Hydrolysis Notes

Hydrolysis

Objectives

• Define the term “Hydrolysis.”
• Illustrate the “Nature” of salts.
• Determine the relationship between Hydrolysis Constant to Ka or Kb.

What is Hydrolysis?

• Hydrolysis is the reaction of an anion and/or cation of a salt with water.
• Salt hydrolysis usually affects the pH of a solution.

Nature of Salts

• When an acid and a base react, an ionic compound called a salt is formed.
• The strength of the acid/base affects the nature of the salt.
• Nature of Salts:
  • Neutral
  • Acidic
  • Basic

Neutral Salts

• Salts that have a pH of 7 and an equal concentration of H^+ and OH^- ions.
• These salts can be formed from:
  • Strong acids and strong bases.
  • E.g. NaCl

Neutral Salts - Ions that do not react with water

• The following ions do not react appreciably with water to produce either H_3O^+ or OH^- ions:
  • Cations from strong bases:
    • Alkali metal cations of group 1A (Li^+, Na^+, K^+)
    • Alkaline earth cations of group 2A (Mg^{2+}, Ca^{2+}, Sr^{2+}, Ba^{2+}) except for Be^{2+}
  • Anions from strong monoprotic acids:
    • Cl^-, Br^-, I^-, NO3^-, and ClO4^-
  • Salts that contain only these ions give neutral solutions in pure water (pH 7).

Basic Salts

• Basic salts are formed from reactions of a weak acid and a strong base.
  • E.g. CH_3COONa (sodium ethanoate)
  • CH3COOH + NaOH \rightleftharpoons CH3COONa + H_2O
• The reaction for the dissociation of this salt is:
  • CH3COONa(s) \rightarrow CH3COO^-(aq) + Na^+(aq)
• The conjugate base of the weak acid can then act as a base and react with water as follows:
  • CH3COO^- + H2O \rightleftharpoons CH_3COOH + OH^-

Basic Salts - Hydrolysis Constant

• CH3COO^- + H2O \rightleftharpoons CH_3COOH + OH^-
• From this reaction, we can write an expression for the “Hydrolysis Constant.”
  • Kh = \frac{[CH3COOH][OH^-]}{[CH_3COO^-]}
• This expression also represents the K_b expression.
  • Thus, Kb = Kh
• Now Kw = Ka \times K_b
  • So: Kb \equiv Kh = \frac{Kw}{Ka}

Acidic Salts

• Acidic salts are formed from a reaction of a weak base and a strong acid.
  • E.g. NH_4Cl
  • NH3(aq) + HCl(aq) \rightarrow NH4Cl(aq)
• The reaction for the dissociation of this salt is:
  • NH4Cl(aq) \rightarrow NH4^+(aq) + Cl^-(aq)
• The conjugate acid of the weak base can then act as an acid and react with water as follows:
  • NH4^+(aq) + H2O(l) \rightleftharpoons NH3(aq) + H3O^+(aq)

Acidic Salts - Hydrolysis Constant

• NH4^+(aq) + H2O(l) \rightleftharpoons NH3(aq) + H3O^+(aq)
• From this reaction, we can write an expression for the “Hydrolysis Constant.”
  • Kh = \frac{[NH3][H3O^+]}{[NH4^+]}
• This expression also represents the K_a expression.
  • Thus, Ka = Kh
• Now Kw = Ka \times K_b
  • So: Ka \equiv Kh = \frac{Kw}{Kb}

pH of Salt Solutions

• Calculate the pH of a 0.5M solution of NH4Cl. (Kb = 1.8 \times 10^{-5})
• The reaction for the dissociation of this salt is:
  • NH4Cl(aq) \rightarrow NH4^+(aq) + Cl^-(aq)
• The NH_4^+(aq) then reacts with water as follows:
  • NH4^+(aq) + H2O(l) \rightleftharpoons NH3(aq) + H3O^+(aq)
• Kh = Ka = \frac{[NH3][H3O^+]}{[NH_4^+]}
• Ka \equiv Kh = \frac{Kw}{Kb}
  • So K_a = \frac{1.0 \times 10^{-14}}{1.8 \times 10^{-5}} = 5.6 \times 10^{-10}

pH of Salt Solutions - Calculation

• Now K_a = \frac{(x)(x)}{(0.50-x)}
• We assume that x is so small, thus 0.50-x = 0.50
• 5.6 \times 10^{-10} = \frac{x^2}{(0.50)}
• x = \sqrt{(5.6 \times 10^{-10}) \times (0.50)} = 1.67 \times 10^{-5}M
• So pH = -log(1.67 \times 10^{-5}) = 4.77
• pH is below 7 which proves the acidic nature of the salt

Salts formed from weak acids and weak bases

• If the reaction has a “weak acid” and a “weak base,” it can form either acidic, basic, or neutral salt solution

  • E.g. NH4F, CH3COONH4, NH4CN

• When Kb > Ka: Solution is basic

• When Kb < Ka: Solution is acidic

• When Kb = Ka: Solution is neutral

Salts formed from weak acids and weak bases - Example

• To determine whether an (NH4)2CO3 solution is acidic, basic, or neutral, let’s work out the values of Ka for NH4^+ and Kb for CO_3^{2-}.
• Because Ka < Kb, the solution is basic (pH > 7 ).