactant: moles of each ÷ coefficient; smallest = limiting; product from limiting only.
Titration: n = cV both sides, balance ratio, solve.
ppm: mg solute / kg solution.
Energy: E = n × ΔH (watch per-mole vs per-equation).

7. Organic (every paper, 8–12 marks)

Alkene + Br₂(aq): addition → dibromide; brown decolourises.
Alkene + H₂O, H₂SO₄: hydration → alcohol (propan-2-ol from propene).
Alkane + Cl₂, UV: substitution → chloroalkane + HCl.
Benzene + Br₂, AlBr₃: substitution → bromobenzene.
Naming: longest chain containing double bond; number to give double bond lowest; prefixes di-/tri-, methyl/ethyl/bromo/chloro. (4,4-dimethylpent-2-ene; 1,2-dibromobutane; 2-chloro-2-methylbutane.)
Substitution vs addition: substitution replaces an atom (alkane + halogen); addition joins across a double bond (alkene + Br₂/H₂O).

8. Chromatography (TLC/HPLC – every year!)

The polarity rule (say both phases):

TLC: the stationary phase is polar and the mobile phase has low polarity; the most polar component interacts most strongly with the stationary phase, so it moves most slowly / travels the least distance → lowest Rf.
HPLC: the mobile phase is polar, so the most polar component dissolves most readily in it, moves fastest through the column → lowest retention time.
Rf = distance moved by component ÷ distance moved by solvent front.
Don’t confuse retention factor with retention time. State the polarity of both phases.
2D-TLC advantage: amino acids with similar Rf in solvent 1 are clearly separated using a second solvent.

9. Acids & Bases (every paper)

Arrhenius acid: produces / releases H⁺ ions in aqueous solution.
Strong acid: fully ionises (HCl, H₂SO₄, HNO₃). Weak acid: partially ionises (HF, H₃PO₄, CH₃COOH, H₂CO₃).
Monoprotic: donates 1 H⁺ (HCl). Diprotic: 2 H⁺ (H₂SO₄, H₂CO₃). Triprotic: 3 H⁺ (H₃PO₄).
Conductivity: HCl is a strong acid, complete ionisation; HF is a weak acid, partial ionisation; so HCl has a greater concentration of ions → greater conductivity.
Electrolytes: H₂O₂ = non-electrolyte; ethanoic acid = weak; ammonium carbonate = strong.
Indicators: methyl orange: red (acid) / yellow (base); phenolphthalein: colourless (acid) / pink (base).
pH = −log[H⁺]; strong base high pH.

10. Periodic Trends (2021/2022 Q37)

Across period 3 (Na→Cl): first ionisation energy and electronegativity increase because nuclear charge increases while the valence shell is the same (similar shielding), so electrons are held more strongly.
Down group 17: electronegativity decreases because the valence shell is further from the nucleus and experiences more shielding, so the nucleus exerts less force on bonding electrons.
Ionic vs covalent: NaCl is ionic because there is a large electronegativity difference (electron transferred); Cl₂ is covalent because identical atoms share equally.
Group 2 name: alkaline-earth metals. Period 3 elements share: valence electrons in the third energy level.

11. Nanoparticles / Catalysts (recurring)

Nanomaterial: particles with diameter in the range 1–100 nm.
Advantage as catalyst: very large surface area to mass ratio, so more catalytic sites / more collisions → higher rate, and less material is needed.
Economic benefit of catalysts: increase rate → more product in less time → more profit.
Enzyme vs metal nanoparticle: enzymes are organic / biological (proteins), temperature & pH sensitive.

12. pH / Electrolytes / Thermo (small but recurring)

pH of 0.023 M Ca(OH)₂: [OH⁻] = 0.046, pOH = 1.34, pH = 12.66.
Highest pH of 0.5 M solutions: H₂CO₃ (weak acid).
Exothermic: freezing CO₂ (gas→solid releases heat). Melting/boiling/evaporating = endothermic.
Thermochemical eqn: CH₂=CH₂(g) + H₂O(g) → CH₃CH₂OH (exothermic, ΔH negative).
Endothermic = more energy absorbed breaking bonds than released forming bonds.
Law of conservation of energy: energy absorbed breaking bonds; more released forming bonds; the difference is released as heat.

Extended-Answer Scenarios (the 5 big ones)

Every year: context paragraph → (a) explain theory → (b–c) big stoichiometry calc with given numbers → (d) collision theory → (e) “suggest a reason”.

Common scenarios seen: Sabatier/NASA (CO₂+4H₂→CH₄+2H₂O), floatation tanks (Epsom salts, unsaturated proof), water hardness (Ca(HCO₃)₂), car battery acid (H₂SO₄), Mendeleev/periodic table, rock ochres (jarosite %Fe), seawater (Cl⁻/AgNO₃, AAS gold ppm), colour-flame candles (Sr), coal combustion, diamond vs graphite, ZnCO₃+acid, chromatography amino acids.

For each: read the context, extract the given numbers, and apply the universal stoichiometry recipe. The last part is usually an explain/suggest worth 2–4 marks – use collision theory or IMFs/structure.

Final Checklist Before the Test

Say “frequency of collisions” + “proportion of successful collisions” in every rate answer.
Name specific IMFs of both substances and compare.
State symbols in ionic equations; mL→L; subscripts in %mass; units + 3 s.f.
“Show all working” – every line a mark; follow-through marks exist.
Energy profile: label Ea, ΔH, axes, catalyst curve.
Know your shapes table cold (tetrahedral/pyramidal/bent/linear/trigonal planar).