Combustion of alkanes — AQA A-Level Chemistry
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Combustion of alkanes explained
Burning an alkane is strongly exothermic because the bonds made in the products, C=O in carbon dioxide and O–H in water, are stronger overall than the C–C and C–H bonds broken in the fuel together with the O=O bonds broken in oxygen.
Read the full explanation
The energy released per gram is high. Methane is natural gas, propane and butane are bottled as LPG, roughly C₅ to C₁₀ makes petrol, and longer chains give kerosene and diesel. Many are liquid or easily liquefied, making them simple to store and transport. The costs are that crude oil is finite and non-renewable, and complete combustion produces CO₂, a greenhouse gas. Incomplete combustion, when oxygen is limited, produces toxic carbon monoxide (CO) or carbon particulates (soot) instead of CO₂.
Combustion of alkanes and other organic compounds can be complete or incomplete.
With plenty of oxygen an alkane burns completely, so every carbon becomes CO₂ and every hydrogen becomes H₂O, as in C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. With a limited oxygen supply the carbon is only partly oxidised, giving carbon monoxide, or not oxidised at all, giving carbon as soot, while the hydrogen still becomes water. Two consequences follow. Less energy is released per mole of fuel, which is why an experimental enthalpy of combustion from a calorimeter comes out less exothermic than the data book value; incomplete combustion is a standard mark scheme answer for that discrepancy, alongside heat loss. And the products are hazardous, since carbon monoxide is a toxic ligand for haemoglobin and soot aggravates respiratory disease.
These gaseous pollutants from internal combustion engines can be removed using catalytic converters.
An engine emits three pollutants that a converter must deal with: carbon monoxide from incomplete combustion, unburnt hydrocarbons, and nitrogen monoxide formed when nitrogen and oxygen from the air combine at the high temperature inside the cylinder. A catalytic converter is a ceramic honeycomb coated with platinum, palladium and rhodium, giving a huge surface area for a small mass of metal. The exhaust gases are adsorbed onto the metal surface, react there, and the products are desorbed. The key reaction pairs two pollutants so that both are removed at once: 2CO + 2NO → 2CO₂ + N₂. Unburnt hydrocarbons are oxidised in the same way, and carbon monoxide can also be oxidised directly: 2CO + O₂ → 2CO₂.
Combustion of hydrocarbons containing sulfur leads to sulfur dioxide that causes air pollution.
Crude oil contains sulfur compounds, and any sulfur left in a fuel is oxidised when that fuel burns: S + O₂ → SO₂. Sulfur dioxide is an acidic gas. Dissolved in atmospheric water it forms sulfurous acid, SO₂ + H₂O → H₂SO₃, and further oxidation in the air gives sulfuric acid, H₂SO₄. This is acid rain. Aqueous sulfur dioxide is acidic enough to register around pH 2 to 3, which is why it attacks carbonate stonework such as limestone and marble, corrodes metal structures, acidifies lakes so that fish die, and damages the leaves and roots of trees. The practical response is to remove sulfur compounds from fuels at the refinery and to scrub sulfur dioxide from flue gases at power stations.
Students should be able to: explain why sulfur dioxide can be removed from flue gases using calcium oxide or calcium carbonate.
This is an acid-base reaction and the explanation must say so. Sulfur dioxide is an acidic oxide, calcium oxide is a basic oxide and calcium carbonate is the salt of a weak acid, so both neutralise it. In flue gas desulfurisation a slurry of powdered calcium oxide or calcium carbonate in water is sprayed into the rising gases, and the solid produced is calcium sulfite: CaO + SO₂ → CaSO₃, or CaCO₃ + SO₂ → CaSO₃ + CO₂. A fine slurry is used so as to maximise the surface area available. Oxidising the sulfite further gives calcium sulfate, sold as gypsum for plasterboard. The carbonate route has an obvious drawback, releasing a mole of carbon dioxide for every mole of sulfur dioxide removed.
Your focus
- Explain why alkane combustion is exothermic by comparing the bonds broken with the bonds made.
- Write a balanced equation for the complete combustion of a named alkane.
- Evaluate the use of alkanes as fuels by giving one advantage and one specific disadvantage.
Show all 15 objectives
- Write balanced equations for both complete and incomplete combustion of a named alkane.
- Explain why an experimental enthalpy of combustion is less exothermic than the data book value.
- Name the products of incomplete combustion and state one hazard of each.
- Name the pollutants a catalytic converter removes and state where each one comes from.
- Write balanced equations for the removal of carbon monoxide and of nitrogen monoxide in a converter.
- Explain the operation of a converter as heterogeneous catalysis on a coated honeycomb.
- Write equations for the formation of sulfur dioxide during combustion and for its reaction with water.
- Describe two specific environmental effects of acid rain.
- State one measure used to reduce sulfur dioxide emissions and explain where in the process it acts.
- Explain the removal of sulfur dioxide from flue gases as an acid-base reaction between an acidic oxide and a base.
