Re: [Phys-L] Energy & Bonds

[Ron Mcdermott <rmcder@gmail.com>]

This is one of the more confusing areas in chemistry.  A lot of the
confusion comes from conventions found in biology and some from positive
versus negative forces.  I see bits and pieces of the picture have been
mentioned already...

There is one overriding tendency behind the process of bonding: Forming a
more stable product; where more stable is more resistant to change.

Within this overarching tendency are two subtendencies:
1, A tendency to lower energy (PE) and...
2. A tendency to greater disorder (more modes of freedom in the material
p>g>l>s).

By convention, we presume the force between bonding atoms to be attractive,
so infinite separation is zero PE, and PE is negative in sign; lower
energy, then, being a more negative PE (explanation covered earlier).

Bonding typically results in lower PE, releasing energy as heat (exothermic
reaction).  Sometimes, however, bonding can result in an increase in the
modes of freedom (solids to gases, for example), and this result is
sufficient to overcome an increase in potential energy in the reactants;
absorbing heat (endothermic).  The assumption made in the diagrams is that
you beginning with reactants which have some stability to begin with; so in
those diagrams, the PE initially RISES in BOTH kinds of reactions.  This
increase in energy is the ACTIVATION ENERGY, and is required to break
existing bonds in order for new bonds to form.  Breaking bonds ALWAYS
absorbs/requires ENERGY.  Forming bonds ALWAYS releases energy.

The overall reaction consists of an initial bond-breaking, endothermic
(energy-absorbing) process, followed by a finishing (new) bond-forming,
exothermic (energy releasing) process.  What you see in the diagrams is the
totality of those two processes.  Whether you end up with products which
are higher or lower in energy depends on the algebraic sum of the two
steps.  Clear so far?

Ok, so here's where the problem typically enters...  In biology, students
are taught that energy is released by atp, so the idea of energy being
'stored' in a bonded entity is born.  Wood has 'stored energy'.  Oil has
'stored energy'.  Etc.  As a result, and as Paul has discovered, the
diagrams make no sense if your thinking has been influenced by the biology
you took (In the US, probably before your chemistry).

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.


> > On 12/11/2013 15:01, Paul Lulai wrote:
> > > Hi.
> > > I am more aware of my conceptual shortcomings after the conversation
> about energy, reactions, and misconceptions.
> > > I would appreciate clarification from the group on:
> > > How do we reconcile the traditional Exothermic & Endothermic graphs of
> Energy vs reaction process with the fact that the products must be more
> bound and have greater potential energy? Typically the endo / exo graphs
> show (for an exothermic reaction) that the reactants have more energy than
> the products.
> > > I may have more follow up questions.
> > > Thanks.
> > > Paul.
> > > _______________________________________________