Burning Fuels

L.O: To be able to explain combustion

Thursday, April 20, 2023

Steam

• Why do cars produce steam from their exhaust (most noticeable on cold days)?

• The internal combustion engine produces water as a product – visible from the exhaust as water vapour…

What is combustion?

A fuel is a substance that reacts with oxygen (combusts) to release useful energy. Hydrocarbons, such as those found in crude oil, make good fuels, particularly if the molecules in the compound are small.

Combustion is a rapid reaction between a substance and oxygen that releases heat and light energy. When a substance burns, it is said to combust.

A hydrocarbon is a compound containing only the elements hydrogen and carbon.

The products of combustion

Oxidation and reduction

A reaction where oxygen is added to an element is known as oxidation. For example:

The opposite process, where oxygen is removed from a compound is known as reduction. For example:

carbon + oxygen carbon dioxide

copper oxide + hydrogen copper + water

During the combustion of a hydrocarbon fuel, the carbon and the hydrogen are oxidized.

Complete combustion

What are the products of the complete combustion of a hydrocarbon?

The blue flame of a Bunsen burner is an example of complete combustion of a hydrocarbon (in this case, almost pure methane), where heat energy is given out.

Complete combustion occurs when there is enough oxygen available for all of the carbon atoms in the fuel to burn and form carbon dioxide.

oxygencarbon dioxidehydrocarbon + + water

Complete combustion of methaneMethane is a hydrocarbon found in natural gas. Methane will combust in oxygen to produce carbon dioxide and water.

oxygencarbon dioxidemethane + + water

2O2 CO2CH4+ + 2H2O

Propane is a hydrocarbon fuel with the formula C3H8. What is the balanced symbol equation for the complete combustion of propane?

Hydrocarbon prefixesNumber of Carbons Prefix

1 Meth

2 Eth

3 Prop

4 But

5 Pent

6 Hex

7 Hept

8 Oct

9 Non

10 Dec

Combustion equations

Experiment

• In groups you will be given 4x different spirit burners containing unknown fuels – how could you set up an experiment to identify which fuel is which?

• The only information given to you is the energy found within the different fuels: -

▫ Methane = 50 MJ per kg▫ Ethane = 48 MJ per kg▫ Propane = 46 MJ per kg▫ Butane = 45 MJ per kg

▫ *Hydrogen = 142 MJ per kg ▫ *Petrol = 47 MJ per kg

Burning unknown fuelsFuel Start C End C Change C Start C End C Change C Average C

A

B

C

D

E

Record the Start, End & Change in temperature when you use each fuel to heat 50 ml of water for 2 minutes

TIP think about making your test fair!!

Experiment

• We can use the small burners of the different fuels to heat some water for a known period of time – we should find that the water reaches a higher temperature when heated with the fuel which releases the most amount of energy (i.e. methane)

• Remember, keep the time of heating, amount of water, distance between the heater and water; type of water; amount of fuel etc… the same to ensure this is a fair and reliable experiment!

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