is pressed, gas comes out spontaneously. When metallic sodium is placed in a
jar filled with chlorine gas, a chemical reaction occurs spontaneously, making
sodium chloride as a product.
However, a rock on the ground does not jump up to waist height sponta-
neously. Hair spray does not spontaneously rush back into the can at high
pressure, and sodium chloride does not spontaneously react into metallic
sodium and diatomic chlorine gas. These are examples ofnonspontaneous
changes. These changes can be made to occur, by performing some sort of
work. For example, sodium chloride can be melted and an electric current run
through it, generating sodium and chlorine, but in such a case we are forcing
a nonspontaneous process to occur. The process is not occurring on its own.
As a final example, consider the isothermal, adiabatic free expansion of an ideal
gas. The process is spontaneous, but it occurs with no change in energy of the
gas in the system.
How can we predict which processes are spontaneous? Consider the three
cases used above. When a rock falls, it goes to a lower gravitational potential
energy. When high-pressure gas goes to a lower pressure, it occurs with a de-
crease in energy. When sodium and chlorine react, the exothermic reaction
means that energy is given off and the overall system has gone to a lower en-
ergy. We therefore make the following suggestion: spontaneous processes
occur if the energy of the system decreases. Is this a sufficient definition and
an able predictor of a spontaneous process? Is this general statement univer-
sally applicable to all spontaneous processes?
Consider the following process:
NaCl (s) →H^2 O Na(aq) Cl(aq)which is the dissolution of sodium chloride in water. The change in enthalpy
for this process is an example of a heat of solution,solnH. This particular
process, which occurs spontaneously (since sodium salts are soluble), has a
solnH(25°C) of3.88 kJ/mol. It is an endothermicprocess, yet it occurs
spontaneously. Consider the chemical reaction of a common chemical demon-
stration:
Ba(OH) 2 8H 2 O (s) + 2NH 4 SCN (s) →
Ba(SCN) 2 (s) + 2NH 3 (g) + 10H 2 O ()
This reaction absorbs so much energy from the surroundings (that is, it is so
endothermic) that it can freeze water into ice, which is the major point of
the demonstration. The system (that is, the chemical reaction) is increasing in
energy, but it too is spontaneous.
The conclusion is that a decrease in the energy of a system is insufficient in
itself to predict whether a process in that system will be spontaneous. Most
spontaneous changes, but not all, are accompanied by a decrease in energy.
Therefore, a decrease in energy for a change is not sufficient to determine
whether or not the change is spontaneous.
Unfortunately, the first law of thermodynamics deals with changes in energy
only. But we have seen that consideration of energy changes alone is insuffi-
cient for determining whether or not changes in the system are spontaneous.
Does this mean that the first law of thermodynamics is wrong? No! It only
means that the first law alonecannot address this particular question.
Thermodynamics does provide other tools with which to study processes.
The consideration of these tools not only broadens the applicability of ther-
modynamics, but goes a long way toward answering the question, “Is this
3.2 Limits of the First Law 67