carbon
C(s) + O 2 (g) → CO 2 (g) + 393.5 kJis –393.5 kJ, also written as ΔHc = –393.5 kJ, and it represents the heat released
in another combustion/redox reaction. The heats of combustion for many common
substances can easily be found in reference tables like the kind found in Appendix
B of this book.
Hydrocarbon Fuel Combustion Reactions
Another common combustion reaction involves the burning of hydrocarbon fuels.
Methane, CH 4 , is a typical hydrocarbon fuel. It is the main component of natural
gas. Analysis of the reaction
shows that carbon is being oxidized and the oxygen is being reduced. This fuel is
used to heat homes and cook food and should be familiar to all chemistry students
as the gas used in their laboratory burners.
Example 1
Describe the reaction that occurs when propane, C 3 H 8 (the hydrocarbon fuel used
in backyard barbecues) combusts, releasing 2,221 kJ when one mole is burned.
It should be noted that when hydrocarbon fuels combust completely in the
presence of oxygen, the products of the reaction are carbon dioxide and water.
Therefore, the reaction is:
Once again, the carbon is oxidized from the fraction –8/3 to +4 (increasing
from negative to positive) and the oxygen is reduced from 0 to −2. Oxygen is the
oxidizing agent and propane is the reducing agent. When this reaction occurs light
and heat are released and the heat of combustion, ΔHc, is −2221 kJ.