Combustion Reactions

AP Comments

The combustion reactions commonly seen on the AP exam are reactions of hydrocarbons (C, H) or C, H, O compounds in excess oxygen. There have also been reactions of nonmetal hydrides with oxygen or metal sulfides.

Some past reactions;

Elements with oxygen.

These are fairly fundamental reactions that we would all predict using the periodic table. However, watch out for the Group IA elements. Lithium forms Li₂O; sodium forms the peroxide, Na₂O₂; while potassium, cesium and rubidium all form the superoxide, MO₂.

Diborane with oxygen

B₂H₆(g) + 3O₂(g) ----> B₂O₃ + 3H₂O(g)

This reaction is interesting because diborane has one of the highest heats of combustion by weight of any fuel except for hydrogen, beyllium dihydride or beryllium di-(tetraborohydride).

Ammonia with oxygen

4NH₃(g) + 3O₂(g) --heat--> 2N₂ + 6H₂O(g)

NH₃(g) + 5O₂(g) --Pt, 800 C--> 4NO + 6H₂O(g)

The NO can react further with oxygen to form NO₂.

Metal Sulfides with oxygen

MS(g) + O₂(g) --heat--> M + SO₂(g)

This reaction of a metal sulfide to produce the pure metal and sulfur dioxide works best for the metals; copper, silver and mercury. Other metals frequently form a sulfate rather than sulfur dioxide. It works best for the three metals above becauee their oxides are much less stable than sulfur dioxide.

Hydrocarbons with oxygen

We've used the reaction between iron and sulfur and between sulfur and oxygen as examples of formation reactions. There was another reaction which occurred when the iron and sulfur mixture was heated. That reaction occurred when we lit the Bunsen burner to heat the mixture of iron and sulfur. The reaction that was used to heat the mixture was a combustion reaction between methane (CH₄) and oxygen (O₂).

CH₄(g) + O₂(g) --heat-->

Methane is an example of a hydrocarbon. It is not the only example of a hydrocarbon. But more about that shortly. The products of this combustion reaction are carbon dioxide gas and water vapor (liquid).

CH₄(g) + O₂(g) --heat--> CO₂(g) + H₂O(g)

Other hydrocarbons you are familiar with include;

You should know the name and formula for the first ten hydrocarbons (memorize).

Methane is a colorless, odorless gas which is used as a fuel in most gas stoves to heat/cook food. Gas companies add a compound which has an odor to help detect gas leaks should they arise. The compound I mentioned in class that was added to natural gas was CH₃SH. This is an error, the compound that is added to natural gas is call t-butyl mercaptan and has the formula C₄H₉SH. So lets write the combustion reaction of this compound.

C₄H₉SH(g) + O₂(g) --heat--> CO₂(g) + H₂O(g) + SO₂(g)

Hey we get more practice balancing equations!