Acids and Bases Notes

Introduction to Acids and Bases

Brønsted-Lowry Definition

• Brønsted-Lowry Acid: A substance that donates a proton (H^+).
• Brønsted-Lowry Base: A substance that accepts a proton (H^+).

Examples of Acid-Base Reactions

• Acetic Acid + Water:
  • Acetic acid acts as the acid, donating a proton to water.
  • Water acts as the base, accepting a proton from acetic acid.
  • Acetate is the conjugate base (formed from the acid).
  • Hydronium ion (H_3O^+) is the conjugate acid (formed from the base).

CH3COOH + H2O \rightleftharpoons CH3COO^- + H3O^+

• Ethylamine + Water:
  • Ethylamine acts as the base, accepting a proton from water.
  • Water acts as the acid, donating a proton to ethylamine.
  • Ethylammonium is the conjugate acid (formed from the base).
  • Hydroxide ion (OH^−) is the conjugate base (formed from the acid).

CH3CH2NH2 + H2O \rightleftharpoons CH3CH2NH_3^+ + OH^-

Amphiprotic Substances

• Definition: A substance that can act as both an acid and a base. Water is a common example.

Quantifying Acid/Base Strength

• Acid Dissociation Constant (KA):

KA = \frac{[A^-]{eq}[H3O^+]{eq}}{[HA]_{eq}}

*   Where:
    *   [A^-]_{eq} is the equilibrium concentration of the conjugate base.
    *   [H_3O^+]_{eq} is the equilibrium concentration of the hydronium ion.
    *   [HA]_{eq} is the equilibrium concentration of the acid.

• Base Dissociation Constant (KB):

KB = \frac{[BH^+]{eq}[OH^-]{eq}}{[B]{eq}}

*   Where:
    *   [BH^+]_{eq} is the equilibrium concentration of the conjugate acid.
    *   [OH^-]_{eq} is the equilibrium concentration of the hydroxide ion.
    *   [B]_{eq} is the equilibrium concentration of the base.

• Strength Correlation:
  • Large K_A value = Strong acid.
  • Large K_B value = Strong base.
• Example Values (from the slides):
  • K_A = 1.8 \times 10^{-5}
  • K_B = 5.0 \times 10^{-4}

Identifying Acids, Bases, Conjugates, and Strength

General Instructions

• Use arrows to show bond breaking and formation during the reaction with water.
• Identify the acid, base, conjugate acid, and conjugate base in the reaction.
• Classify the acid or base as weak or strong.
• Write the corresponding KA or KB expression.

Examples (given with incomplete information, completed based on chemical knowledge)

• Hydrochloric Acid (HCl):

K_A = 1.3 \times 10^6

HCl + H2O \rightarrow H3O^+ + Cl^-

• Benzoate (C6H5COO^−):

K_B = 1.5 \times 10^{-10}

C6H5COO^- + H2O \rightleftharpoons C6H_5COOH + OH^-

• Ammonium (NH_4^+):

K_A = 5.6 \times 10^{-10}

NH4^+ + H2O \rightleftharpoons H3O^+ + NH3

Guidelines for Acid/Base Strength based on K values

• K_A > 1: Strong acid.
• K_B < 1: Weak base.
• K_A < 1: Weak acid.