Acids and Bases: Models, Strength, and Calculations

Three Models of Acids and Bases

Definitions and Comparisons

• Arrhenius Model: Defines acids as substances that produce H+ ions in water and bases as substances that produce OH- ions.

• Brønsted-Lowry Model: Defines acids as proton (H+) donors and bases as proton acceptors. This model is broader than Arrhenius, as it includes reactions not limited to aqueous solutions.

• Lewis Model: Defines acids as electron-pair acceptors and bases as electron-pair donors. This is the most general model, encompassing all Arrhenius and Brønsted-Lowry acids and bases.

Model	Definition of Acid	Definition of Base
Arrhenius	H+ producer	OH- producer
Brønsted-Lowry	H+ donor	H+ acceptor
Lewis	Electron-pair acceptor	Electron-pair donor

Example: In the reaction NH3 + H2O → NH4+ + OH-, NH3 is a Brønsted-Lowry base (accepts H+), and H2O is a Brønsted-Lowry acid (donates H+).

Acid-Base Reactions and Conjugate Pairs

Brønsted-Lowry Acids and Bases

• Any reaction that transfers a proton (H+) from one molecule to another is an acid-base reaction.

• Each acid has a conjugate base, and each base has a conjugate acid.

Example: HCO3- + H2O → H2CO3 + OH-

Acid Strength and Periodic Trends

Binary Acids

• Acid strength increases down a group and across a period (left to right) in the periodic table.

• For binary acids (HX), the more polar and weaker the H–X bond, the stronger the acid.

Order of acidity: H–F < H–Cl < H–Br < H–I (increasing acid strength)

Acid	Electronegativity of Y	Ka
H–O–Cl	3.0	2.9 × 10-8
H–O–Br	2.8	2.0 × 10-9
H–O–I	2.5	2.3 × 10-11

Factors Affecting Acid Strength:

• Bond strength (weaker H–X bond = stronger acid)

• Electronegativity (greater difference = stronger acid)

Strong and Weak Acids and Bases

Definitions

• Strong acid: Completely or nearly completely ionizes in water; strong electrolyte.

• Weak acid: Partially ionizes in water; weak electrolyte.

• Strong base: Completely dissociates in water (e.g., NaOH).

• Weak base: Partially ionizes in water (e.g., NH3).

Acid Ionization Constant (Ka) and Calculations

Equilibrium Expression

The acid ionization constant, Ka, measures the strength of an acid in solution:

$HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)$ $K_a = \frac{[H_3O^+][A^-]}{[HA]}$

The larger the Ka, the stronger the acid.

Ionic Product of Water (Kw)

$H_2O(l) + H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$ $K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$ at 25°C

In pure water, $[H_3O^+] = [OH^-] = 1.0 \times 10^{-7}$ M, so $pH = 7$.

pH and pOH Calculations

• $pH = -\log[H_3O^+]$

• $pOH = -\log[OH^-]$

• $pH + pOH = 14$ (at 25°C)

Example: If $[H_3O^+] = 1.0 \times 10^{-4}$ M, $pH = 4$.

Percent Ionization of Weak Acids

Percent ionization indicates the fraction of acid molecules that ionize in solution:

$\text{Percent Ionization} = \frac{[\text{Ionized Acid}]}{[\text{Initial Acid}]} \times 100\%$

Polyprotic Acids

Ionization in Steps

• Polyprotic acids (e.g., H2SO4, H2CO3) ionize in successive steps, each with its own Ka.

• The first ionization constant (Ka1) is always larger than the second (Ka2).

Example: For carbonic acid,

$H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$ $K_{a1} = 4.5 \times 10^{-7}$ $HCO_3^- \rightleftharpoons H^+ + CO_3^{2-}$ $K_{a2} = 4.7 \times 10^{-11}$

Lewis Acids and Bases

• Lewis acids accept electron pairs; Lewis bases donate electron pairs.

• All Brønsted-Lowry acids and bases are also Lewis acids and bases, but not all Lewis acids/bases are Brønsted-Lowry acids/bases.

Example: $BF_3$ (Lewis acid) + $F^-$ (Lewis base) → $[BF_4]^-$ (acid-base adduct)

Summary Table: Acid and Base Strengths

Acid	Structure	Acid Strength
HClO4	O=Cl(OH)3	Strong
HClO2	O=Cl(OH)O	Weak
HClO	O=Cl(OH)	Very Weak

Practice Problems and Applications

• Identify acids, bases, conjugate pairs in reactions.

• Calculate pH, pOH, [H3O+], [OH-] for strong and weak acids/bases.

• Arrange acids/bases by strength using periodic trends and molecular structure.

• Determine the acidity/basicity of salt solutions based on their constituent ions.

Additional info: These notes include both conceptual explanations and worked examples, as well as summary tables for acid/base strength and periodic trends. The content is suitable for General Chemistry students preparing for exams on acids and bases.