Chapter 17: Acids and Bases – General Chemistry Study Notes

Acids and Bases: Definitions and Concepts

Definitions of Acids and Bases

Acids and bases are fundamental chemical species with several definitions, each broadening the scope of what can be considered an acid or a base.

• Arrhenius Definition:

  • Acid: Increases the concentration of H+ ions in aqueous solution.

  • Base: Increases the concentration of OH- ions in aqueous solution.

• Brønsted-Lowry Definition:

  • Acid: Proton (H+) donor.

  • Base: Proton (H+) acceptor.

• Lewis Definition:

  • Acid: Electron pair acceptor.

  • Base: Electron pair donor.

Example: HCl in water acts as an Arrhenius acid, a Brønsted-Lowry acid, and a Lewis acid.

Brønsted-Lowry Acids and Bases

The Brønsted-Lowry model is widely used to describe acid-base reactions in aqueous solutions.

• General acid formula: HA

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

• HA (acid) donates H+ to water, forming H3O+.

• H2O (base) accepts H+ from HA.

• A- is the conjugate base of acid HA.

• H3O+ is the conjugate acid of base H2O.

Acid and Base Strength

Acid Dissociation Constant (Ka)

The strength of an acid is measured by its acid dissociation constant, Ka.

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

• Simplified: $\mathrm{HA \rightleftharpoons H^+ + A^-}$

• Equilibrium expression: $\mathrm{K_a = \frac{[H^+][A^-]}{[HA]}}$

Interpretation:

• Ka >> 1: Strong acid (almost complete dissociation)

• Ka << 1: Weak acid (partial dissociation)

Acid/Base Strength Table

The following table summarizes the relative strengths of common acids and their conjugate bases:

Acid	Formula	Conjugate Base	Ka
Hydrochloric	HCl	Cl-	Very large
Sulfuric	H2SO4	HSO4-	Very large
Hydronium Ion	H3O+	H2O	1.0
Hydrofluoric	HF	F-	7.2 × 10–4
Acetic	CH3COOH	CH3COO-	1.8 × 10–5
Carbonic (1)	H2CO3	HCO3-	4.3 × 10–7
Ammonium ion	NH4+	NH3	5.6 × 10–10
Carbonic (2)	HCO3-	CO32-	4.8 × 10–11
Water	H2O	OH-	1.0 × 10–14

Additional info: The smaller the Ka value, the weaker the acid and the stronger its conjugate base.

The pH Scale and Calculations

The pH Scale

The pH scale is a logarithmic scale used to express the concentration of hydrogen ions in solution.

• pH = –log[H+]

• pOH = –log[OH-]

• pKa = –log(Ka)

• At 25°C, pH + pOH = 14

• pH < 7: Acidic solution

• pH = 7: Neutral solution

• pH > 7: Basic solution

Calculating pH

• For strong acids (e.g., HCl): Dissociate completely, so [H+] = initial acid concentration.

• For weak acids (e.g., CH3COOH): Use an ICE table and Ka expression to solve for [H+].

Example:

• 0.10 M HCl (strong acid): [H+] = 0.10 M, pH = 1.0

• 0.10 M CH3COOH (weak acid): [H+] = 1.34 × 10–3 M, pH = 2.9

Percent Dissociation

Percent dissociation measures the fraction of acid molecules that ionize in solution.

• Formula: $\mathrm{\%\ dissociation = \frac{[H^+]}{[HA]_{initial}} \times 100}$

Base Strength and Calculations

Base Dissociation Constant (Kb)

The strength of a base is measured by its base dissociation constant, Kb.

• General reaction: $\mathrm{B + H_2O \rightleftharpoons BH^+ + OH^-}$

• Equilibrium expression: $\mathrm{K_b = \frac{[BH^+][OH^-]}{[B]}}$

Relationship Between Ka and Kb

For a conjugate acid-base pair in water:

• $\mathrm{K_a \times K_b = K_w = 1.0 \times 10^{-14}}$ at 25°C

• If you know Ka for an acid, you can calculate Kb for its conjugate base and vice versa.

