Combustion Reactions
In this lesson you will learn
• to write balanced equations for both complete and incomplete
combustion reactions
• define exothermic and endothermic reactions
• draw energy diagrams representing exothermic and endothermic
reactions
Hydrocarbon Combustion
• We will be examining
combustion (burning)
of Hydrocarbons!!
What are Hydrocarbons?
• Hydrocarbons
– Substances
that consist of
hydrogen, (H)
and carbon, (C)
atoms ONLY.
• Propane - C3H8
• Octane - C8H18
• Candle wax C25H52
What happens during combustion?
• During combustion, Oxygen (O2) combines with a
hydrocarbon to produce an oxide of carbon and water.
General Equation for Combustion:
hydrocarbon + oxygen → oxide of carbon + water
There are TWO types of Hydrocarbon
Combustions.
Complete Combustion:
•
Reaction evidence is the presence of a nice blue flame – clean
burning
hydrocarbon + oxygen → carbon dioxide + water
Incomplete Combustion:
• Carbon monoxide and carbon in the form of soot are
produced when combustion is incomplete resulting in an
orange, smoky flame.
hydrocarbon + oxygen → carbon monoxide + carbon + water
Writing and Balancing Combustion Reactions
Balancing Combustion Reactions
•
A great method to use when balancing combustion
reactions is The Method of C H O. That is:
1. balance C
2. balance H
3. balance O last.
Example: Balance the complete combustion reaction of
Propane (C3H8).
___C3H8(g) + ____ O2(g)  ___CO2(g) + ___ H2O(g)
Write a Balanced Chemical equation for the following situation:
A propane fireplace malfunctions and burns its gas incompletely to produce
deadly carbon monoxide gas and water.
a. Write a balanced chemical equation for the complete combustion of
Pentane, C5H12
Endothermic and Exothermic Reactions
• These are reactions that involve ENERGY!!
• MOST ALL reactions are EITHER endothermic
or exothermic.
Exothermic Reactions
• Any reaction which releases energy is
said to be exothermic.
– Examples:
•
•
•
•
•
Burning/combustion
“hot packs”
Digestion of food
Bombs exploding
Adding group 1 metals to water
Endothermic Reactions
• Any reaction in which energy is absorbed
is said to be endothermic.
– Examples:
• Adding Ammonium Nitrate to water.
• "Cold Packs" used by athletes contain chemicals
which absorb heat from the body.
Energy Diagrams
• These are diagrams that show what happens to the energy
in a reaction.
• There are TWO types of Energy Diagrams.
Energy Diagrams for EXOTHERMIC Reactions
Energy
NOTE: Energy of Products is LESS than the energy of the Reactants.
ENERGY IS LOST!!!
Energy Diagram for Endothermic Reactions
Energy
NOTE: Energy of Products is GREATER than the energy of the Reactants.
ENERGY IS GAINED!!!