ATARMAxxing
← All subjects
Ch
VCE · VCE Units 3 & 4

Chemistry

Energy, redox, organic and analysis — full exams with worked solutions and mark-by-mark guides.

20full-length model exams with mark-by-mark answer guides
20detailed note sets — ~120 pages across every topic
64exam-style practice questions with worked solutions
60flashcards for every key term & formula
10official past papers

Preview it all free. Unlock when you're ready.

Read every note, sit every exam, check every answer — the moment you unlock it. From $20 once for one subject — yours for life.

See all plans

One-time payment in AUD · lifetime access · by purchasing you agree to our Terms.

What's inside

Sample revision note

Energy Calculations: Stoichiometry of Fuels

Begin With a Balanced Equation: Mole Ratios in Combustion

Every energy question on the VCE paper, no matter how elaborate, starts with a balanced combustion equation. The coefficients are not decoration — they are the mole ratios that connect the amount of fuel burnt to the oxygen consumed, the carbon dioxide released and, through ΔH, the energy supplied. An unbalanced equation poisons every step that follows, so this is worth ten seconds of care every single time.

For any hydrocarbon burning completely in plentiful oxygen, the only products are CO2 and H2O. Balance in the order carbon, then hydrogen, then oxygen — oxygen last, because O2 stands alone and can absorb any coefficient. If oxygen lands on a half, double the whole equation:

  • CH4 + 2O2 -> CO2 + 2H2O (natural gas)
  • C3H8 + 5O2 -> 3CO2 + 4H2O (LPG)
  • 2C8H18 + 25O2 -> 16CO2 + 18H2O (octane in petrol — doubled to clear the half)
  • C2H5OH + 3O2 -> 2CO2 + 3H2O (ethanol — count the oxygen the fuel brings with it)

In incomplete combustion (restricted oxygen supply) the carbon ends up as CO and/or solid carbon (soot), e.g. 2CH4 + 3O2 -> 2CO + 4H2O. This releases less energy per mole of fuel and produces toxic carbon monoxide — a classic one-mark explanation. The conversion rule for every ratio problem is: n(target) = n(known) × (coefficient of target ÷ coefficient of known).

Worked example. A patio heater burns 11.0 g of propane completely. Find the mass of oxygen consumed and of carbon dioxide produced.

  • Step 1 — equation: C3H8 + 5O2 -> 3CO2 + 4H2O.
  • Step 2 — moles of fuel: M(C3H8) = 3 × 12.0 + 8 × 1.0 = 44.0 g/mol, so n(C3H8) = 11.0 ÷ 44.0 = 0.250 mol.
  • Step 3 — apply ratios: n(O2) = 0.250 × 5 = 1.25 mol, so m(O2) = 1.25 × 32.0 = 40.0 g. n(CO2) = 0.250 × 3 = 0.750 mol, so m(CO2) = 0.750 × 44.0 = 33.0 g.
  • Step 4 — sanity check: n(H2O) = 0.250 × 4 = 1.00 mol = 18.0 g. Mass in = 11.0 + 40.0 = 51.0 g; mass out = 33.0 + 18.0 = 51.0 g. Conservation of mass holds, so the working is internally consistent.

Spot the trap: M(CO2) = 44.0 g/mol happens to equal M(C3H8). They are different substances — label every line of working so you never divide by the wrong 44.0.

Turning ΔH Into Numbers: Mass–Energy Calculations

A thermochemical equation is a balanced equation with states shown and an enthalpy change attached: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l), ΔH = -890 kJ/mol. The negative sign says the reaction is exothermic; the magnitude says 890 kJ leaves the system for the molar amounts exactly as written. Because methane's coefficient is 1, that is 890 kJ per mole of CH4 — but double the equation and ΔH doubles with it: 2CH4(g) + 4O2(g) -> 2CO2(g) + 4H2O(l), ΔH = -1780 kJ/mol. ΔH belongs to the equation, not to any single substance.

