18.3 – Salt Hydrolysis
18.3.1 – Deduce whether salts form acidic, alkaline or neutral aqueous solution
Strong Acid and Strong Base
The salt that is formed from a strong acid and a strong base will be neutral in solution.
The Na+ and Cl– ions do not act as acids or bases – they cannot release or accept H+ ions, so they do not affect the pH.
Strong Acid and Weak Base
The salt formed from a strong acid and a weak base will form an acidic solution.
The NH4+ ion is the conjugate acid of ammonia and can react with water to produce H3O+ ions, thereby decreasing the pH.
Weak Acid and Strong Base
The salt formed from a weak acid and a strong base will form a basic solution.
The CH3COO– ion is the conjugate base of ethanoic acid and can react with water to produce OH– ions, thereby increasing the pH.
Weak Acid and Weak Base
It is impossible to generalise the behaviour of these salts, since it depends on the Ka and Kb values of the ions. These must be calculated for the given salt and compared to determine what the pH of the solution would be.
Both the anion and cation will have acid-base properties. The conjugate acid will produce H3O+ ions, decreasing the pH.
The conjugate base will simultaneously produce OH– ions and increase the pH.
The ions will essentially be pushing the pH in opposite directions. The one that “wins” is the one with the larger equilibrium constant. If the Ka is larger, then the solution will be acidic, whereas if the Kb is larger, the solution will be basic. If they are equal, then the pH will be 7.
You can calculate the K values of the ions using the K values of their parent acid/base.
Summary
Metal Ions
Small metal ions with a larger charge have a high charge density. As a result, they can attract water molecules to act as ligands, forming complex ions. These act as weak acids, creating an acidic solution. The high charge on the cation increases the polarity of the bonds in the water molecule, pulling the electrons towards itself until it breaks the O-H bonds, releasing a H+ ion.
The higher the charge on the cation, the more acidic the solution, since more H+ ions are released. Aluminium and iron are both examples of this.
The metals in group 1 of the periodic table only have a 1+ charge, so they do not have a high enough charge density to do this. Similarly, the larger group 2 metals are too big, so their size prevents them from having a high enough charge density. Group 3 and transition metals will decrease the pH of a solution.