ENERGY IN GEOCHEMICAL PROCESSES - Chemical Fundamentals of Geology and Environmental Geoscience - page 8

Box 1.3 Some properties of entropy

the concept of disorder is of fundamental importance in thermodynamics, because it allows us to distinguish those pro-
cesses and changes that occur naturally - 'spontaneous' processes - from those that do not. We are accustomed to
seeing a cup shattering when it falls to the floor, but we never see the fragments reassemble themselves spontaneously
to form a cup hanging on the dresser hook. Nor is it a natural experience for the air in a cold room to heat up a warm radia-
tor. the direction of change that we accept as natural always leads to a more disordered state than we began with.
to apply such reasoning to the direction of chemical change, we need a variable that quantifies the degree of
disorder in a chemical system. In thermodynamics this is defined by the entropy of the system. to define entropy
rigorously lies beyond the scope of this topic, but it is worth identifying the processes that lead to an increase of
entropy. the entropy of a system depends upon:

(i) the distribution of matter or of individual chemical species in the system; and
(ii) the distribution of energy .

Entropy and the distribution of matter
(a) entropy increases as a substance passes from the solid state to the liquid state to the gaseous state.*

(a)

Crystal Liquid/glass * Gas

(b) entropy increases when a gas expands.

(b)

Ordered state Disordered state

(c) entropy increases when pure substance are mixed together.

(c)

Ordered state Disordered state

* a glass is a solid having the disordered structure of a liquid, but deprived of atomic mobility (no flow). Its entropy is intermediate
between liquid and crystalline solid.