Potential Energy and Chemical Bonds
Date: December 2006
How does potential energy change when a chemical
bond is formed? when a chemical bond is broken?
How does the potential energy of two atoms in a chemically bonded
condition compare to their energy when separated?
This is a difficult question to answer exactly since (1) potential
energy is not something that is directly measured - it can only be
deduced from the heats produced or absorbed in a transformation, and
(2) the heat produced or absorbed (enthalpy) in a chemical
transformation vary from substance to substance.
In general, we expect that when chemical bonds are formed, energy is
released - imagine the individual atoms as having energy and have to
be slowed down in order that chemical bonds can form. Alternatively,
and more accurately, when two atoms spontaneously form a chemical
bond it must mean that the entropy of this system has increased,
since two separate atoms have more disorganization (entropy) then
one complete molecule, then in order for the process to be
spontaneous (entropy increases), there must be some heat released.
The problem now is relating heat to potential energy. I would rather
you relate this to internal energy rather than potential energy
(which is not quite directly applicable to chemical systems). If we
think of internal energy, we know, by definition, that internal
energy is a function of the heat and work that goes in and out of
the system. Since most chemical transformations do not involve work,
then internal energy is mostly a function of the heat that enters or
leaves the system.
Thus, when a chemical bond is formed spontaneously, heat leaves the
system, the internal energy of the system goes down. You may then
think of internal energy as a kind of potential energy and say that
because the system is less energetic (since heat left the system)
that it must now have a lower potential energy.
Greg (Roberto Gregorius)
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Update: June 2012