The data book lists heats of combustion two ways, and you should hop between them with kJ/g = (kJ/mol) ÷ M. Typical data-book-style values (complete combustion at SLC forming H2O(l); always read the current data book rather than trusting memory):

FuelHeat of combustion (kJ/mol)M (g/mol)Heat of combustion (kJ/g)
Hydrogen, H22862.0143
Methane, CH489016.055.6
Ethanol, C2H5OH136046.029.6
Octane, C8H185460114.047.9

The master relationship for mass–energy questions is energy released = n(fuel) × molar heat of combustion, with n = m ÷ M. Run it forwards (mass -> energy) or backwards (energy -> mass).

Worked example (backwards). What minimum mass of ethanol must be burnt to release 500 kJ?

  • Step 1: n(C2H5OH) = energy ÷ heat of combustion = 500 ÷ 1360 = 0.3676 mol (keep guard digits in the calculator).
  • Step 2: M(C2H5OH) = 2 × 12.0 + 6 × 1.0 + 16.0 = 46.0 g/mol.
  • Step 3: m = 0.3676 × 46.0 = 16.9 g (3 significant figures).

Worked example (forwards). Energy from a 250 g tank of octane: n = 250 ÷ 114.0 = 2.193 mol; energy = 2.193 × 5460 = 11 974 kJ, which rounds to 1.20 × 10,000 kJ — report it as 12.0 MJ. Note the unit discipline: combustion data arrive in kJ, while q = mcΔT (coming up) produces joules. Convert one of them before they ever meet on the same line.

Sample exam question

A wind farm operator is comparing fuels for backup generators. Which statement best distinguishes a biofuel such as bioethanol from a fossil fuel such as petrodiesel?

  • Biofuels release no carbon dioxide when combusted, whereas fossil fuels do
  • Biofuels are renewable because the carbon released was recently absorbed from the atmosphere by growing crops
  • Biofuels have a higher energy density per gram than all fossil fuels
  • Only fossil fuels undergo complete combustion to form carbon dioxide and water
Show the worked answer

Answer: B

Bioethanol is renewable: the CO2 released on combustion was recently taken up from the atmosphere by photosynthesis in the crop, giving a short carbon cycle. It does release CO2 (A wrong), generally has lower energy density than fossil fuels (C wrong), and both fuel types can combust completely (D wrong).

