The Foundations of Chemistry

The system (consisting of the water, the solid NH 4 NO 3 , and the resulting hydrated NH 4 
and NO 3 ions) absorbs heat from the surroundings as the endothermic process occurs.
Nevertheless, the process is spontaneous because the system becomes more disordered as
the regularly arranged ions of crystalline ammonium nitrate become more randomly
distributed hydrated ions in solution (Figure 15-10). An increase in disorder in the system
favors the spontaneity of a reaction. In this particular case, the increase in disorder over-
rides the effect of endothermicity.

Two factors affect the spontaneity of any physical or chemical change:

1.Spontaneity is favoredwhen heat is releasedduring the change (exothermic).

2.Spontaneity is favoredwhen the change causes an increase in disorder.

The balance of these two effects is considered in Section 15-15.

THE SECOND LAW OF THERMODYNAMICS

We now know that two factors determine whether a reaction is spontaneous under a given
set of conditions. The effect of one factor, the enthalpy change, is that spontaneity is
favored (but not required) by exothermicity, and nonspontaneity is favored (but not
required) by endothermicity. The effect of the other factor is summarized by the Second
Law of Thermodynamics.

In spontaneous changes, the universe tends toward a state of greater disorder.

The Second Law of Thermodynamics is based on our experiences. Some examples
illustrate this law in the macroscopic world. When a mirror is dropped, it can shatter.
When a drop of food coloring is added to a glass of water, it diffuses until a homoge-
neously colored solution results. When a truck is driven down the street, it consumes fuel
and oxygen, producing carbon dioxide, water vapor, and other emitted substances.
The reverse of any spontaneous change is nonspontaneous, because if it did occur, the
universe would tend toward a state of greater order. This is contrary to our experience.
We would be very surprised if we dropped some pieces of silvered glass on the floor and
a mirror spontaneously assembled. A truck cannot be driven along the street, even in
reverse gear, so that it sucks up CO 2 , water vapor, and other substances and produces fuel
and oxygen.

ENTROPY, S

The thermodynamic state function entropy, S,is a measure of the disorder of the system.
The greater the disorder of a system, the higher is its entropy. For any substance, the
particles are more highly ordered in the solid state than in the liquid state. These, in turn,

15-14

15-13

Figure 15-10 As particles leave a

crystal to go into solution, they

become more disordered. This

increase in disorder favors the

dissolution of the crystal.

15-14 Entropy, S 621

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 20.4, Entropy: Matter Dispersal

or Disorder.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 20.6, The Second Law of

Thermodynamics.

Suppose we shake a beaker

containing marbles. A disordered

arrangement (left) is more likely than

an ordered arrangement (right), in

which all marbles of the same color

remain together.

H 2 O