## 9.9 Salt Hydrolysis ### Introduction to Salt Hydrolysis When a salt is dissolved in water, it dissociates into its constituent cations and anions. These ions can react with water in a process called **hydrolysis**. This interaction can lead to the formation of $H^+$ or $OH^-$ ions, causing the resulting solution to be acidic, basic, or neutral. Consider the following observations: 1. An aqueous solution of ammonium chloride ($\mathrm{NH_4Cl}$) turns blue litmus paper red, indicating an **acidic** solution. 2. An aqueous solution of potassium carbonate ($\mathrm{K_2CO_3}$) turns red litmus paper blue, indicating a **basic** solution. 3. An aqueous solution of sodium chloride ($\mathrm{NaCl}$) has no effect on litmus paper, indicating a **neutral** solution. The general reactions of a salt's ions ($M^+$ and $X^-$) with water are: $$M^+_{(aq)} + H_2O_{(l)} \rightleftharpoons MOH_{(aq)} + H^+_{(aq)}$$ $$X^-_{(aq)} + H_2O_{(l)} \rightleftharpoons HX_{(aq)} + OH^-_{(aq)}$$ **Hydrolysis** is defined as the chemical reaction of cations and anions of a salt with water, which can alter the pH of the solution. **Note:** *Hydrolysis is different from hydration. In hydrolysis, the H-OH bond in the water molecule is broken. In hydration, water molecules surround the ions without any bonds being broken.* ### Hydrolysis of Anions (from Weak Acids) Anions derived from *weak acids* are strong conjugate bases. They react with water to produce hydroxide ions ($OH^-$), making the solution basic. **Examples:** - Acetate ion ($\mathrm{CH_3COO^-}$): $$\mathrm{CH_3COO^-_{(aq)} + H_2O_{(l)} \rightleftharpoons CH_3COOH_{(aq)} + OH^-_{(aq)}}$$ - Cyanide ion ($\mathrm{CN^-}$): $$\mathrm{CN^-_{(aq)} + H_2O_{(l)} \rightleftharpoons HCN_{(aq)} + OH^-_{(aq)}}$$ - Carbonate ion ($\mathrm{CO_3^{2-}}$): $$\mathrm{CO_3^{2-}_{(aq)} + 2H_2O_{(l)} \rightleftharpoons H_2CO_3_{(aq)} + 2OH^-_{(aq)}}$$ Anions from *strong acids* (e.g., $\mathrm{Cl^-}$, $\mathrm{NO_3^-}$, $\mathrm{SO_4^{2-}}$) are very weak conjugate bases and **do not** react with water. ### Hydrolysis of Cations (from Weak Bases) Cations derived from *weak bases* are strong conjugate acids. They react with water to produce hydronium ions ($H^+$ or $\mathrm{H_3O^+}$), making the solution acidic. **Examples:** - Copper(II) ion ($\mathrm{Cu^{2+}}$): $$\mathrm{Cu^{2+}_{(aq)} + 2H_2O_{(l)} \rightleftharpoons Cu(OH)_{2(aq)} + 2H^+_{(aq)}}$$ - Ammonium ion ($\mathrm{NH_4^+}$): $$\mathrm{NH_4^+_{(aq)} + H_2O_{(l)} \rightleftharpoons NH_4OH_{(aq)} + H^+_{(aq)}}$$ Cations from *strong bases* (e.g., $\mathrm{Na^+}$, $\mathrm{K^+}$, $\mathrm{Ca^{2+}}$) are very weak conjugate acids and **do not** react with water. ### Types of Salts and Their Hydrolysis Salts can be categorized into four types based on the strength of their parent acid and base. 1. **Salts of Strong Acids and Strong Bases** - *Examples:* $\mathrm{NaCl}$, $\mathrm{Na_2SO_4}$, $\mathrm{KNO_3}$ - Neither the cation nor the anion hydrolyzes. - The solution remains **neutral** ($pH = 7$). 2. **Salts of Weak Acids and Strong Bases** - *Examples:* $\mathrm{CH_3COONa}$, $\mathrm{NaCN}$, $\mathrm{Na_2S}$ - The anion hydrolyzes to produce $OH^-$ ions. - The solution is **basic** ($pH > 7$). 3. **Salts of Strong Acids and Weak Bases** - *Examples:* $\mathrm{CuSO_4}$, $\mathrm{NH_4Cl}$, $\mathrm{NH_4NO_3}$ - The cation hydrolyzes to produce $H^+$ ions. - The solution is **acidic** ($pH < 7$). 4. **Salts of Weak Acids and Weak Bases** - *Examples:* $\mathrm{NH_4CN}$, $\mathrm{NH_4NO_2}$ - Both the cation and anion hydrolyze. - The nature of the solution (acidic, basic, or neutral) depends on the relative dissociation constants ($K_a$ and $K_b$) of the parent acid and base. - If $K_a > K_b$, the solution is acidic. - If $K_b > K_a$, the solution is basic. - If $K_a \approx K_b$, the solution is nearly neutral. ### Summary of Salt Hydrolysis The table below summarizes the behavior of different types of salts in aqueous solutions. | Salt Type (Acid-Base) | Common Example | Ions which Hydrolyze | Solution pH (Nature) | | :------------------------ | :----------------- | :------------------------- | :---------------------------------------- | | Strong Acid - Strong Base | $\mathrm{NaCl}$, $\mathrm{KBr}$ | None | $= 7.0$ (Neutral) | | Strong Acid - Weak Base | $\mathrm{NH_4NO_3}$, $\mathrm{NH_4Cl}$ | Cations ($\mathrm{NH_4^+}$) | $< 7.0$ (Acidic) | | Weak Acid - Strong Base | $\mathrm{NaCN}$, $\mathrm{K_2CO_3}$ | Anions ($\mathrm{CN^-}$, $\mathrm{CO_3^{2-}}$) | $> 7.0$ (Basic) | | Weak Acid - Weak Base | $\mathrm{NH_4CN}$, $\mathrm{NH_4NO_2}$ | Both Cations & Anions | Depends on relative $K_a$ and $K_b$ values | ### Significance of Hydrolysis Hydrolysis is a crucial process in both plants and animals. - In living systems, many biological reactions are hydrolysis reactions, including the hydrolysis of ATP (adenosine triphosphate) to release energy. - Enzymes act as catalysts to facilitate the hydrolysis of complex molecules like fats, proteins, and carbohydrates into smaller units that can be used by the body. ### Summary - **Salt Hydrolysis** is the reaction of the ions of a salt with water, which can result in an acidic, basic, or neutral solution. - Ions from **strong** acids or bases are weak conjugate partners and **do not** hydrolyze. - Ions from **weak** acids or bases are strong conjugate partners and **do** hydrolyze. - **Anions** from weak acids react with water to form $OH^-$, leading to a **basic** solution. $$X^- + H_2O \rightleftharpoons HX + OH^-$$ - **Cations** from weak bases react with water to form $H^+$, leading to an **acidic** solution. $$M^+ + H_2O \rightleftharpoons MOH + H^+$$ - The pH of a salt solution is determined by the nature of its parent acid and base.