How Does Paraffin Combustion Work?

Paraffin wax burns in oxygen to release heat. The visible flame is not burning solid wax directly: heat from the flame melts and vaporizes the wax, and the wax vapour then reacts with oxygen in the surrounding air.

The word paraffin can mean different things in different countries. In candle chemistry, paraffin usually means paraffin wax, a mixture of long-chain hydrocarbons. Because it is a mixture, it has no single exact molecular formula. A useful representative formula is $\mathrm{C_{25}H_{52}}$, although real wax contains many hydrocarbons with different chain lengths.

Complete combustion of paraffin

For any alkane with formula $\mathrm{C_nH_{2n+2}}$, complete combustion produces carbon dioxide and water:

$$\mathrm{C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O}$$

This equation assumes that oxygen is abundant and that the carbon is oxidized completely to $\mathrm{CO_2}$. The hydrogen becomes water.

If a fraction is inconvenient, multiply every coefficient by 2:

$$\mathrm{2C_nH_{2n+2} + (3n+1)O_2 \rightarrow 2nCO_2 + 2(n+1)H_2O}$$

The product water may be written as liquid or vapour depending on the conditions and the purpose of the equation. In a hot candle flame it is initially water vapour, so $\mathrm{H_2O(g)}$ is often appropriate. After cooling, it can condense to liquid water.

Worked example using C25H52

Take $n=25$, giving the representative wax molecule $\mathrm{C_{25}H_{52}}$.

1. Balance carbon

There are 25 carbon atoms on the left, so place 25 molecules of carbon dioxide on the right:

$$\mathrm{C_{25}H_{52} + O_2 \rightarrow 25CO_2 + H_2O}$$

2. Balance hydrogen

There are 52 hydrogen atoms in the fuel. Each water molecule contains two hydrogen atoms, so the water coefficient is $52/2=26$:

$$\mathrm{C_{25}H_{52} + O_2 \rightarrow 25CO_2 + 26H_2O}$$

3. Balance oxygen

The products contain

• $25\times2=50$ oxygen atoms in carbon dioxide;
• $26\times1=26$ oxygen atoms in water.

That is 76 oxygen atoms in total. Since each $\mathrm{O_2}$ molecule supplies two oxygen atoms, the oxygen coefficient is $76/2=38$:

$$\boxed{\mathrm{C_{25}H_{52} + 38O_2 \rightarrow 25CO_2 + 26H_2O}}$$

The atom count confirms the result:

Element	Reactants	Products
Carbon	25	25
Hydrogen	52	26 × 2 = 52
Oxygen	38 × 2 = 76	25 × 2 + 26 = 76

For every mole of this representative paraffin molecule, complete combustion consumes 38 moles of oxygen, produces 25 moles of carbon dioxide, and produces 26 moles of water.

Why a candle flame can be yellow

A candle flame has several regions. Near the wick, liquid wax is drawn upward by capillary action and vaporizes. In the inner region, the wax vapour has relatively little oxygen available. Some of it breaks down into smaller molecules and carbon-rich particles.

Hot carbon particles can glow, producing the familiar yellow luminosity. In an ideal, well-mixed flame with enough oxygen, the carbon would be converted mainly to carbon dioxide. In an actual candle, mixing is imperfect, so a small amount of soot and carbon monoxide can form. Some soot may escape the flame; this is why a candle can blacken a cool surface held above it.

The outer part of the flame has better access to oxygen and is where much of the final oxidation occurs. The flame temperature and oxygen supply determine how completely the vapour burns.

Incomplete combustion

When oxygen is limited, carbon does not necessarily end up entirely as carbon dioxide. Carbon monoxide can form instead:

$$\mathrm{2C_{25}H_{52} + 51O_2 \rightarrow 50CO + 52H_2O}$$

Check the equation: there are 50 carbon atoms on each side, 104 hydrogen atoms on each side, and 102 oxygen atoms on each side.

Under even more oxygen-starved conditions, elemental carbon, represented by soot, may be produced. A simplified equation is

$$\mathrm{C_{25}H_{52} + 13O_2 \rightarrow 25C + 26H_2O}$$

This is a bookkeeping equation rather than a complete description of every reaction in a smoky flame. Real incomplete combustion can produce a mixture containing carbon monoxide, unburned hydrocarbons, soot, and other partially oxidized compounds.

Carbon monoxide is particularly hazardous because it is toxic and cannot be reliably detected by smell. Paraffin should therefore be burned only with adequate ventilation and in equipment designed for combustion.

Heat released by combustion

Combustion is exothermic: the products have lower chemical energy than the reactants, and the difference is released mainly as heat. The flame supplies enough heat to melt more wax and vaporize it, allowing the cycle to continue.

The exact heat released depends on which hydrocarbons are present in the wax and on whether the water product is treated as liquid or vapour. It is therefore not correct to assign one exact combustion enthalpy to “paraffin wax” without specifying a composition or a representative formula.

Common mistakes

• Writing a formula for paraffin as though it were a pure compound. Paraffin wax is normally a mixture. State the approximation, such as $\mathrm{C_{25}H_{52}}$.
• Balancing oxygen first. Balance carbon, then hydrogen, and oxygen last; this avoids unnecessary recalculation.
• Writing carbon and hydrogen as the only products. Complete combustion of a hydrocarbon gives carbon dioxide and water, not solid carbon and hydrogen gas.
• Assuming every flame has only complete combustion. A real candle flame can form carbon monoxide and soot because oxygen and fuel vapour do not mix perfectly.
• Confusing paraffin wax with a single alkane. The general alkane equation is useful for an idealized component, but it describes an approximation for commercial wax.