All 20 practice exams

  1. Exam 1 — U3 AOS1 thermochemistry/electrochemistry (calorimetry, Faraday electrolysis, fuel cells, galvanic) — ~30 marks; U3 AOS2 rate & equilibrium (Kc, Le Chatelier, green chemistry/atom economy) + U4 AOS1 organic synthesis — ~30 marks; U4 AOS2 analysis (redox titration, combustion analysis, IR/NMR/MS structure determination, HPLC) — ~30 marks
  2. Exam 2 — U3 AOS1 thermochemistry/calorimetry, electrolysis & Faraday's laws, galvanic/fuel cells; U3 AOS2 equilibrium Kc, Le Chatelier, rate vs yield, atom economy & green chemistry; U4 AOS1 organic nomenclature, isomers & reaction pathways; U4 AOS2 redox titration, combustion analysis, IR/NMR/MS spectroscopy & HPLC
  3. Exam 3 — U3 AOS1 energy & electrochemistry (fuels, calorimetry, Faraday, galvanic) — 31 marks; U3 AOS2 rate, equilibrium & green chemistry — 28 marks; U4 AOS1 organic synthesis & U4 AOS2 analysis/spectroscopy/titration — 31 marks
  4. Exam 4 — Electrochemistry & energy (galvanic cells, fuel cells, electrolysis, Faraday, thermochemistry) — U3 AOS1; Rate & equilibrium and green chemistry (collision theory, catalysts, Kc, Le Chatelier, atom economy) — U3 AOS2; Organic nomenclature, isomerism & synthesis pathways — U4 AOS1
  5. Exam 5 — U3 AOS1 energy: thermochemistry/calorimetry, galvanic & fuel cells, Faraday electrolysis (green hydrogen); U3 AOS2 + U4 AOS1: equilibrium Kc/Le Chatelier, rate & atom economy, organic nomenclature/isomers/pathways/polymers; U4 AOS2 analysis: redox titration, IR/NMR/MS structure determination, HPLC calibration
  6. Exam 6 — Balanced VCAA-weighted coverage across all four AOS: U3 AOS1 energy/electrochemistry (Q1 calorimetry, Q2 Faraday, Q9-style EMF in Q5), U3 AOS2 rate/equilibrium/green chem (Q3, Q5), U4 AOS1 organic synthesis/isomers (Q4, Q7), U4 AOS2 analysis (Q6 titration, Q8 spectroscopy); Two extended quantitative items fully python-verified: Q1 calorimetry with electrical calibration (-1335 kJ/mol, -2.2% vs data book) and Q6 redox titration of iron tablets (102.5 mg/tablet, 97.6% of label); plus Q8 multi-spectrum ester structure determination; Pharmaceutical-lab context flavour woven through (iron-supplement assay, aspirin-type ester synthesis, enzyme inhibition, drug-purity chromatography) with realistic data and clean answers
  7. Exam 7 — U3 AOS1 energy: calorimetry with calibration, galvanic cells, thermochemistry/efficiency, and electrolysis/Faraday (Q1-Q4); U3 AOS2 rate & yield: equilibrium Kc, Le Chatelier, atom economy, green chemistry and % yield (Q6-Q7); U4 AOS2 analysis: redox titration (extended calculation), combustion/empirical formula, and IR/NMR/MS structure determination (Q5, Q8, Q9)
  8. Exam 8 — U3 AOS1 thermochemistry/calorimetry, galvanic & fuel cells, Faraday electrolysis; U3 AOS2 equilibrium Kc & Le Chatelier, green chemistry atom economy; U4 AOS1+2 organic nomenclature/pathways, titration, spectroscopy structure determination, combustion analysis
  9. Exam 9 — U3 AOS1 energy: thermochemistry/calorimetry, galvanic vs fuel cells, electrolysis & Faraday's laws; U3 AOS2 + U4 AOS1: equilibrium/Kc/Le Chatelier, green chemistry & atom economy, organic nomenclature/isomers/pathways; U4 AOS2 analysis: redox titration, combustion analysis, IR/NMR/MS structure determination, enzymes
  10. Exam 10 — Thermochemistry, electrochemistry and Faraday calculations (U3 AOS1); Equilibrium, rate-yield trade-offs and green chemistry (U3 AOS2); Organic structure, isomerism and synthesis (U4 AOS1)
  11. Exam 11 — U3 AOS1 energy (thermochemistry/calorimetry, electrolysis & Faraday, fuel cells/galvanic); U3 AOS2 rate & yield (equilibrium Kc, Le Chatelier, green chemistry & atom economy); U4 AOS1 organic synthesis (nomenclature, isomers, reaction pathways)
