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!
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3$\begingroup$ Note also that ethane has more hydrogens than ethene and ethyne, which will also contribute to a more negative energy of combustion. $\endgroup$– a-cyclohexane-moleculeJul 17, 2018 at 23:51
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@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.
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$\begingroup$ I see, your explanation makes a lot of sense, thank you! I'm still wondering, though, what to make of my book's explanation. How is a lower number of bonds a factor in releasing more energy? $\endgroup$ Jul 18, 2018 at 0:22
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3$\begingroup$ It applies where all other parameters are kept the same. If you were to burn one mole of ethylene plus one mole of H2, the same amount of each species as in one mole of ethane, you'd get more energy than from burning one mole of ethane. $\endgroup$ Jul 18, 2018 at 2:01