My book explains that since ethane contains a single bond, ethene contains a double bond, and ethyne contains a triple bond, and that when burned ethane releases the most energy because "the fact that the lowest energy is released by ethyne is taken to indicate that this compounds bonds are already in the highest energy configuration. The high energy of combustion for ethane on the other hand indicates that its bonds have lower energy to start with and can release more energy on combustion." This seems counter intuitive to me. Wouldn't the double and triple bonds release more energy because they're stronger and it takes more energy to break them? Any insight would be appreciated!
@a-cyclohexane-molecule has it right. Ethane generates more water. Here is a diagram that helps explain the issue in more detail. The black numbers and arrows are the heats of formation of the three hydrocarbons. Ethane is the only one more stable than the elements from which it is formed. Blue numbers and arrows are the heats of formation of CO2 and H2O from graphite and hydrogen. The red arrows and numbers are the heats of combustion of the three hydrocarbons. Although acetylene and ethylene have a headstart in combustion because the are less stable than ethane, all three compounds produce two CO2's but ethane generates more water than acetylene and ethylene. Ergo, ethane generates the greatest amount of heat. All values are in kcal/mol.