Oxygen and the Process of Combustion
Combustion is a rapid chemical reaction in which a fuel combines with oxygen and releases heat and usually light. Oxygen (O2) is the reactant that allows burning to take place. In brief: a fire needs three things, called the fire triangle: fuel, oxygen and heat; remove any one and the fire goes out. Burning methane is written CH4 + 2O2 → CO2 + 2H2O.
Combustion powers cars, heats homes and generates electricity, and it is also the cause of dangerous fires. This article describes the properties of oxygen, explains how combustion works, distinguishes complete from incomplete burning, shows how to put out fires, works through a mass calculation and gives the history of the oxygen theory of burning.
What is oxygen?
Oxygen is a colorless, odorless gas that makes up about 21% of air by volume. Its molecules consist of two atoms, O2. It boils at −183 °C and is only slightly soluble in water, which is just enough for fish and other aquatic life. Oxygen is the most abundant element in Earth’s crust by mass, but most of it is chemically combined in oxides and silicates.
Oxygen itself does not burn. It supports burning, which means that fuels burn faster in oxygen-rich air. A glowing splint relights when placed in a test tube of oxygen, and this is the standard laboratory test for the gas.
What happens during combustion?
In combustion, the bonds of the fuel and of oxygen are broken, and new bonds form in the products, mainly carbon dioxide and water for a hydrocarbon fuel. The new bonds are stronger than the old ones, so energy is released as heat and light. Combustion is therefore an exothermic reaction.
Many fuels need a starting source of heat, called the ignition temperature, before they burn. Once burning starts, the heat released keeps the reaction going. Typical equations are:
- Methane (natural gas): CH4 + 2O2 → CO2 + 2H2O
- Hydrogen: 2H2 + O2 → 2H2O
- Magnesium: 2Mg + O2 → 2MgO
- Carbon (charcoal): C + O2 → CO2
What is the fire triangle?
A fire needs fuel, oxygen and heat at the same time. Firefighters use this idea to select a method of extinguishing.
| Element removed | How it is done | Example |
|---|---|---|
| Heat | Cool with water | Water on a wood fire |
| Oxygen | Smother the flame | Fire blanket, foam, carbon dioxide extinguisher |
| Fuel | Remove or cut off the fuel | Turn off the gas supply, create a firebreak |
Complete and incomplete combustion
When oxygen is plentiful, hydrocarbons burn completely to form carbon dioxide and water, with a clean blue flame. When oxygen is limited, combustion is incomplete. Carbon monoxide (CO) and soot (carbon particles) are formed, and the flame is yellow and smoky.
| Feature | Complete combustion | Incomplete combustion |
|---|---|---|
| Oxygen supply | Plentiful | Limited |
| Main products | CO2 and H2O | CO, C (soot), H2O, some CO2 |
| Flame | Blue, hot | Yellow, smoky, cooler |
| Energy released | Maximum | Less |
| Hazard | Greenhouse gas | Carbon monoxide is toxic |
Carbon monoxide is colorless and odorless, and it binds to hemoglobin in blood much more strongly than oxygen does. For this reason gas heaters and boilers must be serviced regularly and have proper ventilation.
How are fires put out?
The right method depends on the fuel. Water is suitable for wood, paper and cloth. It must not be used on burning oil or fat, because the water sinks, boils violently and spreads the burning liquid. Use a lid, a fire blanket or a foam extinguisher for kitchen oil fires. Electrical fires require a non-conducting extinguisher such as carbon dioxide, and the power should be switched off if it is safe to do so.
Worked example: mass of oxygen needed
How much oxygen is needed to burn 16 g of methane completely, and how much carbon dioxide forms?
- Equation. CH4 + 2O2 → CO2 + 2H2O.
- Moles of methane. The molar mass of CH4 is 12 + 4 × 1 = 16 g/mol, so 16 g is 1.0 mol.
- Moles of oxygen. The ratio is 1 : 2, so 2.0 mol of O2 is needed. The molar mass of O2 is 32 g/mol, so the mass is 2.0 × 32 = 64 g.
- Carbon dioxide. 1.0 mol of CO2 forms. Its molar mass is 12 + 2 × 16 = 44 g/mol, so the mass is 44 g.
- Water. 2.0 mol of H2O (molar mass 18 g/mol) forms: 2.0 × 18 = 36 g.
- Check. Reactants: 16 + 64 = 80 g. Products: 44 + 36 = 80 g. The mass is conserved.
How was the role of oxygen discovered?
In the 17th and 18th centuries many scientists explained burning with the phlogiston theory, in which burning substances release a substance called phlogiston. This theory could not explain why metals gain mass when they burn. Joseph Priestley and Carl Wilhelm Scheele prepared oxygen in the early 1770s, and Antoine Lavoisier showed by careful weighing that burning is a combination with oxygen. Lavoisier named oxygen in the 1770s, and his work overturned the phlogiston theory.
Which mistakes are common?
Mistake 1: Saying that oxygen burns
Oxygen supports combustion but is not itself a fuel.
Mistake 2: Believing that burning destroys matter
The mass of the products equals the mass of the reactants. The gases escape into the air, which makes the solid fuel seem to disappear.
Mistake 3: Using water on every fire
Water is unsafe on oil, fat and electrical fires.
Mistake 4: Ignoring ventilation when burning fuel indoors
Burning in a room with limited air produces carbon monoxide, which can be fatal.