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4.3.1Reactivity 1 — What drives chemical reactions?

Combustion Reactions — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 6 of 8 Unit quiz

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Part 1 of 6Complete Combustion

4.3.1 Combustion Reactions

Fuels are substances burned to provide useful energy.

Definition
  • Complete combustion is a rapid exothermic reaction with plenty of oxygen, in which carbon is oxidised to CO₂ and hydrogen to H₂O.
Hydrocarbons
  • Alkanes burn as in CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), releasing heat and giving only carbon dioxide and water.
Alcohols
  • Alcohols also burn completely, for example C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l), with their own oxygen contributing to the reaction.
Observation
  • Complete combustion gives a blue, non-smoky flame, whereas a yellow, sooty flame shows that some carbon has not fully oxidised.

Part 2 of 6Balancing Combustion Equations

1
Write the fuel and oxygen on the left, and carbon dioxide and water on the right.
2
Balance carbon first by putting the number of carbon atoms of the fuel in front of CO₂.
3
Balance hydrogen second by halving the number of hydrogen atoms to find the coefficient of H₂O.
4
Balance oxygen last by counting all oxygen atoms in the products and subtracting any already present in the fuel, then halve for O₂.
5
Double all coefficients if a fraction is left, and then add state symbols.

Part 3 of 6Energy Released

Energy Released
Per mole
  • Larger molecules release more energy per mole because they have more bonds to be converted into C=O and O–H bonds.
Homologous series
  • Within a series such as the alkanes, the enthalpy of combustion becomes more negative with every added CH₂ group.
Per gram
  • Hydrocarbons generally release more energy per gram than alcohols because alcohols already contain oxygen.
Selection of fuels
  • Fuels are chosen for energy density, cost, ease of storage and the pollutants produced.

Part 4 of 6Other Combustion

Other Combustion
Metals
  • Reactive metals such as magnesium burn in oxygen to form oxides, as in 2Mg(s) + O₂(g) → 2MgO(s), with bright flames.
Hydrogen
  • Hydrogen burns to form only water, so no carbon dioxide is produced at the point of use.
Activation energy
  • A flame or spark is needed to start combustion, but the heat released then sustains the reaction.
Fire triangle
  • Combustion needs fuel, oxygen and heat; removing any of these puts out a fire.

Part 5 of 6Worked Example

Question Write the balanced equation for the complete combustion of butan-1-ol, C₄H₉OH(l).

  1. Butan-1-ol is C₄H₁₀O, so 4 carbons gives 4CO₂ and 10 hydrogens gives 5H₂O.
  2. Oxygen atoms needed in the products = 8 + 5 = 13, and one of them comes from the alcohol itself.
  3. Oxygen atoms needed from O₂ = 12, so 6O₂ are required.
  4. Add state symbols.

Answer C₄H₉OH(l) + 6O₂(g) → 4CO₂(g) + 5H₂O(l)

Part 6 of 6Exam Checklist & Tips

4.3.1 – Key Points for the Exam

  • Combustion A rapid exothermic reaction of a substance with oxygen.
  • Complete combustion Combustion with excess oxygen giving carbon dioxide and water as the only products of an organic fuel.
  • Fuel A substance burned to release useful energy.
  • Enthalpy of combustion The enthalpy change when one mole of a substance burns completely in oxygen.

Common Mistakes

  • Balancing oxygen before carbon and hydrogen, which makes errors likely.
  • Forgetting that an alcohol supplies one oxygen atom of its own.
  • Writing CO as a product of complete combustion.
  • Leaving out state symbols or using H₂O(g) when the data refer to liquid water.

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