Final 2010 question 1C

[104607508]

The question asks whether ethane or ethene has the higher heat capacity but i don't understand why ethane has a higher heat capacity when ethene has a double bond and would therefore require more energy to break? Also is there any specific way that the calculations ought to be done?

[Chem_Mod]

Check out https://en.wikipedia.org/wiki/Heat_capacity#Factors_that_affect_specific_heat_capacity

Ethane is not only more massive (it has 2 more hydrogens), but it also has more vibrational modes, since it can rotate, stretch, scissor, etc. Ethene on the other hand is much more rigid since it has a double bond, and double bonds don't rotate.

[Aoife Galvin 2D]

The heat capacity of a molecule increases when the molecule is more complex. Because ethane consists of carbon-carbon single bonds and hydrogens, the atoms have more freedom to move around than those in ethene do because the double bond creates a fixed structure. Because the atoms have more freedom, more energy is required to heat them. Ethane also has more atoms than ethene, the extra hydrogens, again making it more complex. Breaking bonds does not really have to do with determine heat capacity, you just compare the complexity of the molecules.