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.