BackAcid-Base Equilibria: Definitions, Strengths, and Reactions
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Chapter 14: Acid-Base Equilibria
Overview
This chapter introduces the fundamental concepts of acid-base equilibria, including classical and modern definitions, the behavior of acids and bases in water, and the distinction between strong and weak acids. It also explores the practical implications of acid strength through chemical reactions and provides visual examples.
Arrhenius and Brønsted-Lowry Acid-Base Definitions
Arrhenius Acid-Base Definition
The Arrhenius definition is the earliest classification of acids and bases, focusing on their behavior in aqueous solutions.
Arrhenius acid: A substance with hydrogen (H) in its formula that dissociates in water to yield hydronium ions (H3O+).
Arrhenius base: A substance with hydroxide (OH) in its formula that dissociates in water to yield hydroxide ions (OH-).
Example reaction:
Acids and Bases in Water
Strong and Weak Acids
Acids are classified by their extent of ionization in water:
Strong acid: Dissociates completely into ions in water.
Weak acid: Dissociates slightly to form ions in water.
General equations:
Strong acid:
Weak acid:
Equilibrium constant for weak acids:
(for weak acids, is very small)
Visualizing Dissociation
Strong acids: After dissociation, almost all HA is converted to ions; solution contains mostly H3O+ and A-.
Weak acids: Most HA remains undissociated; only a small fraction forms H3O+ and A-.
Weak Acids and Bases & Equilibria
Reaction of Zinc with Strong and Weak Acids
The reactivity of metals with acids depends on acid strength:
Strong acid (e.g., 1 M HCl(aq)): Zinc reacts rapidly due to high [H3O+].
Weak acid (e.g., 1 M CH3COOH(aq)): Zinc reacts slowly because [H3O+] is much lower.
Example: The difference in reaction rates is visually evident by the amount of gas produced when zinc is added to each solution.
Acid-Base Properties of Salt Solutions
Overview
Salt solutions can be acidic, basic, or neutral depending on the nature of the ions produced when the salt dissolves in water. The acid-base properties are determined by the parent acid and base from which the salt is derived.
Neutral salts: Derived from strong acids and strong bases (e.g., NaCl).
Acidic salts: Derived from strong acids and weak bases (e.g., NH4Cl).
Basic salts: Derived from weak acids and strong bases (e.g., CH3COONa).
Electron-Pair Donation and the Lewis Acid-Base Definition
Lewis Acid-Base Definition
The Lewis definition broadens the concept of acids and bases:
Lewis acid: Any species that accepts an electron pair to form a covalent bond.
Lewis base: Any species that donates an electron pair to form a covalent bond.
Example: Ammonia (NH3) donates a lone pair to boron trifluoride (BF3), forming a Lewis acid-base adduct.
Summary Table: Strong vs. Weak Acids
Type | Dissociation in Water | Example |
|---|---|---|
Strong Acid | Complete | HCl, HNO3 |
Weak Acid | Partial | CH3COOH, HF |
Key Equations
Arrhenius acid-base reaction:
Strong acid dissociation:
Weak acid dissociation:
Equilibrium constant:
Applications
Understanding acid strength is crucial for predicting reaction rates and equilibrium positions in aqueous solutions.
Acid-base definitions are foundational for further study in chemical equilibria, titrations, and buffer systems.
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