  12. Exam 12 — U3 AOS1 energy: thermochemistry/calorimetry, Faraday electrolysis, galvanic cells; U3 AOS2 + U4 AOS1: equilibrium Kc/Le Chatelier, atom economy & yield, combustion-derived formulae, reaction pathways/isomers; U4 AOS2 analysis: redox titration, IR/NMR/MS structure determination, HPLC calibration
  13. Exam 13 — U3 AOS1 energy (thermochemistry, electrolysis/Faraday, galvanic + fuel cells, fuel comparison); U3 AOS2 rate & yield (Kc, Le Chatelier, green chemistry, atom economy); U4 AOS1 organic (nomenclature, isomers, reaction pathways) + U4 AOS2 analysis (titration, HPLC, spectroscopy)
  14. Exam 14 — Thermochemistry & electrochemistry (U3 AOS1): calorimetry calibration, Faraday electrolysis, galvanic EMF; Rate, equilibrium & green chemistry (U3 AOS2): Kc, Le Chatelier, atom economy and percent yield; Organic synthesis & analysis (U4 AOS1/AOS2): nomenclature, isomerism, spectroscopy structure determination, redox titration
  15. Exam 15 — U3 AOS1 energy: thermochemistry/calorimetry, Faraday electrolysis, galvanic & fuel cells; U3 AOS2 rate & yield: equilibrium Kc/Le Chatelier, green chemistry & atom economy; U4 organic & analysis: nomenclature/isomers/pathways, redox titration, IR/NMR/MS structure determination
  16. Exam 16 — U3 AOS1 energy: combustion, calorimetry, galvanic/fuel cells, Faraday electrolysis; U3 AOS2 + U4 AOS1: rate/equilibrium (Kc, Le Chatelier), green chemistry, organic nomenclature/isomers/synthesis/polymers; U4 AOS2 analysis: spectroscopy structure determination (MS/IR/NMR), redox titration, HPLC, enzymes
  17. Exam 17 — U3 AOS1 energy (fuels, calorimetry with calibration, galvanic/fuel cells, Faraday electrolysis) — 37 marks; U3 AOS2 rate & yield (collision theory, dynamic equilibrium, Kc, Le Chatelier, green chemistry atom economy) — 19 marks; U4 AOS1 organic synthesis (IUPAC, isomers incl. chirality, IMF, reaction pathways, esterification/polymers) — 12 marks
  18. Exam 18 — U3 AOS1 energy: bomb calorimetry calibration and combustion enthalpy; galvanic cells, electrochemical series and Faraday's laws of electrolysis; U3 AOS2 + U4 AOS1 reactions: equilibrium Kc and Le Chatelier; green chemistry % yield and atom economy; combustion analysis to empirical/molecular formula; U4 AOS2 analysis: redox (permanganate) volumetric titration, HPLC calibration curves, and full IR/NMR/MS structure determination
  19. Exam 19 — U3 AOS1 energy: extended calorimetry/Faraday calcs, galvanic & fuel cells; U3 AOS2 + U4 AOS1: equilibrium Kc/Le Chatelier, green chemistry, organic synthesis & isomers; U4 AOS2 analysis: extended redox titration, combustion-analysis spectroscopy structure determination, HPLC & enzymes
  20. Exam 20 — U3 AOS1 energy: calorimetry calibration, Faraday electrolysis, galvanic/fuel cells, fuel energy density (35 marks); U3 AOS2 rate/yield + U4 AOS1 organic: Kc & Le Chatelier, atom economy, IUPAC/isomers/synthesis pathways, combustion analysis (34 marks); U4 AOS2 analysis: KMnO4 redox titration of an iron tablet and full IR/NMR/MS structure determination of an ester (21 marks)

All 20 revision notes

  • Energy Calculations: Stoichiometry of Fuels
  • Fuel Cells, Secondary Cells & Electrolysis (incl. Faraday)
  • Fuels: Fossil Fuels, Biofuels & Comparing Energy Sources
  • Galvanic Cells & the Electrochemical Series
  • Thermochemistry & Calorimetry
  • Collision Theory & Reaction Rates
  • Dynamic Equilibrium & Kc
  • Equilibrium & Electrolysis in Industry: Exam Technique
  • Green Chemistry & Atom Economy
  • Le Chatelier's Principle Applied
  • IUPAC Nomenclature & Functional Groups
  • Isomers: Structural & Stereoisomers
  • Organic Properties & Intermolecular Forces
  • Reaction Types & Synthesis Pathways
  • Yield, Atom Economy & Organic Calculations
  • Chromatography (HPLC) & Interpreting Results
  • Empirical Formula & Combustion Analysis
  • Medicinal Chemistry: Enzymes & Drug Action
  • Spectroscopy: IR, NMR & Mass Spectrometry
  • Volumetric Analysis: Acid-Base & Redox Titrations