Page 192 - Modern Analytical Chemistry

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Chapter 6 Equilibrium Chemistry 175

6J Two Final Thoughts About Equilibrium Chemistry

In this chapter we have reviewed and extended our understanding of equilibrium
chemistry. We also have developed several tools for evaluating the composition of a
system at equilibrium. These tools differ in how accurately they allow us to answer
questions involving equilibrium chemistry. They also differ in their ease of use. An
important part of having several tools available to you is knowing when to use
them. If you need to know whether a reaction is favorable, or the approximate pH
of a solution, a ladder diagram may be sufficient to meet your needs. On the other
hand, if you require an accurate estimate of a compound’s solubility, a rigorous cal-
culation using the systematic approach and activity coefficients is necessary.
Finally, a consideration of equilibrium chemistry can only help us decide what
reactions are favorable. Knowing that a reaction is favorable does not guarantee that
the reaction will occur. How fast a reaction approaches its equilibrium position
does not depend on the magnitude of the equilibrium constant. The rate of a chem-
ical reaction is a kinetic, not a thermodynamic, phenomenon. Kinetic effects and
their application in analytical chemistry are discussed in Chapter 13.

6K KEY TERMS

acid (p. 140) enthalpy (p. 137) mass balance equation (p. 159)
acid dissociation constant (p. 140) entropy (p. 137) Nernst equation (p. 146)
activity (p. 172) equilibrium (p. 136) oxidation (p. 146)
activity coefficient (p. 172) equilibrium constant (p. 138) oxidizing agent (p. 146)
amphiprotic (p. 142) formation constant (p. 144) pH (p. 142)
base (p. 140) Gibb’s free energy (p. 137) precipitate (p. 139)
base dissociation constant (p. 141) Henderson–Hasselbalch redox reaction (p. 145)
buffer (p. 167) equation (p. 169) reducing agent (p. 146)
charge balance equation (p. 159) ionic strength (p. 172) reduction (p. 146)
common ion effect (p. 158) ladder diagram (p. 150) solubility product (p. 140)
cumulative formation constant (p. 144) Le Châtelier’s principle (p. 148) standard state (p. 137)
dissociation constant (p. 144) ligand (p. 144) stepwise formation constant (p. 144)

6L SUMMARY

Analytical chemistry is more than a collection of techniques; it is Acid–base reactions occur when an acid donates a proton to a
the application of chemistry to the analysis of samples. As you will base. The equilibrium position of an acid–base reaction is de-
see in later chapters, almost all analytical methods use chemical re- scribed using either the dissociation constant for the acid, K a , or
activity to accomplish one or more of the following—dissolve the the dissociation constant for the base, K b . The product of K a and
sample, separate analytes and interferents, transform the analyte K b for an acid and its conjugate base is K w (water’s dissociation
to a more useful form, or provide a signal. Equilibrium chemistry constant).
and thermodynamics provide us with a means for predicting Ligands have electron pairs that they can donate to a metal ion,
which reactions are likely to be favorable. forming a metal–ligand complex. The formation of the metal–
The most important types of reactions are precipitation reac- ligand complex ML 2, for example, may be described by a stepwise
tions, acid–base reactions, metal–ligand complexation reactions, formation constant in which each ligand is added one at a time;
and redox reactions. In a precipitation reaction two or more solu- thus, K 1 represents the addition of the first ligand to M, and K 2
ble species combine to produce an insoluble product called a pre- represents the addition of the second ligand to ML. Alternatively,
cipitate. The equilibrium properties of a precipitation reaction are the formation of ML 2 can be described by a cumulative, or overall
described by a solubility product. formation constant, b 2, in which both ligands are added to M.