Combustion as oxidation

A combustion reaction is an exothermic reaction where a substance reacts with oxygen, usually after heating or ignition. A fuel is a substance that releases useful energy when it reacts, most often by combustion.

Combustion is a redox process. An oxidizing agent causes another species to be oxidized by accepting electrons; in combustion, oxygen is usually the oxidizing agent. A reducing agent causes another species to be reduced by donating electrons; in combustion, the fuel is usually the reducing agent.

That is why it helps if a fuel has a reasonably high activation energy. Activation energy is the minimum energy barrier that reacting particles must overcome before a reaction can proceed. If the barrier is high enough, the fuel can be stored and transported without reacting with air at room temperature. But we still need it to burn when we choose to ignite it. Petrol that ignites in an engine is useful; petrol that ignites on the shelf is not.

Combustion of metals and non-metals

Reactive metals burn in oxygen to form metal oxides. For example:

2Mg(s) + O₂(g) → 2MgO(s)

Magnesium is oxidized because magnesium atoms lose electrons to form magnesium ions. Oxygen is reduced because oxygen atoms gain electrons to form oxide ions. Written as electron transfer:

Mg → Mg²⁺ + 2e⁻

O₂ + 4e⁻ → 2O²⁻

Non-metals can burn in oxygen too, forming non-metal oxides. Sulfur, which can be present as an impurity in fossil fuels, burns to form sulfur dioxide:

S(s) + O₂(g) → SO₂(g)

Sulfur dioxide can be further oxidized in air and then react with water to form acidic products. So combustion is not just about getting energy; it also affects air quality.

Complete combustion of hydrocarbons

A hydrocarbon is an organic compound composed only of carbon and hydrogen atoms. An alkane is a saturated hydrocarbon with only carbon–carbon single bonds and the general formula CₙH₂ₙ₊₂, where n is the number of carbon atoms in one molecule of the fuel (dimensionless whole number).

A complete combustion reaction is a combustion reaction in which carbon-containing fuel burns in excess oxygen to form carbon dioxide and water only as carbon-containing and hydrogen-containing products. For an alkane:

CₙH₂ₙ₊₂ + ((3n + 1)/2)O₂ → nCO₂ + (n + 1)H₂O

For example, methane burns completely like this:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

The state of water depends on the conditions. Hot exhaust gases may contain steam, H₂O(g), while cooler conditions may give liquid water, H₂O(l). In equations, balance the atoms first; add state symbols afterwards if the conditions are known.

Complete combustion of alcohols

An alcohol is an organic compound containing a hydroxyl group, –OH, bonded to a saturated carbon atom. Straight-chain alcohols often met in this topic can be written as CₙH₂ₙ₊₁OH, where n is the number of carbon atoms in one molecule of the alcohol.

Alcohols also undergo complete combustion, producing carbon dioxide and water:

CₙH₂ₙ₊₁OH + (3n/2)O₂ → nCO₂ + (n + 1)H₂O

For ethanol:

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

When deducing combustion equations, I tell students to use this order: balance carbon, then hydrogen, then oxygen last. Fractions for oxygen are acceptable, but you may multiply the whole equation by 2 if whole-number coefficients are preferred.

What changes when oxygen is limited?

Incomplete combustion is a combustion reaction where there is not enough oxygen for every carbon atom in the fuel to be fully oxidized to carbon dioxide. Some of the carbon forms carbon monoxide, CO, or elemental carbon, C, instead.

A limiting reactant is the reactant that gets used up first and so limits how much product can form. In complete combustion, the fuel is often the limiting reactant because oxygen is plentiful. In a poorly ventilated flame, a blocked boiler, a charcoal burner indoors, or a badly adjusted engine, oxygen can become limiting.

When methane burns with a limited oxygen supply, the flame may look yellow or smoky rather than clean blue. You might also see black soot deposited on a cool surface, with less heat released. Carbon monoxide may form too, and that’s the dangerous part: it is colourless and odourless, so you cannot rely on your senses to detect it.

