ochem W.2 slide flashcard

Acid-Base Chemistry Overview

General Concepts:

  • Acids and Bases:

    • Acids donate protons (H+).

    • Bases accept protons.

  • Key Definitions:

    • Bronsted-Lowry Acids: Substances that donate protons.

    • Bronsted-Lowry Bases: Substances that accept protons.

    • Lewis Acids: Electron pair acceptors.

    • Lewis Bases: Electron pair donors.

Learning Objectives of Chapter 2:

  • Confident use of acid/base chemistry concepts.

  • Use electronegativity/atomic size for acidity predictions.

  • Understand resonance in predicting acidity.

  • Draw resonance structures.

  • Analyze acid/base reactions (identify acid, base, CA, CB).

  • Differentiate between Bronsted-Lowry and Lewis systems.

  • Comprehend arrow-pushing mechanisms in acid/base chemistry.

  • Understand and memorize pKa values for common acids/bases.

  • Predict the spontaneity of acid/base reactions using pKa values.

  • Assess acid/base strength based on structure, size, and electronegativity.

  • Understand solubility of acids vs. conjugate bases.

  • Recognize how solvents affect acid/base reactions (levelling effect).

Electronegativity and Bond Polarity:

  • Polar bonds occur when atoms with differing electronegativities are bonded together:

    • C-F, C-Cl, C-N, and C-O are polar bonds.

    • C-C and C-H are nonpolar bonds.

  • Dipole Moment: A separation of charges that occurs due to differences in electronegativity.

Molecular Geometry and Polarity:

  • Non-polar molecules result when bond dipoles cancel each other out due to symmetric geometry.

  • Examples:

    • $CO_2$ is non-polar due to linear geometry.

    • $CH2Cl2$ is polar.

Resonance:

  • Definition: Delocalization of electrons within molecules.

  • Key Features:

    • Lone pairs, double bonds, and triple bonds are capable of movement.

    • Example of resonance structure: acetate $CH_3COO^{-}$.

  • Stability Rules for Resonance Structures:

    1. More covalent bonds = more stable.

    2. Less charge separation = more stable.

    3. Complete valence shells = more stable.

  • Unrealistic Resonance Structures:

    • Structures with atoms having more than 4 bonds (Texas Carbocations).

    • Incorrect formal charges.

Acid-Base Reaction Dynamics:

  • Reactions can be reversible.

  • Example: Hydrofluoric acid in water:
    $HF + H2O \rightleftharpoons H3O^+ + F^{-}$

  • Acids and bases react favorably to form weaker acids and bases.

The Concept of pKa:

  • Defines the strength of an acid; lower pKa = stronger acid.

  • Calculation:
    pKa=−log⁡(Ka)pKa = -\log(Ka)

  • Strong acids have large $Ka$ and low pKa (e.g., $HCl$, $HI$).

  • Weak acids have small $Ka$ and high pKa.

Solubility and pH Effects:

  • Solute solubility can be impacted by pH.

  • Example: Phenylethylamine is more soluble at a lower pH due to protonation.

Predicting Acid-Base Strength:

  • Consider electronegativity and size of conjugate bases for acidity:

    • Inductive effect: electronegative atoms stabilize the negative charge.

    • Larger anions are generally more stable.

Acid-Base Equilibrium Constant (Keq):

  • Ratio of product concentrations to reactant concentrations at equilibrium.
    Keq=[products][reactants]K_{eq} = \frac{[products]}{[reactants]}

  • Pure solids and solvents are excluded from the expression because their concentrations do not change.

Application of Acid/Base Concepts:

  • Predicting strengthening acids and bases:

    • Example: Compare HA and HB based on pKa values.

  • Identify solvent effectiveness based on pKa.

  • Understanding and manipulating acid/base reactions in various environments.

Examples of Resonance Structures:

  • Resonance structures for $1,3$-butadiene and indanone involve moving electrons and pi bonds.

  • Curved Arrow Mechanism: Essential for demonstrating electron flow in acid/base reactions.

Inductive Effects and Stability:

  • Electron withdrawing groups stabilize anions (higher acidity).

  • Electron donating groups destabilize anions (lower acidity).

Summary of Key Relationships:

  • Strong acids yield weak conjugate bases and vice versa.

  • pKa values are essential for determining relative acidity and stability of species.