# Salt Hydrolysis

## Salt hydrolysis

Does neutralisation mean that the pH of the solution is $$\text{7}$$? No. At the equivalence point of a reaction, the pH of the solution need not be $$\text{7}$$. This is because of the interaction of the salt (formed by the reaction) and water.

At the equivalence point of an acid-base neutralisation reaction there is salt and water. The ions of water interact with the salt present and form a small quantity of excess hydronium ions ($$\text{H}_{3}\text{O}^{+}$$) or hydroxide ions ($$\text{OH}^{-}$$). This leads to pH values that are not equal to $$\text{7}$$.

A simple rule for determining the likely pH of a solution is as follows:

#### Tip:

A neutralisation reaction does not imply that the pH is neutral ($$\text{7}$$).

• A $$\color{red}{\textbf{strong }\text{acid}} + \color{blue}{\textbf{strong }\text{base}}$$ form a $$\color{darkgreen}{\textbf{neutral}}$$ salt and water $$\color{darkgreen}{\textbf{solution}}$$:

$$\to$$ pH = $$\text{7}$$.

$$\color{red}{\text{H}_{2}{\text{SO}}_{4}{\text{(l)}}} + \color{blue}{\text{NaOH(s)}} \to \color{darkgreen}{\textbf{Na}_{2}{\textbf{SO}}_{4}{\textbf{(aq)}}} + \color{darkgreen}{\text{H}_{2}{\text{O(l)}}}$$

• A $$\color{red}{\textbf{weak }\text{acid}} + \color{blue}{\textbf{strong }\text{base}}$$ form a $$\color{blue}{\textbf{weak basic}}$$ salt and water $$\color{blue}{\textbf{solution}}$$:

$$\to$$ pH = approximately $$\text{9}$$.

$$\color{red}{\text{HF(l)}} + \color{blue}{\text{NaOH(s)}} \to \color{blue}{\textbf{NaF(aq)}} + \color{blue}{\text{H}_{2}{\text{O(l)}}}$$

• A $$\color{red}{\textbf{strong }\text{acid}} + \color{blue}{\textbf{weak }\text{base}}$$ form a $$\color{red}{\textbf{weak acidic}}$$ salt and water $$\color{red}{\textbf{solution}}$$:

$$\to$$ pH = approximately $$\text{5}$$.

$$\color{red}{\text{H}_{2}{\text{SO}}_{4}{\text{(l)}}} + \color{blue}{\text{NH}_{3}{\text{(l)}}} \to \color{red}{\textbf{(NH}_{4}{\textbf{)}}_{2}{\textbf{SO}}_{4}{\textbf{(aq)}}} + \color{red}{\text{H}_{2}{\text{O(l)}}}$$

 $$\color{red}{\textbf{acid}}$$ $$\color{blue}{\textbf{base}}$$ solution approximate pH strong strong $$\color{darkgreen}{\textbf{neutral}}$$ $$\text{7}$$ weak strong $$\color{blue}{\textbf{weak basic}}$$ $$\text{9}$$ strong weak $$\color{red}{\textbf{weak acidic}}$$ $$\text{5}$$

Table: The approximate pH of neutralisation reaction solutions based on the strength of the acid and base used.