Chapter 16 Study Guide

Acids and Bases

Definitions and Properties

Acids and bases are fundamental chemical species with distinct properties and behaviors in aqueous solutions. Acids typically taste sour, turn blue litmus paper red, and increase the concentration of hydrogen ions ([H+]) in solution. Bases taste bitter, feel slippery, turn red litmus paper blue, and increase the concentration of hydroxide ions ([OH-]).

• Arrhenius Definition: Acids produce H+ in water; bases produce OH- in water.

• Neutralization Reaction: Acid + Base → Salt + Water (e.g., HCl + NaOH → NaCl + H2O).

Brønsted-Lowry Acids and Bases

Proton Transfer Concept

The Brønsted-Lowry theory broadens the definition of acids and bases:

• Acid: Proton (H+) donor.

• Base: Proton (H+) acceptor.

• Amphoteric: A substance that can act as either an acid or a base (e.g., water).

In aqueous solution, the H+ ion is often associated with water molecules, forming hydronium ions (H3O+), and even larger clusters such as H5O2+ and H9O4+.

Conjugate Acid-Base Pairs

Definition and Examples

When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid. Conjugate acid-base pairs differ by one proton.

• Example: HA + H2O ⇌ A- + H3O+

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

• H2O and H3O+ are a conjugate acid-base pair.

Relative Strengths of Acids and Bases

Strength and Conjugate Relationships

The strength of an acid or base is inversely related to the strength of its conjugate. The stronger the acid, the weaker its conjugate base, and vice versa. In aqueous solution, H3O+ is the strongest acid and OH- is the strongest base that can exist in equilibrium.

Review of Strong Electrolytes

Strong vs. Weak Acids and Bases

Strong electrolytes (such as strong acids, strong bases, and most salts) dissociate completely in water. Weak acids and bases only partially ionize, often less than 10%.

• Strong acids: HCl, HNO3, H2SO4

• Strong bases: NaOH, KOH

• Weak acids: CH3COOH, HCN

• Weak bases: NH3

The Autoionization of Water

Equilibrium and the Ion Product

Water undergoes slight autoionization:

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

• At 25°C, $[\mathrm{H_3O^+}] = [\mathrm{OH^-}] = 1.0 \times 10^{-7}\ \mathrm{M}$

• The equilibrium constant is $K_w = [\mathrm{H_3O^+}][\mathrm{OH^-}] = 1.0 \times 10^{-14}$ at 25°C.

The pH and pOH Scales

Definitions and Calculations

The pH and pOH scales provide a convenient way to express acidity and basicity:

• $\mathrm{pH} = -\log[\mathrm{H_3O^+}]$

• $\mathrm{pOH} = -\log[\mathrm{OH^-}]$

• At 25°C, $\mathrm{pH} + \mathrm{pOH} = 14$

In neutral water at 25°C, pH = 7.00. Acidic solutions have pH < 7, and basic solutions have pH > 7.

Ionization Constants for Weak Monoprotic Acids and Bases

Equilibrium Expressions

Weak acids and bases establish equilibrium in water. The acid ionization constant ($K_a$) and base ionization constant ($K_b$) quantify their strengths:

• For a weak acid, HA: $\mathrm{HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)}$

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

• For a weak base, B: $\mathrm{B(aq) + H_2O(l) \rightleftharpoons BH^+(aq) + OH^-(aq)}$

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

Polyprotic Acids

Stepwise Ionization

Polyprotic acids have more than one ionizable hydrogen. Each ionization step has its own $K_a$, and $K_{a1} > K_{a2} > K_{a3}$, etc. For example, phosphoric acid (H3PO4):

• $K_{a1} = 7.5 \times 10^{-3}$

• $K_{a2} = 6.2 \times 10^{-8}$

• $K_{a3} = 3.6 \times 10^{-13}$

Relationship Between Ka and Kb

Conjugate Acid-Base Pairs

For a conjugate acid-base pair:

• $K_a \times K_b = K_w$

• $pK_a + pK_b = pK_w = 14$ at 25°C

The stronger the acid (larger $K_a$), the weaker its conjugate base (smaller $K_b$).

pH of Salt Solutions

Acidic, Basic, and Neutral Salts

The pH of a salt solution depends on the strengths of the acid and base from which the salt is derived:

• Strong acid + strong base: Neutral solution (e.g., NaCl).

• Strong base + weak acid: Basic solution (e.g., NaC2H3O2).

• Weak base + strong acid: Acidic solution (e.g., NH4Cl).

• Weak base + weak acid: pH depends on relative $K_a$ and $K_b$ values.

Carboxylic Acids

Structure and Properties

Carboxylic acids are organic acids containing the carboxyl group (COOH). They are generally weak acids. Acetic acid (CH3COOH) is a common example.

Lewis Acids and Bases

Electron Pair Concept

The Lewis definition is the most general:

• Lewis acid: Electron pair acceptor (e.g., BF3, metal ions).

• Lewis base: Electron pair donor (e.g., NH3, OH-).

Lewis acids often have incomplete octets or vacant orbitals, while Lewis bases have lone pairs available for bonding.

Summary Table: Relative Strengths of Acids and Bases

The following table summarizes the relative strengths of selected acids and bases, as well as their conjugate pairs:

Acid	Ka	Base	Kb
HNO3	Strong acid	NO3-	Negligible basicity
HF	6.8 × 10-4	F-	1.5 × 10-11
HC2H3O2	1.8 × 10-5	C2H3O2-	5.6 × 10-10
HCO3-	4.3 × 10-7	CO32-	1.8 × 10-4
NH4+	5.6 × 10-10	NH3	1.8 × 10-5
HCO3-	4.3 × 10-7	CO32-	1.8 × 10-4
OH-	Negligible acidity		Strong base

Summary Table: Strengths of Acids and Bases

Acid	Base
HCl, HNO3, H2SO4	Cl-, NO3-, HSO4-
H3PO4, HF, CH3COOH	H2PO4-, F-, CH3COO-
HCO3-, H2O	CO32-, OH-
H2	H-

Additional info: This guide covers all major aspects of acid-base equilibria, including definitions, calculations, and conceptual relationships, suitable for college-level general chemistry.