 # Acids and Bases: Salts Home Table of Contents Introduction Molecular Structure Ionization Constants Salts Buffers Lewis Theory

### GOAL(s):

• Recognize salts as products as acid-base reactions
• Determine if an aqueous salt solution will be acidic, basic, or neutral
• Understand how pH can affect solubility

The ingredient list for almost any household product is sure to inlude at least one compound that chemists classify as a salt. While you may think of salt as the white granules used to flavor foods (known by chemists as sodium chloride, NaCl), chemists classify many compounds as salts. To chemists, a salt is any ionic compound that could have been formed by an acid-base reaction. Sodium chloride fits this definition since it could be formed by the reaction of hydrochloric acid and sodium hydroxide:

HCl + NaOH NaCl + H2O

When any acid and base are combined, an exchange reaction occurs, producing a salt and water. A generic equation for this process is shown below:

HA + BOH BA + H2O

The salt's negative ion (A-) is the conjugate base of the acid HA while the salt's positive ion (B+) is the conjugate acid of the base BOH. Depending on the strength of the acid and base, the resulting solution may be acidic, basic, or neutral.

If the strengths of the original acid (HA) and base (BOH) are known, the strengths of their conjugate acid and base can be determined since the ionization constants of conjugate acid-base pairs are related:

Ka x Kb = Kw = 1.0 x 10-14 Complete the following statements: 1 If HA is a strong acid, A- will be a strong weak very weak   base and will cause the pH of the solution to increase decrease neither increase nor decrease 2 If HA is a weak acid, A- will be a strong weak very weak   base and will cause the pH of the solution to increase decrease neither increase nor decrease Good! This relationship is also seen with bases and their conjugate acids. If BOH is a very strong base, B+ will be a very weak acid and will not affect the pH of a solution. If BOH is a weak base, B+ will be a weak acid and will cause the pH of a solution to decrease. These relationships are summarized in the table below:

 Example Conjugate base Strength ofconjugate base Affect on pH Strong acid HNO3 NO3- Very weak None Weak acid HCO2H HCO2- Weak Increase
 Example Conjugate acid Strength ofconjugate acid Affect on pH Strong base KOH K+ Very weak None Weak Base NH3 NH4+ Weak Decrease A strong acid is one that completely ionizes in water. Its ionization constant is too large to measure. If this is the case, the conjugate base will have an ionization constant that is incredibly small (too small to measure!) and can be classified as very weak. Suppose a weak acid has an ionization constant of 1.0 x 10-4. The ionization constant of its conjugate base would be 1.0 x 10-10. How would you classify the strength of this base? Now suppose a weak acid has an ionization constant of 1.0 x 10-11. The ionization constant of its conjugate base would be 1.0 x 10-3. Would the strength of this base be in the same classification as the first example? Remember that a base (unless it is very weak) will cause the pH of a solution to increase. A very weak base will not affect the pH of the solution.   Salts  