Does the pH increase, decrease, or remain the same when the substances are added to the solutions? (a) LiF to an HF solution (b) KI to an HI solution (c) NH4Cl to an NH3 solution

Verified step by step guidance

1. Identify the nature of each substance added and the solution it is being added to. Determine if the added substance is an acid, base, or a salt and what it dissociates into in solution.
2. Analyze the common ion effect for each scenario. This effect occurs when adding a salt that shares a common ion with a dissolved substance, potentially shifting the equilibrium of the reaction.
3. (a) For LiF added to an HF solution, recognize that LiF is a salt that dissociates into Li+ and F-. HF is a weak acid that partially dissociates into H+ and F-. Adding LiF increases the concentration of F- ions, shifting the equilibrium of the HF dissociation reaction to the left, thereby decreasing the concentration of H+ ions and increasing the pH.
4. (b) For KI added to an HI solution, note that KI is a salt that dissociates into K+ and I-. HI is a strong acid that completely dissociates into H+ and I-. Adding KI increases the concentration of I- ions but since HI is a strong acid and fully dissociates, the pH remains largely unaffected.
5. (c) For NH4Cl added to an NH3 solution, understand that NH4Cl is a salt that dissociates into NH4+ and Cl-. NH3 is a weak base that partially reacts with water to form NH4+ and OH-. Adding NH4Cl increases the concentration of NH4+ ions, shifting the equilibrium to the left and decreasing the concentration of OH- ions, thus decreasing the pH.

Key Concepts

Acid-Base Theory

Acid-base theory explains the behavior of acids and bases in solution. According to the Brønsted-Lowry theory, acids donate protons (H+) while bases accept them. The pH of a solution is a measure of its hydrogen ion concentration, with lower pH values indicating more acidic solutions and higher values indicating more basic solutions. Understanding how different substances interact with acids and bases is crucial for predicting pH changes.

Salt Hydrolysis

Salt hydrolysis occurs when an ionic compound (salt) reacts with water to produce an acidic or basic solution. For example, when LiF is added to HF, the fluoride ion can interact with water, affecting the pH. Similarly, NH4Cl can release H+ ions when dissolved, influencing the acidity of the solution. Recognizing how salts derived from weak acids or bases behave in water is essential for predicting pH changes.

Buffer Solutions

Buffer solutions are mixtures that resist changes in pH when small amounts of acid or base are added. They typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid. In the context of the question, the presence of weak acids like HF and NH3 in their respective solutions can create buffer systems that stabilize pH, making it important to analyze how added substances affect these equilibria.