combustion+reactions

A //combustion reaction// conjures images of explosions, fire, and smoke. While these are certainly considered combustion reactions, none are welcome in a chemistry lab.

This class of chemical reactions is one of the most important, for a few reasons: the most important being the production of energy.
 * __transportation:__ Almost every vehicle we use to move around these days relies on a combustion engine. Autos use gasoline or diesel, trains use diesel (used to be coal), planes use jet fuel, etc.
 * __electricity generation:__ A large majority of the world's electricity (almost 70%) is generated by burning fossil fuels.
 * __home heating:__ most homes in the higher latitudes experience cold winters, and must have a heat source. Urban and suburban homes are heated by natural gas furnaces or by electric heat, either of which requires combustion at some end. Some rural homes use propane, wood, or coal furnaces. Some homes also use gas-fired hot water heaters.
 * __industrial uses:__ Many kinds of industrial processes require heat, whether it be for metallurgy or plastics synthesis.



The U.S. uses 100 quadrillion BTU (British thermal units) of energy every year. Here is the breakdown:
 * 40% from petroleum
 * 23% from coal
 * 23% from natural gas
 * 8.4% from nuclear
 * 7.3% from renewables (solar, hydro, wind, etc.)



Here is a link showing energy use per person, you pick the country.

And as for transportation, here's a look at energy efficiency for combustion engines.

As you can see, the majority of our energy comes from **combustion of fossil fuels**, and contributes to man-made carbon dioxide emissions, which have been linked to climate change.

There are many other uses for combustion reactions. But, surprisingly, these reactions can be represented by one chemical equation:

**fuel + oxygen --> carbon dioxide + water**

So, to identify a chemical reaction as a combustion reaction, just look for a fuel, O 2, and the production of CO 2 and H 2 O!

__Example:__

The combustion of ethane, C 2 H 6, is shown below, unbalanced.

C 2 H 6 (g) + O 2 (g) --> CO 2 (g) + H 2 O (g)

To balance a combustion reaction, start by **balancing carbons first**:

C 2 H 6 (g) + O 2 (g) --> 2 CO 2 (g) + H 2 O (g)

Next, balance the hydrogens:

C 2 H 6 (g) + O 2 (g) --> 2 CO 2 (g) + 3 H 2 O (g) Then add up the oxygens in the products, which in this example equals 7.
 * If you **get an odd number**, then that is the number given to oxygen in the reactants side:

C 2 H 6 (g) + 7 O 2 (g) --> 2 CO 2 (g) + 3 H 2 O (g)

And **you must double all the other compounds' coefficients**:

2 C 2 H 6 (g) + 7 O 2 (g) --> 4 CO 2 (g) + 6 H 2 O (g)

This will balance the chemical equation!

Try some practice problems: