Acid-Base Equilibria: General Chemistry Study Notes

Acid-Base Equilibria

Introduction

This chapter explores the fundamental concepts of acid-base equilibria, including definitions, theories, calculations, and the behavior of acids, bases, and their salts in aqueous solutions. Mastery of these topics is essential for understanding chemical reactivity and solution chemistry in general chemistry.

Acids and Bases: A Brief Review

Definitions and Properties

• Acids: Substances that can donate a proton (H+) to another substance.

• Bases: Substances that can accept a proton (H+) from another substance.

• Acids and bases change the color of indicators, neutralize each other, and react to form water and salts.

• Common examples: Acids (citric acid in lemons, ascorbic acid in vitamin C); Bases (NaOH in caustic soda, CaCO3 in antacids).

Acid-Base Theories

Overview of Theories

• Arrhenius Theory: Acids produce H+ ions in water; bases produce OH- ions. Limited to aqueous solutions.

• Brønsted-Lowry Theory: Acids are proton donors; bases are proton acceptors. More general than Arrhenius.

• Lewis Theory: Acids are electron pair acceptors; bases are electron pair donors. Most general definition.

Brønsted-Lowry Acids and Bases

Proton Transfer Reactions

• Acid-base reactions involve the transfer of a proton (H+) from the acid to the base.

• The hydrogen ion (H+) is a bare proton, but in aqueous solution, it exists as the hydronium ion (H3O+).

• Example: $\mathrm{NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$

Conjugate Acid-Base Pairs

• Every acid-base reaction yields a conjugate acid-base pair: two species that differ by one proton.

• Example: $\mathrm{HA + H_2O \rightleftharpoons H_3O^+ + A^-}$

• HA and A- are a conjugate acid-base pair.

Amphiprotic Species

• An amphiprotic (or amphoteric) species can act as either an acid or a base, depending on the reaction partner.

• Example: Water (H2O) can donate or accept a proton.

Relative Strengths of Acids and Bases

Strength and Conjugate Relationships

• Strong acids ionize completely in water; their conjugate bases are very weak.

• Weak acids only partially ionize; their conjugate bases are relatively stronger.

• Equilibrium favors the formation of the weaker acid and base.

The Autoionisation of Water

Self-Ionization and Equilibrium

• Water can act as both an acid and a base, leading to self-ionization:

$\mathrm{2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)}$

• The equilibrium constant for this process is the ion-product constant for water ($K_w$):

$K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$ (at 25°C)

The pH Scale

Definition and Calculations

• pH is a measure of the acidity of a solution: $\mathrm{pH = -\log[H_3O^+]}$

• pOH is defined similarly: $\mathrm{pOH = -\log[OH^-]}$

• Relationship: $\mathrm{pH + pOH = 14}$ (at 25°C)

• Significant figures: The number of decimal places in pH equals the number of significant figures in [H+].

Strong Acids and Bases

Properties and Examples

• Strong acids and strong bases dissociate completely in aqueous solution.

• For strong acids: $[H_3O^+] = [\text{acid}]$

• Common strong acids: HCl, HBr, HI, HNO3, HClO3, HClO4, H2SO4

• Common strong bases: Group 1 and 2 metal hydroxides (e.g., NaOH, KOH, Ca(OH)2)

Weak Acids

Equilibrium and Calculations

• Weak acids only partially ionize in water, establishing an equilibrium.

• General equation: $\mathrm{HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)}$

• The acid dissociation constant ($K_a$):

$K_a = \frac{[H_3O^+][A^-]}{[HA]}$

• The extent of ionization depends on $K_a$ and the initial concentration of the acid.

Percentage Ionization

• Degree of ionization: Fraction of acid molecules that ionize.

• Percentage ionization: $\frac{[\text{ionized acid}]}{[\text{initial acid}]} \times 100\%$

Polyprotic Acids

• Polyprotic acids can donate more than one proton (e.g., H2SO4, H3PO4).

• Each ionization step has its own $K_a$ value, with $K_{a1} > K_{a2} > K_{a3}$.

• Example for carbonic acid (H2CO3):

First ionization: $\mathrm{H_2CO_3 + H_2O \rightleftharpoons HCO_3^- + H_3O^+}$

Second ionization: $\mathrm{HCO_3^- + H_2O \rightleftharpoons CO_3^{2-} + H_3O^+}$

Weak Bases

Equilibrium and Calculations

• Weak bases are poor proton acceptors and only partially react with water.

• General equation: $\mathrm{B(aq) + H_2O(l) \rightleftharpoons HB^+(aq) + OH^-(aq)}$

• The base dissociation constant ($K_b$):

$K_b = \frac{[HB^+][OH^-]}{[B]}$

• Types of weak bases include neutral molecules with lone pairs (e.g., ammonia) and anions of weak acids (e.g., acetate).

Relationship between $K_a$ and $K_b$

Conjugate Acid-Base Pairs

• For a conjugate acid-base pair:

$K_a \times K_b = K_w$

• This relationship allows calculation of $K_b$ from $K_a$ and vice versa.

Acid-Base Properties of Salt Solutions

Hydrolysis and pH of Salts

• A salt is an ionic compound formed from the neutralization of an acid by a base.

• Salts can produce acidic, basic, or neutral solutions depending on the strengths of the parent acid and base.

Type of Salt	Parent Acid	Parent Base	Solution pH
NaCl	Strong	Strong	Neutral
CH3COONa	Weak	Strong	Basic
NH4Cl	Strong	Weak	Acidic
AlCl3	Strong	Weak	Acidic

• Hydrolysis reactions of the ions determine the final pH of the solution.

Summary Table: Key Acid-Base Constants and Relationships

Constant	Expression	Description
$K_a$	$\frac{[H_3O^+][A^-]}{[HA]}$	Acid dissociation constant
$K_b$	$\frac{[HB^+][OH^-]}{[B]}$	Base dissociation constant
$K_w$	$[H_3O^+][OH^-]$	Ion-product constant for water
pH	$-\log[H_3O^+]$	Acidity scale
pOH	$-\log[OH^-]$	Basicity scale
Relationship	$K_a \times K_b = K_w$	Conjugate acid-base pairs

Key Points for Exam Preparation

• Be able to define and identify acids, bases, conjugate pairs, and amphiprotic species.

• Write and use equilibrium constant expressions for acid and base dissociation.

• Calculate pH, pOH, and concentrations of H+ and OH- in solution.

• Distinguish between strong and weak acids/bases and predict the pH of salt solutions.

• Apply significant figure rules in logarithmic calculations.

Additional info: Some examples and tables were inferred and expanded for clarity and completeness.