16. Ions and Equilibrium;
       Acids and Bases
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       Strong Acids and Bases

When salts dissolve in water the attractive forces within the ionic lattice are broken, and are replaced by attractive forces between individual ions and the polar water molecules that surround them in a hydration shell.

As we saw in Chapter 12, the heat of solution is the difference between hydration energy and crystal-lattice energy. In addition, the salt becomes more disordered when it dissolves, so the entropy increases. If the combination of entropy increase and heat of hydration is enough to overcome the crystal-lattice energy, the salt will dissolve.

We can write the overall process as

Salts such as NaCI are 100% ionized in the crystal and in aqueous solution. The symbol (aq) indicates that each ion is hydrated, or surrounded by a shell of polar water molecules in the manner that we saw first in Chapter 5, and in the drawing at the beginning of this chapter.

For the sake of brevity, we will not use the (aq) symbol in equations in this chapter, but you should remember that ions in aqueous solution always are hydrated, and that hydration energy is largely responsible for making the salt dissolve. If there were no hydration energy to balance the loss of energy from crystal attractions, then dissolving NaCI would be as difficult as vaporizing it, which can be accomplished only at temperatures above 1400'C.

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