Chapter 18, Problem 91E

“Hard” water has a high concentration of calcium and magnesium ions. Focusing on the calcium ion, a common home water -softening process is based on a reversible chemical change that can be expressed by ${\text{Na}}_{\text{2}}\text{Ze}\left(\text{s}\right)+{\text{Ca}}^{2+}\left(\text{aq}\right)\rightleftharpoons \text{CaZe}\left(\text{s}\right)+2{\text{Na}}^{\text{+}}\left(\text{aq}\right)$. ${\text{Na}}_{\text{2}}\text{Ze}$ represents a solid resin that is like an ionic compound between sodium ions and zeolite ions, a complex arrangement of silicate and aluminate groups; $\text{CaZe}$ is the corresponding calcium compound. When, during the day, is this system most likely to reach equilibrium? Why doesn’t it reach equilibrium and stay there? Periodically it is necessary to “recharge” the water softner by running salt water, $\text{NaCl}\left(\text{aq}\right)$, through it. Why is this necessary? What concept is discussed in this chapter does the recharging process illustrate?

Zeolite ions are used in home water softners because they have a porous but rigid structure that is weakly attracted to cations, readily allowing an exchange among different ions.