Field of Science

Does bond cleavage require or release energy?

Back from a severe flu spell with a question worth pondering. A commenter on Derek's blog posed the following question, which I think could be a nice trick question for undergrads (or grads for that matter).

Question: "Looks like a bunch of chemists are pooling here. Let me ask an undergrad question as to why bond cleavage in chemistry requires energy whereas bond cleavage in biology releases energy, i.e. from ATP to ADP."

Here's the answer which I could think of on the spur of the moment:

Answer: "Bond cleavage always requires energy, even in biology. The question is whether the net reaction is energetically favorable, which it is in case of ATP. That gives the illusion of energy 'release' by bond cleavage. There are such cases in organic chemistry too; for instance the cleavage of a bond that expels nitrogen as a leaving group is usually very favorable (nitrogen is sometimes called "the world's best leaving group"). That does not mean that you don't need energy to cleave this bond, it's just that the unusual stability of nitrogen makes the net reaction favorable.

Fair enough?


  1. I just put up a post which riffs on this one a bit.



Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="">FoS</a> = FoS