Overview of Acids and Bases

Definitions and Theories

Arrhenius Definition

• Acids: Produce H+ ions in solution.
  • Example: HCl(aq) → H+(aq) + Cl-(aq)
  • H+ ions form hydronium ion (H3O+) with water.
• Bases: Produce OH- ions in solution.
  • Example: NaOH(aq) → Na+(aq) + OH-(aq)
• Limitations: Does not account for substances that do not contain OH- but make basic solutions (e.g., NH₃).

Brønsted-Lowry Definition

• Acids: Proton (H+) donors.
• Bases: Proton (H+) acceptors.
• Example: HCl(aq) + NH₃(aq) → NH₄+(aq) + Cl-(aq)
• Amphoteric substances: Can act as both acid and base.
  • Example: HPO₄²⁻ can donate or accept a proton.

Lewis Theory

• Acids: Electron pair acceptors.
• Bases: Electron pair donors.
• Example: NH₃ donates electron pair to BF₃ forming H₃N-BF₃.
• Focuses on electron pairs rather than H+ ions.

Acid-Base Reactions

Neutralization

• Acids and bases neutralize to form water and salt.
  • Example: HCl(aq) + NaOH(aq) → H₂O(l) + NaCl(aq)

Titrations

• Used to determine the concentration of an acid or base.
• Equivalence point: Moles of acid = Moles of base.
  • Example: HCl titrated with NaOH.

Strength of Acids and Bases

Strong vs. Weak Acids

• Strong Acids: Ionize completely in solution.
  • Examples: HCl, HNO₃, H₂SO₄
• Weak Acids: Partially ionize in solution.
  • Example: CH₃COOH

Strong vs. Weak Bases

• Strong Bases: Ionize completely (e.g., NaOH).
• Weak Bases: Partially ionize (e.g., NH₃).

pH Scale

• pH indicates acidity or basicity based on H+ ion concentration.
• Formula: pH = -log[H+]
• Scale ranges typically from 0 (acidic) to 14 (basic), 7 being neutral.

Conjugate Acids and Bases

• Each acid has a conjugate base and vice-versa.
• Example: HCl → Cl- (conjugate base); NH₃ → NH₄+ (conjugate acid).

Limitations of Arrhenius Theory

• Cannot explain reactions without water (e.g., NH₃ + HCl in gas phase).
• Does not include non-ionic bases like NH₃ that accept protons.

Exercises

• Identification and categorization of acids and bases.
• Calculation problems related to titration and pH.

Example Questions

1. Identify a strong acid from given compounds.
   • Answer: HNO₃
2. Identify a Brønsted-Lowry base.
   • Answer: HPO₄²⁻
3. Calculate concentration of HCl after titration.
   • Example calculation provided with solution.

References

1. Brent, Lynnette. Acids and Bases.
2. Hulanicki, Adam. Reactions of Acids and Bases in Analytical Chemistry.
3. Oxlade, Chris. Acids & Bases.
4. Petrucci, Ralph H. General Chemistry: Principles and Modern Applications.
5. Vanderwerd, Calvin A. Acids, Bases, and the Chemistry of the Covalent Bond.

These notes cover the fundamental concepts of acids and bases, their various definitions, how they interact, and practical applications such as titration.