UNIT: 8.11 Connecting pH and Salt Solubility

pH and Solubility of Salts

Introduction

  • Subunit 8.11 links pH and salt solubility.
  • It's the last part of Unit 8 on acids and bases.
  • Recap: Solubility of slightly soluble salts (Unit 7), including equilibrium constant Ksp.
  • Topic: Factors affecting solubility, including pH.

Factors Affecting Solubility: pH

  • pH is a factor affecting the solubility of a salt.
  • Salts containing hydroxide are particularly affected.
High pH Environment (Basic)
  • High pH indicates a high concentration of hydroxide ions ($\text{OH}^-$).
  • Adding a salt to a basic environment increases the concentration of hydroxide ions, causing Q > K_{sp}.
  • When Q > K_{sp}, the reaction shifts to consume products (ions) and create more solid reactant, thus decreasing solubility.
  • Example: Iron(III) hydroxide (Fe(OH)3\text{Fe(OH)}_3) has decreased solubility in a high pH environment.
  • This is also known as the common ion effect – the presence of a common ion (hydroxide) decreases solubility.
Low pH Environment (Acidic)
  • Low pH indicates a high concentration of hydronium ions (\text{H}_3");
  • Hydronium ions can react with hydroxide ions in the equilibrium system.
  • Reaction: \text{H}3\text{O}^+ + \text{OH}^- \rightarrow 2\text{H}2\text{O}(Note:balancedequationrequires(Note: balanced equation requires2\text{H}_2\text{O}).
  • The decrease in hydroxide ion concentration causes Q < K_{sp}.
  • When Q < K_{sp}, the reaction shifts to produce more products (ions), increasing solubility.
  • Iron(III) hydroxide is more soluble in an acidic environment.

Other Salts Affected by pH

  • Example: Calcium fluoride (\text{CaF}_2).
  • Dissolving calcium fluoride in an acidic environment:
    • Reaction: \text{H}3\text{O}^+ + \text{F}^- \rightarrow \text{HF} + \text{H}2\text{O}
    • HF is a weak acid and will form, removing fluoride ions from the solution.
    • This causes Q < K_{sp}, shifting the equilibrium to the right and increasing the solubility of calcium fluoride.
  • Calcium fluoride is more soluble in acidic environments because fluoride ions react with hydronium ions.
  • In a basic environment, hydroxide ions would not react with calcium ions to form calcium hydroxide because calcium hydroxide is a strong base.

Salts Unaffected by pH

  • Salts with anions that are not effective bases are unaffected by pH.
  • Example: Lithium chloride ($\text{LiCl}$)
Acidic environment
  • $\text{LiCl}(s) \rightleftharpoons \text{Li}^+(aq) + \text{Cl}^-(aq)$
  • Reaction of chloride with hydronium: \text{H}3\text{O}^+ + \text{Cl}^- \rightarrow \text{HCl} + \text{H}2\text{O}
  • This reaction does not occur because HCl is a strong acid and will remain ionized.
Basic Environment
  • Hydroxide ions do not react with lithium ions because lithium hydroxide is a strong base.

Practice Problems

Question 1
  • Reaction: \text{Cd(OH)}_2(s) \rightleftharpoons \text{Cd}^{2+}(aq) + 2\text{OH}^-(aq)
  • Question: Will the molarity of cadmium ions (\text{Cd}^{2+}) increase or decrease with the addition of hydroxide ions?
  • Answer: The molarity of cadmium ions will decrease.
    • Additional hydroxide ions increase the molarity of products, causing Q > K_{sp}.
    • The reaction shifts in the reverse direction, consuming products to form more solid cadmium hydroxide.
Question 2
  • Comparing solubility of cadmium hydroxide in pure water (A) vs. acidic water (B).
  • Question: Which container will have the highest molarity of cadmium ions?
  • Answer: Container B (acidic water) will have a higher molarity of cadmium ions.
    • Hydroxide ions from cadmium hydroxide react with hydronium ions in container B.
    • This decreases the molarity of products, causing Q < K_{sp}$$.
    • The reaction shifts in the forward direction, increasing the solubility of the salt and the molarity of cadmium ions.