Entropy
In many of the preceding predictions of reactions, we used the concept that
reactions will occur when they result in the lowest possible energy state.
TIP
Entropy is a measure of the degree of disorder.There is, however, a more fundamental driving force to reactions that is
related to their state of disorder or of randomness. This measure of disorder is
called entropy. A reaction is ultimately driven, then, by a need for a greater
degree of disorder. An example is the intermixing of gases in two connected
flasks when a valve is opened to allow the two previously isolated gases to travel
between the two flasks. Because temperature remains constant throughout the
process, the total heat content cannot have changed to a lower energy level, and
yet the gases will become evenly distributed in the two flasks. The system has
thus reached a higher degree of disorder or entropy. A quantitative treatment of
entropy is given in Chapter 10.
THERMOCHEMISTRY
In general, all chemical reactions either liberate or absorb heat. The origin of
chemical energy lies in the position and motion of atoms, molecules, and
subatomic particles. The total energy possessed by a molecule is the sum of all
the forms of potential and kinetic energy associated with it.
The energy changes in a reaction are due, to a large extent, to the changes in
potential energy that accompany the breaking of chemical bonds in reactants to
form new bonds in products.
The molecule may also have rotational, vibrational, and translational energy,
along with some nuclear energy sources. All these make up the total energy of
molecules. In beginning chemistry, the greatest concern in reactions is the
electronic energy involved in the making and breaking of chemical bonds.
Because it is virtually impossible to measure the total energy of molecules,
the energy change is usually the experimental data that we deal with in reactions.
This change in quantity of energy is known as the change in enthalpy (heat
content) of the chemical system and is symbolized by ΔH.
CHANGES IN ENTHALPY
Changes in enthalpy for exothermic and endothermic reactions can be shown
graphically, as in the examples below.