Combustion of alkanes
Lajoy Tucker
Teacher
Introduction & Definitions
Combustion: The reaction of a substance with oxygen, releasing heat and light.
Complete Combustion: Occurs when alkanes react fully with excess oxygen to produce carbon dioxide (CO₂) and water (H₂O).
Incomplete Combustion: Occurs when there is a limited supply of oxygen, producing carbon monoxide (CO) or carbon (C, soot) and water.
Basic Principles
Alkanes are used as fuels because they release large amounts of energy on combustion.
These reactions are exothermic.
Examples
A. Complete Combustion
Example with pentane
Word equation:
Pentane + oxygen → carbon dioxide + water
Balanced symbol equation:
B. Incomplete Combustion
Occurs in restricted oxygen supply (e.g. faulty boilers, vehicle engines).
Example with propane (carbon monoxide produced):
Word equation:
Propane + oxygen → carbon monoxide + water
Balanced symbol equation:
Example with butane (carbon/soot produced):
Word equation:
Butane + oxygen → carbon + water
Balanced symbol equation:
Carbon monoxide is toxic, binds to haemoglobin and reduces oxygen transport.
Soot causes breathing problems and global dimming.
No answer provided.
Check for Understanding
Recall
a) Define complete combustion.
Answer
Reaction with excess oxygen producing CO₂ and H₂O.
b) State two products of incomplete combustion.
Answer
Carbon monoxide; carbon (soot).
c. What is produced when methane burns completely?
Answer
Carbon dioxide and water.
Application
d) Write a balanced equation for complete combustion of propane.
Answer
C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
e) Explain why carbon monoxide is dangerous.
Answer
Toxic, odorless and colourless so hard to detect; binds to haemoglobin better than oxygen; reduces oxygen transport and inhibits respiration; can lead to tiredness, nausea, headaches, shortness of breath, chest and muscle pain and in severe cases death/comas.
Challenge (HT)
f. Write and balance the equation for complete combustion of octane.
Answer
2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O
Summary
Alkanes burn in excess oxygen (complete combustion) to produce carbon dioxide (CO₂) and water (H₂O).
Alkanes burn in a limited supply of oxygen, producing carbon monoxide (CO) or carbon (C, soot) and water.
Balance combustion equations by balancing C then H then O
No answer provided.