Chapter 16: Acid-Base Equilibria – Study Notes

Acid-Base Equilibria

Introduction to Acid-Base Equilibria

Acid-base equilibria are fundamental to understanding chemical reactions in aqueous solutions. This topic explores the definitions, properties, and behaviors of acids and bases, including their equilibrium reactions and the factors that affect their reactivity.

• Acid-base equilibrium reactions: These involve the transfer of protons (H+) between species in solution.

• Characteristics affecting reactivity: Electron lone pairs and molecular structure influence acid/base strength.

• Electron lone pairs: Bases often possess lone pairs that can accept protons or participate in bonding.

Definitions of Acids and Bases

Several models exist to define acids and bases, each with its own criteria and applications.

• Arrhenius Acid: A substance that increases the concentration of H+ ions in aqueous solution.

• Arrhenius Base: A substance that increases the concentration of OH- ions in aqueous solution.

• Brønsted-Lowry Acid: A proton (H+) donor.

• Brønsted-Lowry Base: A proton (H+) acceptor.

• Lewis Acid: An electron pair acceptor.

• Lewis Base: An electron pair donor.

Example: In water, hydrochloric acid (HCl) dissociates as follows:

• $\mathrm{HCl} (aq) \rightarrow \mathrm{H}^+ (aq) + \mathrm{Cl}^- (aq)$

• More accurately, $\mathrm{HCl} (aq) + \mathrm{H}_2\mathrm{O} (l) \rightarrow \mathrm{H}_3\mathrm{O}^+ (aq) + \mathrm{Cl}^- (aq)$

Neutralization Reactions

Acids and bases react to form water and a salt in a process called neutralization.

• $\mathrm{H}^+ (aq) + \mathrm{OH}^- (aq) \rightarrow \mathrm{H}_2\mathrm{O} (aq)$

Hydronium Ion and Proton Transfer

In aqueous solutions, free protons do not exist; instead, they are associated with water molecules as hydronium ions.

• Hydronium ion: $\mathrm{H}_3\mathrm{O}^+$ is formed when water accepts a proton.

• Example: $\mathrm{HCN} (aq) + \mathrm{H}_2\mathrm{O} (l) \rightleftharpoons \mathrm{H}_3\mathrm{O}^+ (aq) + \mathrm{CN}^- (aq)$

Strong and Weak Bases in Water

Bases can be classified as strong or weak depending on their ability to produce hydroxide ions in water.

• Strong base: Dissociates completely in water to produce OH- ions.

• Example: $\mathrm{NaOH} (s) \rightarrow \mathrm{Na}^+ (aq) + \mathrm{OH}^- (aq)$

• Weak base: Reacts with water to produce OH- ions, but does not fully dissociate.

• Example: $\mathrm{NH}_3 (aq) + \mathrm{H}_2\mathrm{O} (l) \rightleftharpoons \mathrm{NH}_4^+ (aq) + \mathrm{OH}^- (aq)$

In weak base reactions, the base accepts a proton from water, generating hydroxide ions.

Key Terms and Concepts

• Proton: Often used to refer to H+ in acid-base chemistry.

• Hydronium ion ($\mathrm{H}_3\mathrm{O}^+$): The actual form of the proton in aqueous solution.

• Conjugate acid-base pairs: When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.

Summary Table: Acid and Base Definitions

Definition	Acid	Base
Arrhenius	Increases [H+]	Increases [OH-]
Brønsted-Lowry	Proton donor	Proton acceptor
Lewis	Electron pair acceptor	Electron pair donor

Applications and Examples

• Neutralization: Used in titrations and to control pH in various chemical processes.

• Acid-base equilibria: Essential for understanding buffer solutions, biological systems, and industrial chemistry.

Additional info: The provided images and slides are introductory materials for a General Chemistry chapter on acid-base equilibria, covering foundational definitions and examples relevant for college-level study.