- Write balanced equations for the reaction of sulfur dioxide with calcium oxide and with calcium carbonate.
- Evaluate the carbonate route by identifying the carbon dioxide released as a disadvantage.
Combustion of alkanes exam tips
Marking Points
- one mark for combustion being exothermic because the bonds formed are stronger than the bonds broken
- one mark for a balanced combustion equation for a named alkane
- one mark for a named alkane fuel matched to its use, such as methane for heating or octane in petrol
- one mark for a disadvantage, such as carbon dioxide emission or the non-renewable supply
- one mark for a balanced equation for complete combustion giving only CO₂ and H₂O
- one mark for a balanced equation for incomplete combustion giving CO or C together with H₂O
- one mark for linking incomplete combustion to a less exothermic experimental value
- one mark for a hazard of an incomplete combustion product, such as the toxicity of carbon monoxide
- one mark for naming carbon monoxide, unburnt hydrocarbons and oxides of nitrogen as the pollutants removed
- one mark for a balanced equation such as 2CO + 2NO → 2CO₂ + N₂
- one mark for identifying the catalysis as heterogeneous, with adsorption onto the metal surface
- one mark for explaining that nitrogen monoxide forms from N₂ and O₂ at the high temperature in the engine
- one mark for the equation S + O₂ → SO₂ for the oxidation of sulfur during combustion
- one mark for sulfur dioxide dissolving in water to give an acidic solution, with an equation such as SO₂ + H₂O → H₂SO₃
- one mark for naming acid rain and a specific effect, such as dissolving limestone or acidifying lakes
- one mark for a measure taken to reduce emissions, such as removing sulfur from the fuel beforehand
- one mark for identifying sulfur dioxide as an acidic oxide and calcium oxide or calcium carbonate as basic
- one mark for a balanced equation such as CaO + SO₂ → CaSO₃ or CaCO₃ + SO₂ → CaSO₃ + CO₂
- one mark for describing the process as neutralisation, or as an acid-base reaction
- one mark for a comment on the products, such as calcium sulfate being sold as gypsum or carbon dioxide being released by the carbonate route
Examiner Tips
- 💡Set out bonds broken and bonds made as two separate totals if asked why combustion is exothermic.
- 💡Name the specific alkane and the specific use rather than writing that oil is used for fuel.
- 💡Give one named pollutant when asked for an environmental disadvantage.
- 💡Balance carbon and hydrogen first, then use a fraction or double the equation to balance oxygen.
- 💡When explaining a low calorimetry result, give incomplete combustion as a reason distinct from heat loss.
- 💡Say limited oxygen supply rather than no oxygen, which would prevent combustion altogether.
- 💡Balance nitrogen last and check it explicitly, because it is the atom examiners find unbalanced most often.
- 💡Name all three platinum group metals if asked what the honeycomb is coated with.
- 💡Say the gases are adsorbed onto the surface, which earns the heterogeneous catalysis mark.
- 💡Write equations for both stages: sulfur to sulfur dioxide, then sulfur dioxide to an acid.
- 💡Name a specific consequence of acid rain, because general environmental harm scores nothing.
- 💡Keep acid rain separate from global warming in your answer, since different gases cause them.
- 💡Name the acid-base relationship explicitly, because they react together is not an explanation.
- 💡Check the oxidation state of sulfur on both sides to confirm this is neutralisation rather than redox.
- 💡Include carbon dioxide as a product whenever the reagent is the carbonate.
Common Mistakes
- saying bond breaking releases energy, when breaking bonds always requires energy
- omitting the O=O bonds of oxygen from a bond enthalpy argument about combustion
- claiming alkane fuels are renewable because more crude oil keeps being found
- giving pollution as a disadvantage without naming a specific pollutant
- writing an incomplete combustion equation that does not balance because the oxygen coefficient was not adjusted
- giving less exothermic without a reason when the question asks why an experimental value is low
- claiming incomplete combustion releases more energy because more products are formed
- naming carbon dioxide as the toxic product instead of carbon monoxide
- writing NO₂ where NO is the gas formed in the engine, or writing N₂O
- claiming the converter removes carbon dioxide, which it actually produces
- leaving an equation unbalanced for nitrogen, the atom most often miscounted
- describing the converter as a filter that traps the gases rather than a catalyst that converts them
- writing SO₃ as the direct product of burning sulfur in an engine, when SO₂ forms first
- saying sulfur dioxide causes the greenhouse effect rather than acid rain
- giving it is bad for the environment instead of a named effect such as damage to limestone
- forgetting that the sulfur comes from impurities in the fuel rather than from the hydrocarbon chain itself
- writing calcium sulfate as the direct product with calcium oxide, when the immediate product is the sulfite
- calling the reaction a redox reaction, when sulfur stays in the +4 oxidation state throughout
- forgetting the carbon dioxide product when calcium carbonate is the reagent used
- describing calcium oxide as acidic because it contains oxygen