Why does ethane release more energy than ethyne when burned?
@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.