Ch 16: Acid-Base Equilibria: Models, Strengths, and Electrolytes

Models of Acids and Bases

Arrhenius Concept

The Arrhenius model defines acids and bases based on their behavior in aqueous solution. This concept is foundational for understanding acid-base chemistry.

• Acids: Substances that produce H+ ions in solution, increasing the concentration of hydrogen ions.

• Bases: Substances that produce OH- ions in solution, increasing the concentration of hydroxide ions.

• Example: $\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$

Brønsted-Lowry Concept

The Brønsted-Lowry model expands the definition of acids and bases to include proton transfer reactions.

• Acids: Proton (H+) donors.

• Bases: Proton acceptors.

• Example: $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+$

Conjugate Acid/Base Pairs

Definition and Identification

Conjugate acid-base pairs are related by the gain or loss of a proton. Understanding these pairs is essential for predicting the direction of acid-base reactions.

• Conjugate base: The species remaining after an acid has donated a proton.

• Conjugate acid: The species formed when a base accepts a proton.

• General reaction: $\text{HA} (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{H}_3\text{O}^+ (aq) + \text{A}^- (aq)$

• Example: $\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-$

Strong and Weak Acids and Bases

Strong Acids and Bases

Strong acids and bases are completely ionized in aqueous solution, making them strong electrolytes.

• Strong acids: HCl, HBr, HI, HNO3, H2SO4, HClO3, HClO4

• Strong bases: Group 1A and heavy Group 2A hydroxides (e.g., NaOH, KOH, Ca(OH)2, Ba(OH)2)

• Reaction example: $\text{HCl} (aq) \rightarrow \text{H}^+ (aq) + \text{Cl}^- (aq)$

Weak Acids and Bases

Weak acids and bases are only partially ionized in solution, resulting in an equilibrium between the ionized and unionized forms.

• Weak acid example: $\text{HF} (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{H}_3\text{O}^+ (aq) + \text{F}^- (aq)$

• Weak base example: $\text{NH}_3 (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{NH}_4^+ (aq) + \text{OH}^- (aq)$

Relative Strengths of Conjugate Acid-Base Pairs

Strength Chart

The strength of an acid is inversely related to the strength of its conjugate base. Strong acids have weak conjugate bases, and vice versa.

Acid	Base
HCl, H2SO4, HNO3, H3O+	Cl-, HSO4-, NO3-, H2O
CH3COOH, H2CO3, H2PO4-	CH3COO-, HCO3-, HPO42-
H2O, NH4+	OH-, NH3

Additional info: The table above is a simplified version of the full chart shown in the slides, focusing on the main strong and weak acids and bases.

Acid Strength

Strong Acids

Strong acids are fully ionized in water, and their conjugate bases are very weak.

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

• Example reaction: $\text{HNO}_3 (aq) + \text{H}_2\text{O} (l) \rightarrow \text{NO}_3^- (aq) + \text{H}_3\text{O}^+ (aq)$

Weak Acids

Weak acids are not fully ionized, and their conjugate bases are relatively stronger than those of strong acids.

• Example: $\text{CH}_3\text{COOH}$ (acetic acid) is a weak acid; its conjugate base is $\text{CH}_3\text{COO}^-$.

Strong Bases

Definition and Examples

Strong bases are substances that are fully ionized in water, typically soluble hydroxides of Group 1A and heavy Group 2A metals.

• Examples: NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

• Other strong bases: O2-, H-, CH3-

Strong Bases	Group
NaOH, KOH, RbOH, CsOH	Group 1A
Ca(OH)2, Sr(OH)2, Ba(OH)2	Heavy Group 2A

Electrolytes and Acid-Base Neutralization

Strong and Weak Electrolytes

Electrolytes are substances that conduct electricity in solution due to the presence of ions. Their strength depends on the degree of ionization.

• Strong electrolytes: Completely ionized in solution. Includes soluble ionic compounds, strong acids, and strong bases.

• Weak electrolytes: Partially ionized; equilibrium lies toward the reactant (molecular) side.

Ionic Equations and Neutralization

In acid-base neutralization reactions, an acid reacts with a base to produce water and a salt. Spectator ions are not included in the net ionic equation.

• General reaction: Acid + Base → Water + Salt

• Example: $\text{HCl} (aq) + \text{NaOH} (aq) \rightarrow \text{NaCl} (aq) + \text{H}_2\text{O} (l)$

Summary Table: Acid and Base Strengths

Type	Examples	Ionization in Water
Strong Acid	HCl, HNO3, H2SO4	Complete
Weak Acid	CH3COOH, HF	Partial
Strong Base	NaOH, KOH, Ba(OH)2	Complete
Weak Base	NH3, CH3NH2	Partial

Additional info: These notes cover the foundational concepts of acid-base equilibria, including models, conjugate pairs, strength, and electrolyte behavior, suitable for General Chemistry students.