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136 Modern Analytical Chemistry

O OH O CH 3 6A Reversible Reactions and Chemical Equilibria
In 1798, the chemist Claude Berthollet (1748–1822) accompanied a French military
expedition to Egypt. While visiting the Natron Lakes, a series of salt water lakes
carved from limestone, Berthollet made an observation that contributed to an im-
portant discovery. Upon analyzing water from the Natron Lakes, Berthollet found
large quantities of common salt, NaCl, and soda ash, Na 2 CO 3 , a result he found sur-
OH OH prising. Why would Berthollet find this result surprising and how did it contribute
(a) (b) to an important discovery? Answering these questions provides an example of
Figure 6.1 chemical reasoning and introduces the topic of this chapter.
Structures of (a) p-hydroxybenzoic acid and Berthollet “knew” that a reaction between Na 2 CO 3 and CaCl 2 goes to comple-
(b) p-hydroxyacetophenone. tion, forming NaCl and a precipitate of CaCO 3 as products.

Na 2 CO 3 + CaCl 2 ® 2NaCl + CaCO 3
Understanding this, Berthollet expected that large quantities of NaCl and Na 2 CO 3
could not coexist in the presence of CaCO 3 . Since the reaction goes to completion,
adding a large quantity of CaCl 2 to a solution of Na 2 CO 3 should produce NaCl and
CaCO 3 , leaving behind no unreacted Na 2 CO 3 . In fact, this result is what he ob-
served in the laboratory. The evidence from Natron Lakes, where the coexistence of
NaCl and Na 2 CO 3 suggests that the reaction has not gone to completion, ran
counter to Berthollet’s expectations. Berthollet’s important insight was recognizing
that the chemistry occurring in the Natron Lakes is the reverse of what occurs in the
laboratory.

CaCO 3 + 2NaCl ® Na 2 CO 3 + CaCl 2
CaCO 3
Using this insight Berthollet reasoned that the reaction is reversible, and that the
relative amounts of “reactants” and “products” determine the direction in which
Grams the reaction occurs, and the final composition of the reaction mixture. We recog-
nize a reaction’s ability to move in both directions by using a double arrow when
writing the reaction.

Ca 2+ Na 2 CO 3 + CaCl 2 t 2NaCl + CaCO 3
Berthollet’s reasoning that reactions are reversible was an important step in
Time
understanding chemical reactivity. When we mix together solutions of Na 2 CO 3
Figure 6.2 and CaCl 2 , they react to produce NaCl and CaCO 3 . If we monitor the mass of
Change in mass of undissolved Ca 2+ and dissolved Ca 2+ remaining and the mass of CaCO 3 produced as a function of
solid CaCO 3 over time during the time, the result will look something like the graph in Figure 6.2. At the start of
precipitation of CaCO 3 . 2+
the reaction the mass of dissolved Ca decreases and the mass of CaCO 3 in-
creases. Eventually, however, the reaction reaches a point after which no further
changes occur in the amounts of these species. Such a condition is called a state
equilibrium of equilibrium.
A system is at equilibrium when the Although a system at equilibrium appears static on a macroscopic level, it is
concentrations of reactants and products
important to remember that the forward and reverse reactions still occur. A reac-
remain constant.
tion at equilibrium exists in a “steady state,” in which the rate at which any species
forms equals the rate at which it is consumed.

6B Thermodynamics and Equilibrium Chemistry
Thermodynamics is the study of thermal, electrical, chemical, and mechanical
forms of energy. The study of thermodynamics crosses many disciplines, including
physics, engineering, and chemistry. Of the various branches of thermodynamics,