Combustion and fuelsIB MYP Chemistry: Revision notes
Section 1
Fossil fuels
Fossil fuels are coal, crude oil and natural gas. They formed over millions of years from the remains of dead plants and tiny sea organisms that were buried and then changed by heat and pressure. They contain carbon, and most are hydrocarbons (compounds of hydrogen and carbon only).
Fossil fuels are non-renewable: we use them much faster than they form, so they will run out. Natural gas is mostly methane, CH₄.
Section 2
Complete combustion
Combustion is burning: a substance reacts with oxygen and gives out energy. When there is plenty of oxygen, a hydrocarbon undergoes complete combustion:
hydrocarbon + oxygen → carbon dioxide + water
Example: methane + oxygen → carbon dioxide + water
CH₄ + 2O₂ → CO₂ + 2H₂O
A Bunsen burner with the air hole open gives a hot, blue flame, with complete combustion.
Section 3
Incomplete combustion
If the oxygen supply is limited, a hydrocarbon undergoes incomplete combustion. The products are water and, instead of (or as well as) carbon dioxide, carbon monoxide and/or carbon (soot):
hydrocarbon + oxygen → carbon monoxide + water
hydrocarbon + oxygen → carbon + water
A Bunsen burner with the air hole closed gives a cooler, yellow, sooty flame. Soot deposits on cold objects held in the flame.
Incomplete combustion does not mean that the fuel only half burns. It means the carbon is not fully oxidised to carbon dioxide.
Section 4
The carbon monoxide hazard
Carbon monoxide is a toxic gas. It has no colour and no smell, so people cannot tell when it is present. It combines with haemoglobin in red blood cells, so the blood carries less oxygen. Symptoms are headaches, dizziness and tiredness; high amounts cause unconsciousness and death.
Carbon monoxide is made by faulty gas appliances, charcoal burned in closed rooms and car engines. Gas appliances must be serviced and rooms ventilated, and carbon monoxide alarms can be fitted.
In exams, say carbon monoxide is toxic because it stops the blood carrying oxygen, and that it cannot be seen or smelled.
Section 5
Testing for carbon dioxide
To test for carbon dioxide, bubble the gas through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater turns cloudy (milky) because insoluble calcium carbonate forms.
When a fuel burns, the products can be tested like this: cool the gases in an ice-cooled tube so that water vapour condenses to a liquid, and pass the rest through limewater.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Combustion and fuels
- A family in Istanbul cooks on a gas hob that burns methane (natural gas). The flame is normally blue. One evening the flame turns yellow and black soot appears on the bottom of the pans. An engineer finds that the air holes of the burner are blocked.Explain why the flame turned yellow and left black soot on the pans.2 marks
- A student in Lagos investigates the products made when ethanol burns in a spirit burner. She uses a pump to draw the waste gases through a U-tube surrounded by ice, and then bubbles the remaining gas through limewater. A colourless liquid collects in the U-tube and the limewater turns cloudy.Write the word equation for the complete combustion of ethanol.2 marks
- Crude oil and natural gas are fossil fuels. They are found under the seabed in countries such as Norway and are burned in power stations, vehicles and homes.Describe how crude oil and natural gas were formed.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).