Combustion involves reaction with oxygen

Combustion reactions involve substances burning in the presence of oxygen gas (O₂).

The reactants can include:

• Metals like lithium and magnesium.
• Non-metals such as sulfur.
• Organic compounds including hydrocarbons and alcohols.

The products formed depend on the type of substance undergoing combustion and the amount of oxygen available.

Combustion of metals produces metal oxides

Reactive metals like lithium and magnesium readily burn in oxygen to form ionic metal oxides. These combustion reactions are exothermic redox processes involving:

• Oxidation of the metal - The metal loses electrons to form positive ions.
• Reduction of oxygen - The oxygen atoms gain electrons to become O^2- ions.

The general equation for metal combustion is:

metal + oxygen ➔ metal oxide

For example:

Sodium combusts in oxygen, releasing heat and forming sodium oxide:

2Na_(s) + 1⁄2O_2(g) ➔ Na₂O_(s)

Magnesium reacts vigorously with oxygen, emitting a bright white light and producing magnesium oxide:

Mg_(s) + 1⁄2O_2(g) ➔ MgO_(s)

The metals lose electrons to form positive metal ions in these reactions:

Na ➔ Na⁺ + e^-

Mg ➔ Mg²⁺ + 2e^-

The oxygen atoms gain electrons to become O^2- ions:

O₂ + 4e^- ➔ 2O^2-

Combustion of non-metals produces non-metal oxides

Non-metals also undergo oxidation when combusted in oxygen, yielding non-metal oxides.

The general equation is:

non-metal + oxygen ➔ non-metal oxide

For example:

Sulfur, a common impurity in fossil fuels like coal, burns in oxygen to mainly form sulfur dioxide (SO₂):

S_(s) + O_2(g) ➔ SO_2(g)

Sulfur dioxide can further react with atmospheric oxygen to produce sulfur trioxide:

SO_2(g) + 1⁄2O_2(g) ⇌ SO_3(g)

Sulfur trioxide then reacts with atmospheric water to generate sulfuric acid, a component of acid rain:

SO_3(g) + H₂O_(l) ➔ H₂SO_4(aq)

Industrially, sulfur dioxide is manufactured in large quantities as a precursor for sulfuric acid synthesis. This acid is predominantly used to make fertilisers, paper, paints, textiles, and many other products.

Complete combustion of alkanes and alcohols

• Alkanes, composed of only carbon and hydrogen, are relatively unreactive due to their low bond polarity and strong covalent bonds. However, they release large amounts of energy when combusted, making them ideal fuels. Shorter-chain alkanes are preferably used as fuels due to their higher volatility.
• Alcohols, which contain a hydroxyl group (-OH), are also used as fuels; ethanol, for example, is used as a renewable biofuel.
• In excess oxygen, both alkanes and alcohols undergo complete combustion - an exothermic reaction producing carbon dioxide and water.

The general equation for complete combustion is:

alkane/alcohol + oxygen ➔ carbon dioxide + water

For example:

Ethane combusts completely as follows:

C₂H_6(l) + 7⁄2O_2(g) ➔ 2CO_2(g) + 3H₂O_(l) ΔH^⦵ = $-$1,560 kJ mol^-1

Similarly, ethanol combusts completely as follows:

C₂H₅OH_(l) + 3O_2(g) ➔ 2CO_2(g) + 3H₂O_(l) ΔH^⦵ = $-$1,367 kJ mol^-1

Incomplete combustion of alkanes and alcohols

In combustion reactions, oxygen is usually in excess. However, when it becomes the limiting reactant, incomplete combustion of alkanes and alcohols occurs, yielding carbon monoxide (CO) and/or carbon (C, soot). Incomplete combustion can produce both carbon monoxide and carbon simultaneously,

The general equations for incomplete combustion are:

alkane/alcohol + oxygen ➔ carbon monoxide + water

alkane/alcohol + oxygen ➔ carbon + water

alkane/alcohol + oxygen ➔ carbon monoxide + carbon + water

For example:

The incomplete combustion of ethane can be represented as follows:

C₂H_6(g) + 5⁄2O_2(g) ➔ 2CO_(g) + 3H₂O_(l)

or

C₂H_6(g) + 2O_2(g) ➔ C_(s) + CO_(g) + 3H₂O_(l)

or

C₂H_6(g) + 3⁄2O_2(g) ➔ 2C_(s) + 3H₂O_(l)

Similarly, the incomplete combustion of ethanol can be represented as follows:

C₂H₅OH_(l) + 2O_2(g) ➔ 2CO_(g) + 3H₂O_(l)

or

C₂H₅OH_(l) + 3⁄2O_2(g) ➔ C_(s) + CO_(g) + 3H₂O_(l)

or

C₂H₅OH_(l) + O_2(g) ➔ 2C_(s) + 3H₂O_(l)

These incomplete combustion reactions:

• Are less exothermic than the corresponding complete combustion.
• Can happen concurrently with complete combustion in varying proportions.
• Produce the toxic gas carbon monoxide and polluting soot particles.

Comparing complete and incomplete combustion

The table below summarises the different types of combustion reactions that can occur with organic compounds, depending on the availability of oxygen: