Combustion Reactions (HSC SSCE Chemistry): Revision Notes

Combustion Reactions

What are combustion reactions?

A combustion reaction occurs when a substance burns in oxygen (or air) at temperatures significantly higher than room temperature. These reactions involve elements or compounds combining rapidly with oxygen, releasing energy as heat and light.

One striking example is the combustion of magnesium metal. When silvery magnesium burns, it produces an extremely bright white flame and forms a white powdered product called magnesium oxide:

$2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)$

A more familiar example is the burning of methane, which is the main component of natural gas. You can observe this reaction on a kitchen stove, where methane burns with a blue flame to produce carbon dioxide and water:

$\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g)$

Another everyday example is a burning candle. The wax, which is a compound containing carbon and hydrogen atoms, undergoes combustion to produce similar products.

The self-sustaining nature of combustion

Most combustible substances need an initial energy input to begin burning. This energy can be provided in several ways:

• Heating a portion of the substance (such as lighting coal)
• Creating a spark in a fuel-air mixture (as in a car engine)
• Using a match or gas lighter (for barbecues or stoves)

Although a few substances, like sodium metal and white phosphorus, burst into flames spontaneously when exposed to oxygen in the air, most combustion reactions require this initial "push" to get started.

For example, when you light a home barbecue fuelled by liquid petroleum gas (LPG), which is mainly propane ($\text{C}_3\text{H}_8$), you need a spark or flame to start the reaction. Once begun, the reaction continues on its own:

$\text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(g)$

Complete combustion of hydrocarbons

Complete combustion occurs when a hydrocarbon burns with a sufficient supply of oxygen. Under these conditions, all the carbon atoms in the fuel are converted to carbon dioxide ($\text{CO}_2$), and all the hydrogen atoms form water ($\text{H}_2\text{O}$).

This is the ideal form of combustion because it:

• Releases the maximum amount of energy from the fuel
• Produces relatively harmless products (compared to incomplete combustion)
• Occurs when there is plenty of air circulation

When methane burns completely on a kitchen stove with adequate air supply, the products are only carbon dioxide and water vapour. Similarly, when propane burns completely in a barbecue with good ventilation, it produces only these two products.

Incomplete combustion

Incomplete combustion happens when the oxygen supply is limited or insufficient. Under these conditions, the carbon in the fuel cannot be fully oxidised to carbon dioxide. Instead, some alternative products form:

• Carbon monoxide ($\text{CO}$) - a toxic, colourless, odourless gas
• Soot (solid carbon, $\text{C}$) - black particles that create smoke

For propane, incomplete combustion can occur in two ways:

Producing carbon monoxide:

$2\text{C}_3\text{H}_8(g) + 7\text{O}_2(g) \rightarrow 6\text{CO}(g) + 8\text{H}_2\text{O}(g)$

Producing solid carbon (soot):

$\text{C}_3\text{H}_8(g) + 2\text{O}_2(g) \rightarrow 3\text{C}(s) + 4\text{H}_2\text{O}(g)$

Notice that incomplete combustion requires less oxygen than complete combustion. The presence of a yellow or orange flame, rather than a blue flame, often indicates incomplete combustion and soot production. This is why proper ventilation is essential when using gas appliances.

Balancing combustion equations

Writing balanced equations for combustion reactions requires a systematic approach. Let's examine the complete combustion of octane ($\text{C}_8\text{H}_{18}$), which is a constituent of petrol.

Remember!