Fire Triangle

There are lots of different words you hear thrown around when talking about fire... oxidation! combustion! burning!

It's important that we have a little look at the difference between some of these words first, as they are quite similar but don't mean the same thing.

IGNITION - the process that starts the burning

COMBUSTION - a type of oxidation reaction when the reaction of the fuel occurs with a fixed amount of oxygen/air (i.e. in a closed space). Heat energy is released.

BURNING - when the reaction of the fuel occurs with an unlimited amount of oxygen/air (i.e. in the open). Heat and light energy is released and a flame/fire accompanies burning.

The Fire Triangle - describes the three things that must be present for a fire to occur.

The fuel is what is 'eaten'. Examples are wood, petrol, diesel, coal

The heat is the source of the ignition.

The oxygen usually comes from the air.

How to extinguish a fire

The chemical reactions that take place.......

When the fuel is a simple element, it is pretty easy to work out the chemical equation for the reaction. For example, what would I make when carbon (e.g. charcoal) burns?

Carbon + oxygen → carbon dioxide

C_(s) + O_2(g) → CO_2(g)

When you react a fuel with oxygen where the fuel contains more than one element, the number of products you make equals the number of elements the fuel contains (other than oxygen)

e.g. 1 burning ethane (C₂H₆)

Ethane contains two elements, carbon and hydrogen... so when it burns/reacts with oxygen, we make two products... one containing the carbon and the other containing the hydrogen. The carbon part turns into carbon dioxide and the hydrogen part turns into water. You basically add oxygen to each of the elements...easy peasy!

ethane + oxygen → carbon dioxide + water

2 C₂H_6(g) + 7 O_2(g) → 4 CO_2(g) + 6 H₂O_(g)

You can see that each of the two elements in the original ethane have separately combined with oxygen to make the two products.

e.g. 2. Let's look at another example. What would be the word and symbol equation if I combusted the compound hydrogen sulphide (H₂S)?

Same idea again... H₂S contains 2 different elements, so when I burn it in oxygen there will be 2 products made, one from hydrogen combining with the oxygen and the other from the sulphur combining with the oxygen...

Hydrogen sulphide + oxygen → water + sulphur dioxide

2 H₂S_(g) + 3 O_2(g) → 2 H₂O_(g) + 2 SO_2(g)

Extension work - complete and incomplete combustion

Ethane, which I used above in example 1, is called a hydrocarbon. This means it is a compound which contains ONLY the elements carbon and hydrogen. The name hydrocarbon sort of make sense...

The Bunsen burners you use in College burn a hydrocarbon called butane - you may have heard of it before. The formula of butane is C₄H₁₀. Have you ever wondered what the difference is between closing and opening the airhole on the Bunsen burner and why the flame changes colour?

A: Airhole open B: Airhole closed

When the airhole is open, a lot of oxygen can mix with the butane gas and the flame goes a lovely blue colour. This gives a hot flame and shows that the type of combustion taking place is complete combustion.

2 C₄H_10(g) + 13 O_2(g) → 8 CO_2(g) + 10 H₂O_(g)

But, if we close the airhole, we reduce the amount of oxygen that can make it to the butane and so it cannot burn completely in the oxygen. Instead, we get what is known as incomplete combustion. This means that instead of making carbon dioxide as a product, we make a mixture of carbon monoxide and soot (carbon residue!) The soot given off causes the flame to go a yellow colour. It also gives off a lot less heat energy because the incomplete combustion reaction is less exothermic.

C₄H_10(g) + 4 O_2(g) → 3 CO_(g) + C(s) + 5 H₂O_(g)

This is why you never heat a container with the airhole closed! Because soot is given off in the flame, it sticks to the bottom of the container and puts a layer of soot on the bottom of it.