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Recently the question was asked, "Is bond formation “strictly” exothermic?" As a corollary to that question, what is the relationship between entropy and bond formation? Does the entropy of the bond formation reaction always decrease when a chemical bond is formed?

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  • $\begingroup$ “Although in defolded state protein has grater entropy, greater degree of disorder, it folds into original conformation (lower entropy), and this seems to be in collision with the II law of thermodynamics. But by refolding molecules of water surrounding him increase its own entropy by forming maximum number of hydrogen bonds (they’re squeezing, pushing in hydrophobic regions from protein surface inside of protein, allowing hydrophilic regions to come out to surface to form H-bonds with water), so net change in entropy of solute and protein is than increase of entropy, and process is spontaneou $\endgroup$ – Dissenter Jun 13 '14 at 15:33
  • $\begingroup$ Entropy of what? Isn't the entropy of the universe always increasing? $\endgroup$ – Dissenter Jun 13 '14 at 15:34
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    $\begingroup$ Yes, entropy of the system could increase. E.g., Chelation en.wikipedia.org/wiki/Chelation $\endgroup$ – user26143 Jun 13 '14 at 16:10
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    $\begingroup$ @user26143 Chelation describes a process of multiple bond formations and breakings. I think the OP is asking about the entropy for the formation of one bond. $\endgroup$ – Martin - マーチン Nov 18 '14 at 3:46
  • $\begingroup$ Are you talking about gas phase? In solution, solvent organization is going to make things very complicated for this question... $\endgroup$ – Zhe Nov 28 '16 at 22:39
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I do not think that an entropy decrease is a requirement to form a bond.

Think about a "normal" case: $\ce{HCl}$ (a gas) and $\ce{NH3}$ (another gas) react together to form $\ce{NH4Cl}$, which is a solid. This is an example of bond formation where entropy clearly decreases.

But now think what would happen if two liquid or solid compounds react together to form a gaseous compound. An increase of entropy has taken place. Can't think of a concrete example, but I think this might be the case of, for example, an esterification between a liquid alcohol and a liquid carboxylic acid that form an ester where, due to the lose of $\ce{-OH}$ groups, intermolecular interactions are lower and it can result with the product molecule being more volatile than the original compounds.

It's all about the number of micro states in which the compounds can exist, and gas formation from solids or liquids importantly increases entropy.

(PS: These are just some thoughts, really, I'm no physical-chemist.)

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