Equations for incomplete combustion of hydrocarbons

For an alkane forming carbon monoxide:

CₙH₂ₙ₊₂ + ((2n + 1)/2)O₂ → nCO + (n + 1)H₂O

For an alkane forming carbon soot:

CₙH₂ₙ₊₂ + ((n + 1)/2)O₂ → nC + (n + 1)H₂O

For methane, the two idealized incomplete combustion equations are:

CH₄(g) + 1.5O₂(g) → CO(g) + 2H₂O(l)

CH₄(g) + O₂(g) → C(s) + 2H₂O(l)

Real flames do not follow just one neat equation. Complete combustion, carbon monoxide formation and soot formation can happen at the same time in different parts of the same flame.

Equations for incomplete combustion of alcohols

For an alcohol forming carbon monoxide:

CₙH₂ₙ₊₁OH + nO₂ → nCO + (n + 1)H₂O

For an alcohol forming carbon soot:

CₙH₂ₙ₊₁OH + (n/2)O₂ → nC + (n + 1)H₂O

For example, incomplete combustion of ethanol to carbon monoxide is:

C₂H₅OH(l) + 2O₂(g) → 2CO(g) + 3H₂O(l)

Health risk

Carbon monoxide is a toxic gas made of one carbon atom and one oxygen atom, and it binds strongly to haemoglobin in red blood cells. Haemoglobin is a protein in red blood cells that transports oxygen around the body. Once carbon monoxide binds to haemoglobin, less oxygen can be carried, so tissues may be starved of oxygen even while the person is breathing.

Carbon soot causes problems as well. Fine carbon particles can be inhaled and are associated with respiratory harm. Limiting oxygen therefore changes the chemistry and the risk: less CO₂ forms, but more CO and particulate carbon are produced, and less useful energy is released than in complete combustion.

What counts as a fossil fuel?

A fossil fuel is a non-renewable carbon-based fuel formed from ancient biological material over geological time. The main examples are coal, crude oil and natural gas.

A non-renewable resource is a resource used faster than natural processes can replace it on a human timescale. Coal, crude oil and natural gas are finite. Once we extract and burn them, carbon that was stored underground moves into the active carbon cycle, mainly as carbon dioxide.

Comparing carbon dioxide released by different fuels

To work out how much carbon dioxide a fuel adds to the atmosphere, start with the balanced combustion equation and the molar masses. The mole ratio between the fuel and CO₂ does the main work.

For propane:

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)

One mole of propane makes three moles of carbon dioxide. One mole of propane has a mass of about 44 g, while three moles of carbon dioxide have a mass of about 132 g. So burning 1 g of propane produces about 3 g of CO₂. Don’t guess from the size of the molecule; use the equation.

Fuels with more carbon per unit energy generally add more CO₂ for the same useful energy output. Hydrogen, if produced without fossil fuels, forms water rather than carbon dioxide when used in a fuel cell or burned. Methane produces less CO₂ per unit energy than coal, but it is still a fossil fuel when obtained from natural gas reserves.

Specific energy and incomplete combustion tendency

Specific energy is the energy released per unit mass of fuel, usually measured in MJ kg⁻¹. A high-specific-energy fuel releases more energy for the same carried mass, which is why this matters in transport.

The standard enthalpy of combustion, ΔHᶿc, is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions, with all substances in their standard states; its usual unit is kJ mol⁻¹. The more negative the value, the more energy is released per mole during complete combustion.

Specific energy and CO₂ production don’t always identify the same “best” fuel. Coal, for example, can be convenient where it is locally available, but it has relatively high greenhouse gas emissions and often more pollutants. Natural gas has high specific energy and burns more cleanly than coal, but it still releases CO₂, and methane leakage is a serious concern.

Larger hydrocarbons are more likely to undergo incomplete combustion. Stronger London dispersion forces give them higher boiling points and lower volatility, so they do not vaporize and mix with oxygen as readily. With poor mixing, some carbon atoms fail to meet enough oxygen during the short time available in a flame or engine cylinder.

