PLENTIFUL ENERGY

stored energy: energy stored in the bonds of atoms and molecules as potential
energy. Reactions result when there is a reduction in the stored chemical energy
from the reactants to the product—like a weight losing height under the force of
gravity. The driving force is the energy transfer.
Energy transfer is the very subject of thermodynamics. The name in fact means
movement of heat or energy. There are two parts to the thermodynamics of this.
First, and most straightforwardly, a reaction can occur if energy is released by the
reaction (felt as heat, a decrease in ―enthalpy‖, another name for total energy).
Second, if the energy of the product is less concentrated, more spread out, less
useful, after the reaction, there is an increase in ―entropy,” a somewhat elusive
concept, but it too is an energy change—an energy loss, in fact. The two, enthalpy
change and entropy change, may not act in the same direction, but if the net of the
two overall is a decrease in energy content, the reaction can take place.
The net change is called the free energy: that is, it is the energy free to drive the
reaction. It is the maximum energy available from a reaction for conversion to other
forms of energy. It can be looked at as a ―chemical potential,‖ or as a voltage, in
fact. For it is a potential energy, and may be thought of as the energy actually
available to ―flow downhill‖ and do useful work. As noted previously, there are
energy barriers to the reaction proceeding spontaneously, but when those are
overcome (we used ignition as an example), reactions are free to proceed.
The classic thermodynamic relationships, elegant and simple, apply. Using them,
precise calculations can be made of the energy exchanges. These in turn define
what redox reaction can occur and under what conditions. Thermodynamics gives
us the quantitative information we need.
For those with some mathematical background, these considerations are
summarized in the expression
G = U + TS. (1)
The free energy is denoted by G (for Gibbs, the originator of the concept).
Enthalpy, U, is a measure of the energy intrinsic to the compound, its ―internal
energy.‖ Entropy, S, is that portion of that energy unavailable for anything useful.
It‘s a loss of energy that comes with the rearrangement of atoms and molecules in
the reaction. T is the absolute temperature in Kelvin, showing that the entropy
effect rises as temperature (and in turn, molecular motion) increases.
At constant temperature the change in G, G, in forming a compound is given
by equation (2) below. If G is negative (think of this as a well, with the depth
given by the magnitude of G), the reaction forming the compound will tend to
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