Acid-Base Properties of Salt Solutions
Introduction
- Focus on the acid-base properties of soluble ionic compounds (salt solutions).
- Understanding required: equilibria of weak acids/bases, relationship between Ka and Kb of conjugate pairs.
- Complex but covers many previous concepts from chapters 15 and 16.
Neutral, Acidic, or Basic Solutions
- Sodium Salts (e.g., NaCl, NaBr, NaF):
- Sodium ions act as spectators – no effect.
- Halide ions' behavior determines solution pH.
Fluoride Ion
- Conjugate base of hydrofluoric acid (weak acid), making solutions basic.
Chloride and Bromide Ions
- Conjugate bases of strong acids (HCl, HBr), resulting in neutral solutions.
Neutral Salt Solutions
- Anion as conjugate base of a strong acid (e.g., chloride, bromide, nitrate).
- Cations as conjugate acids of strong bases (e.g., NaOH, KOH, Sr(OH)₂).
Basic Salt Solutions: Sodium Fluoride Example
- Microscopic View: Fluoride ions react with water to form hydroxide ions, making the solution basic.
- Calculating pH:
- Use chemical equation: fluoride + water → HF + hydroxide.
- Set up an ICE table to find equilibrium concentrations.
- Calculate Kb using given Ka.
- Solve for hydroxide concentration and then pH.
Acidic Salt Solutions
- Ammonium Chloride Example:
- Ammonium ion reacts with water to produce hydronium, making the solution acidic.
- Calculating pH with ICE table and equilibrium concentrations.
Cations Affecting pH
- Ammonium and other cations that are conjugates of weak bases can make solutions acidic.
- Small, highly charged cations (e.g., Al³⁺, Fe³⁺) can also lower pH by forming strong ion-dipole interactions.
Predicting Acidic or Basic Solutions
- Use knowledge of cations and anions from salts to predict solution behavior.
- Practical observation with indicators to verify pH.
Salts with Both Cation and Anion Affecting pH
- Some salts have both anion and cation capable of altering pH.
- Example: Ammonium hypochlorite.
- Compare strengths (Ka vs. Kb) to predict resulting pH (basic if Kb > Ka).
This lecture emphasizes understanding the interactions and equilibrium of ions in solution to predict the acid-base nature of salt solutions and provides practical methods to calculate and verify pH values.