Textbook ExplanationHere is what I think I know: The entropy of dissolution reactions increases as methylene groups are added (i.e. butanol has higher entropy of dissolution than propanol). Also, acyclic saturated hydrocarbons become more hydrophobic as the number of methylene groups increases.

Here is where I get confused:

Larger hydrocarbons have a higher enthalpy of dissolution because they are composed of a greater number of bonds. BUT why does the CHANGE in entropy of dissolution become more negative as the number of carbons increases? Doesn't the dissolution of a larger hydrophobic compound yield a more positive entropy value/become less negative because the initial entropy of the solvent is very low (many large, ordered solvent cages) and then solvent entropy increases after the molecules aggregate by the hydrophobic effect?

Super confused...thank you.

  • $\begingroup$ It is important to know the nature of the solvant, because H-bonds have the main role in water. Is it water or a non-polar solvant ? You also never mention the nature of the solute. In the beginning it is an alcohol. But in the title it is a hydrocarbon. So what ? Please explain ! $\endgroup$ – Maurice Feb 10 at 21:06
  • $\begingroup$ Maurice: sorry for not clarifying. The solvent is indeed water, and I think that I am more interested in the dissolution of hydrocarbons (I used the alcohols misleadingly to illustrate my point). Specifically, I am hoping to figure out the trend in entropy as hydrocarbons with differing number of methylene groups dissolve. $\endgroup$ – Lila Berle Feb 10 at 21:41
  • $\begingroup$ Is it possible to measure the enthalpy of dissolution of hydrocarbons in water ? Their solubility is so weak. Does it make sense ? How can this dissolution be measured ? $\endgroup$ – Maurice Feb 10 at 21:51
  • $\begingroup$ Yes, the solubility of hydrocarbons is low in polar solvents. I am trying to figure out the thermodynamic justification of the hydrophobic effect. I added a picture of the textbook for reference. $\endgroup$ – Lila Berle Feb 11 at 17:26

I think that you should focus on what you wrote first, longer the hydrocarbon chain higher their entropy in solution. When dissolved, they can access a multitude of states both energetically and in spaces (think of all possible conformers).

This said, it does not mean that they must dissolve in, eg, water. Right because the H term.


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