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!!Hydrocarbons!!

What are Hydrocarbons?• Hydrocarbon

s – 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 burninghydrocarbon + 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

• 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)

Balancing Combustion Reactions

A propane fireplace malfunctions and burns its gas incompletely to produce deadly carbon monoxide gas and water.

Write a Balanced Chemical equation for the following situation:

a. Write a balanced chemical equation for the complete combustion of Pentane, C5H12

Endothermic and Exothermic ReactionsEndothermic and Exothermic Reactions

• These are reactions that involve ENERGY!!

• MOST ALL reactions are EITHER endothermic or exothermic.

Exothermic Reactions

• Any reaction which releasesreleases 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

NOTE: Energy of Products is LESS than the energy of the Reactants.

ENERGY IS LOST!!!

Energy

Energy Diagram for Endothermic Reactions

NOTE: Energy of Products is GREATER than the energy of the Reactants.

ENERGY IS GAINED!!!

Energy