Q.Write chemical equations for combustion reactions of the following hydrocarbons:

Combustion Reaction Equations – From Intuition to Precision

What your brain already knows about fire

You've seen a candle burn, a match light, or a gas stove turn blue. Something is being consumed, heat and light are given off, and after a while, the fuel is gone. That's combustion in action.

At its core, combustion is a chemical reaction where a substance (the fuel) reacts rapidly with oxygen gas ($O_2$) from the air. The reaction releases a large amount of energy as heat and light — that's the flame you see.

But what exactly happens to the atoms? That's where the equation comes in.

The simple skeleton: fuel + oxygen → products

Every combustion reaction follows this pattern:

The "oxides" are compounds formed when elements in the fuel combine with oxygen. For most common fuels (which contain carbon and hydrogen), the products are:

• Carbon dioxide ($CO_2$) — from carbon
• Water ($H_2O$) — from hydrogen

If the fuel also contains other elements (like sulfur or nitrogen), you get their oxides too ($SO_2$, $NO_x$), but for now, focus on the carbon-hydrogen case.

The precise statement

A combustion reaction equation is a balanced chemical equation that shows:

1. The fuel and oxygen as reactants
2. The complete oxidation products as products
3. Conservation of mass (same number of each atom on both sides)

For a hydrocarbon fuel with general formula $C_xH_y$, the complete combustion equation is:

This is the master template for any hydrocarbon burning completely.

Example: burning methane (natural gas)

Methane is $CH_4$ — one carbon, four hydrogens.

Using the template: $x = 1$, $y = 4$

• $O_2$ coefficient: $1 + \frac{4}{4} = 2$
• $CO_2$ coefficient: $1$
• $H_2O$ coefficient: $\frac{4}{2} = 2$

So the balanced equation is:

Check atoms:

Left: C=1, H=4, O=4

Right: C=1, H=4, O=2+2=4

Balanced.

Example: burning propane (LPG)

Propane is $C_3H_8$ — three carbons, eight hydrogens.

$x = 3$, $y = 8$

• $O_2$: $3 + \frac{8}{4} = 3 + 2 = 5$
• $CO_2$: $3$
• $H_2O$: $\frac{8}{2} = 4$

Check:

Left: C=3, H=8, O=10

Right: C=3, H=8, O=6+4=10

Balanced.

What about incomplete combustion?

If there isn't enough oxygen, the fuel doesn't burn completely. Instead of $CO_2$, you get carbon monoxide ($CO$) — a poisonous gas — and sometimes even soot (carbon, $C$).

Example: methane with limited oxygen

Notice: less oxygen used, and $CO$ appears instead of $CO_2$. Incomplete combustion is dangerous and less efficient.

How to balance any combustion equation (step-by-step)

If you ever forget the template, here's the reliable method:

1. Write the unbalanced skeleton: Fuel + $O_2$ → $CO_2$ + $H_2O$
2. Balance carbon first: Put the number of carbons from the fuel in front of $CO_2$ …