No biologists, breaking bonds does NOT ‘release energy’

Written by Adrian
On January 22, 2016
Categories: AP | Big Idea 2

An age old problem that I’ve become a little weary of addressing, so let’s clear this up really quickly and really simply.

No biologists, breaking bonds does NOT ‘release energy’, and here’s why.

No biologists, breaking bonds does NOT 'release energy'Bond breaking is an endothermic process (+ve), and bond making is an exothermic process (-ve). All chemical reactions involve both processes, and the enthalpy change for the reaction is always a SUM of those processes. Biologists choose to ignore the individual breaking and making of bonds during a chemical reaction, and they choose to only report the SUM of these processes, i.e., the enthalpy change, which is fine. However, then they go on to incorrectly label the overall enthalpy change as simply ‘bond breaking’ or ‘bond making’ – that’s plain wrong since they are ignoring the fact that BOTH processes contribute to the overall enthalpy change.

I have no problem with the biologists ignoring the individual processes if that suits them and their own needs, but the problem is with their incorrect labeling of a chemical reaction as only one or the other; it isn’t, it’s both.

As an aside, I am in the habit of teaching enthalpy changes that involve bond breaking and bond making in a manner that is slightly different to most chemistry teachers in the USA. By teaching that a positive number is associated with bond breaking, and that a negative number is associated with bond making, and that ∆H is the SUM of the two, I try to emphasize a knowledge of the endothermic/exothermic nature of bond breaking/making. In my experience, most chemistry teachers don’t do that, rather they tend to say that ∆H = bonds broken MINUS bonds made. Of course this ultimately yields the same answer, but I find that my method tends to help cement an idea that would help this biology based misconception.


  1. Pamela Gardner

    Very useful observation.
    I’m interested in physiological chemistry – originating in dietetics – and all aspects of the transference of energy. And, of course, a main energy source comes from the reduction of ATP to ADP: but to describe the energy simply as coming from the breaking of a bond in ATP would be wrong and not further an understanding in any of the complex metabolic, neural or other pathways.

  2. Scott

    I teach enthalpy changes the same way. I find it prevents students from having to try and memorize a formula, instead making sense of what is actually going on. Many of my AP Chemistry students have already taken AP Biology, and therefore it is an uphill battle convincing them that breaking a bond does not release energy. Using complete biological examples with them seems to help. Also, telling them that making such a statement is incorrect and will have a negative impact on their grade, also works.

  3. Leslie Twining

    Thank you for the clarification as this clears up a big debate amongst my students and I can better explain it correctly

    • Adrian

      You’re welcome!


Submit a Comment

Your email address will not be published. Required fields are marked *