Carbon dioxide and the greenhouse effect

A greenhouse gas is an atmospheric gas that absorbs infrared radiation emitted by Earth and re-emits energy in different directions, including back toward Earth’s surface. The greenhouse effect is the warming influence caused when greenhouse gases reduce the rate at which infrared energy escapes to space.

Carbon dioxide matters because CO₂ absorbs infrared radiation linked to molecular vibrations. Nitrogen and oxygen make up most of the atmosphere, but they are not significant infrared absorbers in the same way. A relatively small concentration of CO₂ can still have a large climate effect when its concentration increases.

The link is not “CO₂ makes sunlight hotter”. Sunlight reaches Earth mainly as visible and shorter-wavelength radiation. Earth then emits infrared radiation. Greenhouse gases interact strongly with some of this outgoing infrared radiation, changing the energy balance and raising average global temperature.

Implications of burning fossil fuels

The implications are chemical, but they also reach into environmental, economic, ethical and social questions.

• Environmental: increased atmospheric CO₂, global warming, ocean acidification, habitat loss, particulates, sulfur oxides and nitrogen oxides depending on fuel and conditions.
• Economic: fossil fuels have existing infrastructure and provide reliable energy, but climate damage, healthcare costs and pollution control are expensive.
• Ethical: the benefits and harms are unevenly distributed. Some communities contribute little to emissions but are highly vulnerable to climate impacts.
• Social: energy access supports development, transport, heating and industry, so replacing fossil fuels must consider reliability, affordability and fairness.

Fossil fuels are chemically useful because they store a lot of energy and are easy to use, but the products and scale of their combustion create long-term global problems.

Renewable and non-renewable energy sources

A renewable energy source is an energy source replenished by natural processes at a rate comparable to, or faster than, its use by humans. Solar, wind, hydroelectric, geothermal and sustainably managed biomass are common renewable examples.

A non-renewable energy source has a finite supply and is not replaced on a human timescale. Fossil fuels count as non-renewable because they take millions of years to form, yet humans burn them over years to centuries.

A biofuel is a fuel made from recently living biological material. The word “recently” matters. Biofuels are not automatically pollution-free, but their carbon has usually come from the atmosphere over years or decades, rather than being locked underground for geological time.

Biological carbon fixation and photosynthesis

Biological carbon fixation is the conversion of inorganic carbon, such as carbon dioxide, into organic carbon compounds by living organisms. In green plants, photosynthesis carries this out.

Photosynthesis is a biological process in which plants use light energy to convert carbon dioxide and water into glucose and oxygen. Learn this equation:

6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)

Glucose stores chemical energy in covalent bonds. Organisms can use it directly, convert it into biomass, or ferment it to make alcohol fuels such as ethanol.

Fermentation is an anaerobic biological process in which microorganisms convert glucose into ethanol and carbon dioxide. The equation is:

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)

Ethanol then burns like other alcohols:

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

This releases CO₂. Ethanol can still have a lower net carbon footprint than a petroleum fuel because some of the CO₂ released during combustion was absorbed recently as the plant grew. That depends on the whole life cycle, though: farming, fertilizers, processing, transport and land-use change all count.

Advantages and disadvantages of biofuels

Biofuels are useful because they can fit into parts of existing fuel infrastructure and can be made from crops, plant waste or other biomass. They may also improve energy security for countries that import crude oil.

The drawbacks are real. Biofuels compete for land, water and agricultural resources. If forests are cleared to grow biofuel crops, the carbon balance may get worse instead of better. Monoculture farming can reduce biodiversity, and food prices may rise if food crops or farmland are diverted to fuel production.

A good evaluation does not use the lazy phrase “biofuels are carbon neutral” unless the evidence supports it. Better wording is: biofuels can reduce net CO₂ emissions when the rate of biological carbon fixation and the production pathway offset the emissions from growing, processing and burning the fuel.