Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] Energy & Bonds

For intro courses to correctly establish the concepts, it should be pointed out that:

- a chemically bonded system is stable b/c it is at a relative energy minimum, and
- energy must be added to a bonded system to break one or more of its bonds

The physical analogy of hills and valleys can be very helpful. A boulder in a valley 1000' above sea level may be rolled to the top of the nearest hill 1500' ASL (energy input to the system). If the boulder then rolls into an adjacent valley coming to rest at 1200' ASL the overall process will require net energy input ("endothermic"). However, if it instead rolls down to a new "equilibrium" 500' ASL there will be a net energy release ("exothermic"). In neither case will there be a net change in the energy of the earth/boulder system unless sufficient energy is added to get the boulder to the top of the hill. Same thing with a chemical bond. Overall energy change depends of the difference in heights of the two valleys, not on the height of the initial condition. Not perfect, but far better than talking about "energy stored in bonds".

John Barrere
Fresno, CA

On Tuesday, November 12, 2013 3:09 PM, John Clement <> wrote:

YES, YES, YES.  I have repeately pointed out that it the released energy
comes from hydration, forming new bonds.  This is always ignored in bio
books.  But if you read the Wikipedia account it gets it right!  The bio
books do more than gloss.  They state that breaking the bonds releases
energy, but many books will say that this only true for breaking the bonds
in the Krebs cycle.  In other words they are completely wrong.  This also is
done in physical science books.

John M. Clement
Houston, TX

So here's the thing...  Ultimately, energy is not stored in a
bond.  You can often, however, ADD energy, break that bond,
form a NEW bond, and end up with a NET release in energy. 
That (extra) energy comes from the field between the original
atoms and NEW atoms in their vicinity... BUT... that 'field
energy' can only be accessed by breaking the original bonds. 
Going back to ATP, you have to break those initial bonds,
absorbing energy, and THEN new, stronger bonds form releasing
more energy than was initially added.

Biology typically glosses over the intermediate steps and
leaves kids with a completely incorrect concept.  As a
result, the topic of bonding in chemistry is one of the most
confusing they ever face.

Forum for Physics Educators