1. TYPES OF CHEMICAL RXNS: COMBUSTION

2. So far We have learned 4 types of rxns: Synthesis: X + Y  XY
Decomposition: XY  X + Y
Single Displacement: X + YZ  XZ + Y
Double Displacement: XW + YZ  XZ + YW
Today we learn about Combustion

3. COMBUSTION
This rxn involves a fuel burning or reacting quickly with oxygen.
Oxygen is ALWAYS a reactant in combustion! Without it, the rxn won’t happen.
The products of the rxn are usually an Oxide and energy.
We typically use Hydrocarbons (compounds made up of Carbon & Hydrogen) as the fuel
Ex. C3H8 is known as propane (BBQ) and C4H10 is called butane and is used for cooking or lighters.

4. COMPLETE COMBUSTION OF HYDROCARBONS
In general, if a lot of Oxygen is available, then Hydrocarbons can be burnt completely.
If we have complete combustion, the only products available are CO2, H2O, and Energy
Complete combustion usually produces a bluish flame
The general rxn for Complete Combustion of Hydrocarbons is,
CXHY + O2  CO2 + H2O + Energy
If we don’t have complete combustion, you end up with a sooty residue (carbon), like in a fireplace.

5. 2C2H6(g) + 7O2  4CO2(g) + 6H2O(g) + Energy
EXAMPLE
The complete combustion of Ethane in air is described by the following chemical eqn.
C2H6(g) + O2  CO2(g) + H2O(g) + Energy (Skeleton Eqn.)
Balance the above equation.
2C2H6(g) + 7O2  4CO2(g) + 6H2O(g) + Energy
(Balanced Eqn.)

6. 2C3H8(g) + 10O2(g)  6CO2(g) + 8H2O(g) + Energy (Balanced Eqn.)
EXAMPLE
Propane (C3H8) is completely burned in air. Write the balanced chemical equation for this rxn.
Start with the Skeleton Eqn.
C3H8(g) + O2(g)  CO2(g) + H2O(g) + Energy (Skeleton Eqn.)
Now Balance the Eqn.
2C3H8(g) + 10O2(g)  6CO2(g) + 8H2O(g) + Energy (Balanced Eqn.)

7. INCOMPLETE COMBUSTION OF HYDROCARBONS
If we don’t have enough oxygen, then incomplete combustion occurs.
The general products of incomplete combustion are CO (carbon monoxide), C (soot), as well as CO2 and H2O
Usually you will see an orange, flickering flame when incomplete combustion is happening.
General rxn eqn. for Incomplete Combustion of Hydrocarbons is,
CXHY + O2  CO2 + H2O + CO + C + Energy

8. 2C8H18(l) + 11O2(g)  CO2(g) + 18H2O(g) + 2CO(g) + 13C(s)
example
Octane is a primary component of gasoline in our cars. Predict the incomplete combustion chemical eqn. for this process and balance it.
Write down the skeleton eqn. first.
C8H18(l) + O2(g)  CO2(g) + H2O(g) + CO(g) + C(s) + Energy (Skeleton Eqn.)
Now you can balance the eqn.
2C8H18(l) + 11O2(g)  CO2(g) + 18H2O(g) + 2CO(g) + 13C(s)
+ Energy
(Balanced Eqn.)

9. WHAT IF IT ISN’T A HYDROCARBON?
We can burn other things, like metals in the presence of Oxygen.
The general eqn. for combustion of other substances is,
Element + Oxygen  Oxide
X + O2  XO
If you haven’t noticed, Combustion of other elements is also a Synthesis rxn!

10. EXAMPLE
Write the word equation for burning sodium in air. Also create the balanced eqn. associated with this rxn.
Start by writing the word eqn.
Sodium + Oxygen  Sodium Oxide
Next, write the skeleton eqn.
Na(s) + O2(g)  Na2O(s) (Skeleton Eqn.)
Finally, balance the eqn.
4Na(s) + O2(g)  2Na2O(s) (Balanced Eqn.)

11. HOMEWORK
Read Section 6.9
Pg. 251, #2 – 5