Pass the Salt, Acid, and Base Please

In this section we'll talk about the dirty little secret that some molecules disguised as innocuous salts can have acidic or basic tendencies. Here, a salt will refer to a molecule that dissociates entirely when added to an aqueous solution, which in our case will be water.

Most people are familiar with good ole' table salt or as chemists like to say, NaCl. (Actually, table salt is way more nuanced and frankly more delightful than just plain old NaCl—in your face, chemists.) NaCl is an example of a salt that does not have acidic or basic properties when dissolved into solution.

The reason NaCl doesn't change the pH of a solution is because the cation (Na⁺) and anion (Cl^-) that are produced when NaCl is dissolved are really weak acids and bases. We know this because their conjugate acid and base partners are really strong acids and bases. The conjugate base of Na⁺ is NaOH and the conjugate acid of Cl^- is HCl.



Look how tiny those dissociation constants are. Those dissociation constants tell us that the reaction will lie very far to the left. If we give these equations a 180° spin, this might make more sense because NaOH and HCl are a strong base and strong acid, respectively. They will be nearly fully dissociated in solution. Incidentally, combining HCl and NaOH in equal proportions results in a neutral solution of Na⁺ and Cl^-. Combining any strong acid and strong base results in a neutral solution.

For your reference pleasure, here are some cations and anions that do not react with water to give acidic or basic solutions:

1. Cations:
   
   • Alkali metals from group 1A: Li⁺, Na⁺, and K⁺
     
   • Alkaline earth metals from group 2A: Ca²⁺, Sr²⁺, and Ba²⁺
2. Anions:
   • Cl^-, Br^-, I^-, NO₃^-, and ClO₄^-

Things get dicey when a particular cation or anion from a salt does significantly react with water. One example of a salt that produces an acid solution is the salt NH₄Cl (ammonium chloride). When NH₄Cl is dissolved into water the molecule splits up into NH₄⁺ and Cl^- anions. We showed above that Cl^- will not react significantly with water because its conjugate partner is a strong acid, but NH₄⁺ will react with water to some small extent by acting as an acid and transferring a proton:



If we wanted to, we could calculate all the species present in a solution of NH₄Cl by treating the NH₄⁺ released by the salt as a weak acid solution. Don't worry. You'll have a chance to do this later on in the example problems.

It turns out that salts can also have basic properties when they release ions that react with water to form OH^-. For example, the salt NaCN releases both a Na⁺ cation, which doesn't react with water because its conjugate base is really strong, and a CN^- anion, which does react with water because its conjugate acid is weak. Say what?

Take a look at the chemical equation for CN^- acting as a base:



Notice that the conjugate acid, HCN, is a weak acid that does not like to be totally dissociated. Some of the CN^- will react to form HCN. Again, to calculate equilibrium concentrations and pH values you can use the same procedure that we used before.

Lastly, it's possible that the dilution of a salt into water could result in the production of anions and cations that react with water to produce H₃O⁺ and OH^-. Let the battle over the protons begin.

If the cation (typically acting as the acid) is stronger than the anion (typically acting as the base), then the cation will win and the solution will be acidic. If the anion wins and is acting as base, then the solution will be basic. Here's an example of the salt NH₄F reacting with water when it dissolves in solution:



Comparing the dissociation constants above shows that NH₄⁺ is the stronger of the two ions. Since it is an acid, it will beat out the F^- anion and the solution will be acidic.

Here are the rules for determining the pH of any salt solution:

1. If K_a > K_b the solution will have more H₃O⁺ and the pH will be less than 7.
2. If K_a < K_b the solution will have more OH^- and the pH will be greater than 7.
3. If K_a ≈ K_b the solution will contain nearly equal amounts of H₃O⁺ and OH^- and the pH will be approximately 7. If the dissociation constant values are approximately equal to or less than the dissociation constant for water (1 × 10^-14) then the solution will have a pH value of approximately 7.