Chemistry 123 Midterm 2 Review

This cheat sheet summarizes key concepts from a Chemistry 123 midterm review session, covering thermodynamics, chemical equilibrium, acid-base chemistry, and amino acid properties. It focuses on understanding principles rather than memorization.

Core Principles

• Spontaneous processes occur without continuous energy input from surroundings.
• Spontaneity depends on enthalpy (ΔH) and entropy (ΔS).
• Gibbs free energy (ΔG) determines spontaneity; ΔG < 0 for spontaneous processes.
• Equilibrium constant (K) relates to Gibbs free energy (ΔG = -RT ln K).
• Acids donate protons; bases accept protons (Brønsted-Lowry definition).
• Acid/base strength relates to Ka/Kb values; strong acids/bases have Ka/Kb > 1.
• Buffer systems resist pH changes by neutralizing added acids or bases.
• Amino acid properties (acidity/basicity) are influenced by side chain structure and resonance stabilization.

Action Steps

• Identify spontaneous vs. non-spontaneous processes based on ΔG.
• Calculate ΔG using ΔH, ΔS, and temperature.
• Determine spontaneity at different temperatures based on signs of ΔH and ΔS.
• Calculate equilibrium constant (K) from ΔG°.
• Apply Henderson-Hasselbalch equation for buffer solutions.
• Identify acids, bases, conjugate acids, and conjugate bases in reactions.
• Relate pKa/pKb values to acid/base strength.
• Analyze how structural features (e.g., resonance, atom size) affect acid/base strength.

Formulas

• $ \Delta G = \Delta H - T \Delta S $
• $ \Delta G = -RT \ln K $
• $ K = e^{-\Delta G^{\circ}/RT} $
• $ pH = -\log [H_3O^+] $
• $ pOH = -\log [OH^-] $
• $ pH + pOH = 14 $
• $ pK_a = -\log K_a $
• $ pK_b = -\log K_b $
• $ pK_a + pK_b = 14 $
• $ \Delta G = \Delta G^{\circ} + RT \ln Q $
• $ pH = pK_a + \log \frac{[A^-]}{[HA]} $

Key Terms

• Spontaneous Process: A process that occurs without continuous energy input from the surroundings.
• Enthalpy (ΔH): The heat change of a process at constant pressure.
• Entropy (ΔS): A measure of the dispersion or disorder of energy in a system.
• Gibbs Free Energy (ΔG): The energy available to do work; determines spontaneity (ΔG < 0 is spontaneous).
• Equilibrium Constant (K): A ratio of products to reactants at equilibrium, indicating the extent of a reaction.
• Brønsted-Lowry Acid: A proton (H+) donor.
• Brønsted-Lowry Base: A proton (H+) acceptor.
• Conjugate Acid-Base Pair: Two species that differ by a single proton (H+).
• Buffer System: A solution that resists changes in pH upon addition of acid or base.
• Zwitterion: A molecule with both positive and negative charges, resulting in a net neutral charge.