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Syllabus
Syllabus
The combustion of Hydrocarbon fuels releases energy.
During combustion, the Carbon and Hydrogen in the fuels are oxidised.
The complete combustion of a Hydrocarbon produces Carbon Dioxide and water.
Students should be able to write balanced equations for the complete combustion of Hydrocarbons with a given formula.
What does this mean?
What does this mean?
Combustion
Combustion
Combustion is the chemical name for burning.
Burning involves adding Oxygen from the air to a substance.
The substance burning is the fuel.
Alkanes only contain Carbon and Hydrogen atoms.
So these elements are Oxidised - they gain the Oxygen.
Exhaust is a word used to describe the gases made.
Though we would usually just call these products.
Complete Combustion
Complete Combustion
When Hydrogen atoms react with Oxygen it always forms Water Vapour (H2O)
If there is enough Oxygen for complete combustion then each Carbon atom will bond to two Oxygen atoms to form Carbon Dioxide (CO2).
We can show this by drawing the gases from a burning Hydrocarbon (a candle) through a cold U-tube and another filled with Limewater.
The Limewater turns milky, proving the presence of Carbon Dioxide.
The water vapour condenses to form a colourless liquid. This liquid will turn dry Anhydrous Copper Sulphate powder from white to blue.
Incomplete Combustion
Incomplete Combustion
If Oxygen is limited then some Carbon atoms will bond to only one Oxygen atom to form Carbon Monoxide (CO)
Some Carbon atoms may not pick up any Oxygen atoms at all - in which case we will see them as black soot (pure Carbon powder)
Carbon Monoxide is toxic - it sticks permanently to our red blood-cells, stopping us from transporting Oxygen round the body.
Balanced equations.
Balanced equations.
We only need to be able to write symbol equations for complete combustion.
eg 1
Complete Combustion
Methane + (lots of) Oxygen --> Carbon Dioxide + Water
Unbalanced : CH4 + O2 --> CO2 + H2O
Balance the H's: CH4 + O2 --> CO2 + 2 H2O
Balance the O's: CH4 + 2 O2 --> CO2 + 2 H2O
Finished! All elements balance.
eg 2
Complete Combustion
Propane + (lots of) Oxygen --> Carbon Dioxide + Water
Unbalanced : C3H8 + O2 --> CO2 + H2O
Balance the H's: C3H8 + O2 --> CO2 + 4 H2O
Balance the C's: C3H8 + O2 -->3CO2 + 4 H2O
Balance the O's: C3H8 + 5O2 -->3CO2 + 4 H2O
Finished! All elements balance.
The syllabus isn't limited to these two alkanes so it isn't impossible that we may have an equation to balance that involves a higher number of atoms.
eg C9H20 + O2 -->CO2 + H2O becomes C9H20 + 14O2 -->9CO2 + 10H2O
And we might have an awkward number of Oxygens to balance at the end that needs us to use halves
eg C2H6 + O2 -->CO2 + H2O becomes C2H6 + 3.5 O2 -->2CO2 + 3H2O
though we could also double everything in the equation and it would still be correct:
2 C2H6 + 7 O2 -->4 CO2 + 6 H2O
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