IB Chemistry SLTopic 4 — Energy from FuelsPaper 1 & 2Core idea~12 min read
Combustion Reactions
Burning is the reaction civilisation runs on, and chemically it is nothing more than oxidation happening fast enough to glow. What comes out the other end depends entirely on what went in — and on how much oxygen was available.
📚 What you need to know
Combustion is a rapid, exothermic oxidation reaction. It needs a fuel, oxygen, and a source of heat.
Metal + oxygen → metal oxide. These oxides are ionic and basic.
Non-metal + oxygen → non-metal oxide. These oxides are covalent and acidic.
Complete combustion of a hydrocarbon or alcohol gives carbon dioxide and water only.
Balance combustion equations in the order C, then H, then O last.
If the fuel is an alcohol, the oxygen it already contains must be subtracted before you set the O2 coefficient.
What combustion actually is
Combustion
a rapid, exothermic reaction between a fuel and oxygen, releasing heat and light
Every part of that definition earns its place. Exothermic, because the bonds formed in the oxides are stronger than the bonds broken in the fuel. Rapid, because slow oxidation — iron rusting, for instance — releases exactly the same energy without ever producing a flame. And light, because the reaction gets hot enough for the products to glow.
Fuel, oxygen, heat. Every method of putting out a fire works by removing one of the three.
Fuels do not burst into flame on the shelf even though the reaction is strongly exothermic. That is because combustion has a high activation energy — a spark is needed to start it. Handy for anyone who stores petrol.
Burning metals
All metals can be oxidised, but not all of them combust, which means burning with a flame. Two things decide it:
Reactivity. Reactive metals such as the s-block metals burn readily in air. Unreactive metals such as copper never produce a flame — heat copper and it simply darkens as black copper(II) oxide forms on the surface.
Surface area. A block of iron will not burn, but iron filings will throw sparks, which is exactly what a sparkler is. Finely divided metal exposes far more atoms to the oxygen at once.
The standard example is magnesium, which burns with a fierce white light:
A metal burning
2Mg(s) + O2(g) → 2MgO(s)
metal + oxygen → metal oxide
Metal oxides are ionic, and those that dissolve give alkaline solutions — they are basic oxides. Magnesium oxide in water gives a pH comfortably above 7.
A few metals are awkward: sodium burnt in plenty of oxygen gives the peroxide Na2O2, and iron gives Fe3O4. This is beyond what you are examined on — write the ordinary oxides, Na2O and Fe2O3, and you will be given the marks.
Burning non-metals
Non-metals burn too, and the products behave in the opposite way. Sulfur burns with a blue flame:
These oxides are covalent and molecular, and they dissolve to give acids — sulfur dioxide is the origin of acid rain. So the two halves of the periodic table give you two opposite kinds of oxide, and that is a favourite one-mark question.
Which side of the table the element came from tells you what its oxide will do in water.
Complete combustion of organic fuels
Hydrocarbons and alcohols are the fuels that matter commercially, and burnt in excess oxygen they undergo complete combustion: every carbon ends up fully oxidised to carbon dioxide, and every hydrogen ends up as water.
Complete combustion
fuel + oxygen → carbon dioxide + water
Nothing else appears. If you have written anything other than CO2 and H2O on the right-hand side, either the equation is wrong or the combustion was not complete.
Oxygen goes last because it is the only element that appears in a molecule on its own, so it can absorb whatever number is left over.
🧩 Balancing a combustion equation
Write the products as CO2 and H2O.
Carbon: the number of C atoms in the fuel is the coefficient of CO2.
Hydrogen: half the number of H atoms in the fuel is the coefficient of H2O.
Oxygen: count all the O atoms in the products, subtract any oxygen already in the fuel, then halve what is left to get the O2 coefficient.
A half is allowed. Double everything if the question insists on whole numbers.
WORKED EXAMPLE
Write balanced equations for (a) aluminium burning to aluminium oxide, Al2O3, and (b) lithium burning to lithium oxide, Li2O.
(a) start from the formula of the productAl + O₂ → Al₂O₃Three O on the right, two on the left. Take 2 Al₂O₃ to make six O, which needs 3 O₂.4Al(s) + 3O₂(g) → 2Al₂O₃(s)(b) same routineLi₂O has one O, so two of them use up one O₂, needing 4 Li.4Li(s) + O₂(g) → 2Li₂O(s)The oxide formula comes from the charges: Al³⁺ with O²− gives Al₂O₃, Li⁺ with O²− gives Li₂O.
WORKED EXAMPLE
Write the equation for the complete combustion of butane, C4H10.
Step 1 — carbon4 C → 4CO₂Step 2 — hydrogen10 H → 5H₂OStep 3 — oxygen last(4 × 2) + 5 = 13 O atoms → 6½ O₂C₄H₁₀ + 6½O₂ → 4CO₂ + 5H₂ODoubling gives whole numbers: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. Keep the half if the question asks for one mole of fuel.
WORKED EXAMPLE
Write the equation for the complete combustion of methanol, CH3OH.
Steps 1 and 2 — carbon and hydrogen1 C → CO₂ 4 H → 2H₂OStep 3 — oxygen, minus what the fuel broughtProducts contain 2 + 2 = 4 O atoms. Methanol already supplies 1.4 − 1 = 3 → 3 ÷ 2 = 1½ O₂CH₃OH + 1½O₂ → CO₂ + 2H₂OForgetting the oxygen inside the alcohol gives 2O₂ and an unbalanced equation. It is the single most common slip here.
💡 Exam tip
Always balance C, then H, then O. Any other order means going back and redoing the oxygen anyway.
Half coefficients are fine and are often what an enthalpy of combustion question needs, since ΔHc is defined per mole of fuel.
Get the formula of the metal oxide from the ion charges before trying to balance anything.
Asked about the oxide’s pH? Metal → basic, non-metal → acidic. One line, one mark.
Include state symbols where the question uses them, and remember water is (l) at room temperature but (g) in a hot flame.
⚠️ Common mix-up
Ignoring the oxygen in an alcohol when balancing. It counts.
Assuming all metals burn. Copper oxidises without ever catching fire.
Writing CO or C as a product of complete combustion. Those belong to the next page.
Balancing oxygen first and then having to change it again after fixing the carbon.
Confusing oxidation with combustion. All combustion is oxidation; most oxidation is not combustion.
Up next: Incomplete Combustion — what happens when the oxygen runs short, why the flame turns yellow, and why a faulty boiler can kill you.
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