Calculating pH of Base Solutions

• Strong bases (e.g., NaOH): [OH-] = initial base concentration, then calculate pOH and pH.

• Weak bases (e.g., NH3): Use ICE table and Kb to solve for [OH-], then find pOH and pH.

Example: 0.2 M NH3 (Kb = 1.8 × 10–5): [OH-] = 1.9 × 10–3 M, pH = 11.27

Polyprotic Acids

Definition and Properties

Polyprotic acids can donate more than one proton per molecule (e.g., H2SO4, H2CO3).

• Each dissociation step has its own Ka value (Ka1, Ka2, etc.).

• The first dissociation usually contributes most to the pH.

Example: H2CO3:

• First dissociation: $\mathrm{H_2CO_3 \rightleftharpoons H^+ + HCO_3^-}$, Ka1 = 4.3 × 10–7

• Second dissociation: $\mathrm{HCO_3^- \rightleftharpoons H^+ + CO_3^{2-}}$, Ka2 = 5.6 × 10–11

Auto-Ionization of Water and the pH Scale

Auto-Ionization of Water

Water can act as both an acid and a base, undergoing auto-ionization:

• $\mathrm{2H_2O \rightleftharpoons H_3O^+ + OH^-}$

• Equilibrium constant: $\mathrm{K_w = [H^+][OH^-] = 1.0 \times 10^{-14}}$ at 25°C

Acid/Base Properties of Salts

Salts in Solution

Salts are ionic compounds that can affect the pH of their aqueous solutions depending on the acid-base properties of their constituent ions.

• Salts from strong acids and strong bases (e.g., NaCl) yield neutral solutions.

• Salts from strong bases and weak acids (e.g., NaCH3COO) yield basic solutions.

• Salts from weak bases and strong acids (e.g., NH4Cl) yield acidic solutions.

Example: 0.1 M NaCl solution is neutral; 0.1 M NH4Cl solution is acidic.

Calculating Overall K for Acid/Base Reactions

Combining Equilibria

When combining acid-base reactions, the overall equilibrium constant is the ratio of the product acid's Ka to the reactant acid's Ka:

• $\mathrm{K_{overall} = \frac{K_{a,product}}{K_{a,reactant}}}$

Example: For the reaction CH3COOH + F- → HF + CH3COO-, use the Ka values of HF and CH3COOH to determine K.

Acids and Bases in Everyday Life

Common Examples

Acids and bases are found in many household products and foods.

• Acids: Lemon juice, vinegar, vitamin C tablets, baking powder.

• Bases: Ammonia solution, baking soda, oven cleaner, drain cleaner.

Environmental Context: Carbon Dioxide and Water

CO2 in Water

Carbon dioxide dissolves in water, forming carbonic acid and contributing to the acidity of natural waters.

• $\mathrm{CO_2 (aq) + H_2O (l) \rightleftharpoons H_2CO_3 (aq)}$

• $\mathrm{H_2CO_3 (aq) \rightleftharpoons H^+ (aq) + HCO_3^- (aq)}$

• Overall: $\mathrm{CO_2 (aq) + H_2O (l) \rightleftharpoons H^+ (aq) + HCO_3^- (aq)}$

This process is important in environmental chemistry, such as ocean acidification and the dissolution of calcium carbonate (CaCO3).

Summary Table: Key Concepts in Acid-Base Chemistry

Concept	Key Points
Acid/Base Definitions	Arrhenius, Brønsted-Lowry, Lewis
Acid Strength	Measured by Ka; strong acids dissociate completely
Base Strength	Measured by Kb; strong bases dissociate completely
pH Scale	pH = –log[H+]; pH + pOH = 14 at 25°C
Polyprotic Acids	More than one ionizable proton; each step has its own Ka
Salt Solutions	pH depends on the acid/base properties of the ions
CO2 in Water	Forms carbonic acid, affects environmental pH