Unit 8 Acids and Bases

8.1: Introduction to Acids and Bases

Defining Acids and Bases

Arrhenius Definition

• Acids: Substances that produce $ H^+ $ ions in aqueous solution.
• Bases: Substances that produce $ OH^- $ ions in aqueous solution.
  • $$ HCl(aq) \rightarrow H^+(aq) + Cl^-(aq) $$ * $$ NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq) $$ *Limitation: only valid in aqueous solutions, and doesn’t account for bases that don’t directly contain $ OH^- $

Brønsted-Lowry Theory

• Acids: Proton ( $ H^+ $ ) donors.
• Bases: Proton ( $ H^+ $ ) acceptors.
  • $$ Acid \rightleftharpoons Conjugate,Base + H^+ $$ * $$ Base + H^+ \rightleftharpoons Conjugate,Acid $$

Conjugate Acid-Base Pairs

• A conjugate acid-base pair consists of two species that differ by the presence or absence of a proton ( $ H^+ $ ).

Lewis Definition

• Acids: Electron-pair acceptors.
• Bases: Electron-pair donors.
  • $ BF_3 $ (Lewis acid) accepts an electron pair from $ NH_3 $ (Lewis base).
  • $$ BF_3 + NH_3 \rightarrow F_3B-NH_3 $$

Strengths of Acids and Bases

Strong Acids

• Strong acids completely dissociate into ions in solution.
• Examples: $ HCl $ , $ HBr $ , $ HI $ , $ H_2SO_4 $ , $ HNO_3 $ , $ HClO_4 $ .
• $$ HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq) $$

Weak Acids

• Weak acids only partially dissociate into ions in solution.
• Establish an Equilibrium between the acid and its conjugate base.
• Examples: $ HF $ , $ CH_3COOH $ (acetic acid).
• $$ HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq) $$

Strong Bases

• Strong bases completely dissociate into ions in solution.
• Examples: Group 1 hydroxides ( $ LiOH $ , $ NaOH $ , $ KOH $ , etc.), Group 2 hydroxides ( $ Ca(OH)_2 $ , $ Sr(OH)_2 $ , $ Ba(OH)_2 $ , etc.).
• $$ NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq) $$

Weak Bases

• Weak bases only partially react with water to produce hydroxide ions.
• Examples: $ NH_3 $ (ammonia), amines ( $ CH_3NH_2 $ ).
• $$ NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq) $$

Acid and Base Strength and Water

• Water as an Acid and Base
• Water Autoionization

Acid Dissociation Constant ( $ K_a $ )

• Represents the equilibrium constant for the dissociation of a weak acid in water.
• $$ HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq) $$ * $$ K_a = \frac{[H_3O^+][A^-]}{[HA]} $$ * Larger $ K_a $ values indicate stronger acids (more dissociation).

Acid Strength and Conjugate Base Strength

Base Dissociation Constant ( $ K_b $ )

• Represents the equilibrium constant for the reaction of a weak base with water.
• $$ B(aq) + H_2O(l) \rightleftharpoons HB^+(aq) + OH^-(aq) $$ * $$ K_b = \frac{[HB^+][OH^-]}{[B]} $$ * Larger $ K_b $ values indicate stronger bases (more $ OH^- $ production).

Base Strength and Conjugate Acid Strength

$ K_a $ and $ K_b $ Relationship

• For a conjugate acid-base pair in water:
• $$ K_a \times K_b = K_w $$ * Where $ K_w $ is the Ionic product of water Kw. At 25°C, $ K_w = 1.0 \times 10^{-14} $ .

pH Scale

• A measure of the acidity or basicity of a solution.
• Defined as: $$ pH = -log[H_3O^+] $$ * pH and pOH:
  • $ pH < 7 $ : Acidic
  • $ pH = 7 $ : Neutral (at 25°C)
  • $ pH > 7 $ : Basic