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4.3.1Reactivity 1 — What drives chemical reactions?

Combustion Reactions — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 7 of 12 Unit quiz

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5What Combustion Is

4.3.1 Combustion Reactions

Combustion is the main source of energy for transport, heating and electricity generation.

Definition
  • Combustion is a rapid exothermic reaction of a substance with oxygen that releases energy as heat and often light.
Fuels
  • A fuel is a substance burned to release useful energy, such as methane, octane, ethanol, hydrogen or coal.
Exothermic
  • The products lie lower in energy than the reactants, so the surroundings gain energy and ΔH is negative.
Activation energy
  • Most fuels need a spark or flame to start burning, but once started the released heat sustains the reaction.

Part 2 of 5Complete Combustion Equations

Balanced equations with state symbols are expected in reaction-energy questions.

1
Complete combustion needs a plentiful supply of oxygen so that every carbon atom ends up in carbon dioxide.
2
A hydrocarbon burns according to hydrocarbon + oxygen → carbon dioxide + water, as in CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).
3
An alcohol such as ethanol also forms only carbon dioxide and water: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).
4
To balance, adjust carbon first, then hydrogen, and leave oxygen until last because it appears in several species.
5
Standard enthalpy of combustion refers to one mole of fuel burned completely, and its value is negative.

Part 3 of 5Comparing Fuels

Comparing Fuels
Specific energy
  • Energy released per gram of fuel, in kJ g⁻¹, matters when mass is limited, as for rockets and aircraft.
Energy density
  • Energy released per unit volume, in kJ dm⁻³, matters when storage space is limited.
Hydrogen
  • Releases a large amount of energy per gram but needs bulky storage as a gas under pressure.
Practical choice
  • Cost, safety, availability and pollution all influence which fuel is used in a given application.

Part 4 of 5Worked Example

Question

  • The standard enthalpy of combustion of methane is −890 kJ mol⁻¹.
  • Calculate the energy released when 1.00 g of methane burns completely (M = 16.05 g mol⁻¹).

  1. Moles of methane = 1.00 g ÷ 16.05 g mol⁻¹ = 0.0623 mol.
  2. Energy released = 0.0623 mol × 890 kJ mol⁻¹ = 55.5 kJ (using 0.06231 mol).
  3. This is the specific energy of methane, about 55 kJ g⁻¹.

Answer 55.5 kJ released per gram of methane

Part 5 of 5Exam Checklist & Tips

4.3.1 – Key Points for the Exam

  • Combustion A rapid exothermic reaction of a substance with oxygen.
  • Fuel A substance that releases useful energy when it reacts, usually by burning.
  • Specific energy Energy released per unit mass of fuel.
  • Energy density Energy released per unit volume of fuel.

Common Mistakes

  • Writing H₂O(g) instead of H₂O(l) when the standard enthalpy of combustion is requested.
  • Balancing oxygen first, which usually forces the other coefficients to be changed again.
  • Giving combustion a positive ΔH, forgetting that heat is released.
  • Confusing specific energy (per gram) with energy density (per unit volume).

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