Balancing the equation for burning an alkane, what happens with too little oxygen, cracking long alkanes into shorter ones and alkenes, and the bromine water test.
What you will learn
- Balance the equation for the complete combustion of an alkane
- Find the missing product of a cracking equation by counting atoms
- Describe the bromine water test for alkenes
Burning hydrocarbons
Hydrocarbon fuels release energy when they burn.
In plenty of oxygen, complete combustion oxidises the carbon to carbon dioxide and the hydrogen to water:
hydrocarbon + oxygen → carbon dioxide + water
Balance the symbol equation in a fixed order: carbon, then hydrogen, then oxygen.
- Propane: C₃H₈ + O₂ → CO₂ + H₂O
- 3 C atoms: 3 CO₂
- 8 H atoms: 4 H₂O
- O on the right: 3 × 2 + 4 × 1 = 10 atoms, so 5 O₂
- C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
If the oxygen count comes out as a half (ethane needs 3½ O2), double every number: 2 C2H6 + 7 O2 → 4 CO2 + 6 H2O.
With too little oxygen, incomplete combustion also makes carbon monoxide (a toxic gas you cannot see or smell) and carbon (soot).
To show the products of combustion, pull the gases from the flame through a cooled U-tube and then through limewater. Water condenses in the U-tube and turns white anhydrous copper sulfate blue; carbon dioxide turns limewater milky.
Never change a formula to balance an equation. Only change the big numbers in front.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Burning hydrocarbons
- Try it: Balance a combustion equation
- Cracking long alkanes
- Testing for alkenes
10 practice questions, marked as you go; about 22 minutes in all.
Key terms
Complete combustion, Incomplete combustion, Cracking, Alkenes, Colourless, Stays orange
In this unit: Organic chemistry
Crude oil and the alkanes, combustion and cracking, then alkenes, alcohols, carboxylic acids, esters and polymers for separate Chemistry, with the formula and mass calculations that go with them.