TYPES OF CHEMICAL RXNS: COMBUSTION

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

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.

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.

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.)

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.)

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

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.)

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!

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.)

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