Exam 3 Review Notes for CHEM 1160

Overview of Exam 3 Content

Exam covers Chapters 7 and 8, focusing on essential concepts that you'll need to master to excel on the test.

Definition of acids and bases: Understand the fundamental differences between acids (substances that donate protons) and bases (substances that accept protons).
Strength Trends across the Periodic Table:
- Left to Right: Electronegativity increases (e.g., from carbon to fluorine), resulting in increased acid strength.
- Tip: Remember that stronger electronegative elements hold onto their protons more tightly, showcasing stronger acidic behavior.
- Example: Hydrofluoric acid (HF) is stronger than other hydrohalic acids up to iodine, illustrating this trend.
- Top to Bottom: Acid strength increases not just due to electronegativity but also because of atomic size and bond strength.
- Trick: Use the acronym "B.E.S.T." where B stands for Bonds, E for Electronegativity, S for Size, and T for Trends to remember this.
- Example: Iodine is larger than fluorine, leading to weaker HX bonds (e.g., HI is stronger than HF).

Enthalpic Effect: Breaking bonds requires energy. Larger atoms lead to weaker bonds, which generally contributes to stronger acidity.
- Tip: Focus on the size of the atom when considering bond strength; larger atoms often have weaker bonds.
Entropic Effect: Larger ions (like iodide) have higher entropy, which is more thermodynamically favorable.
- Example: Iodide ion allows more freedom for surrounding water molecules compared to fluoride.

Resonance Structures: Acids like acetic acid can exhibit resonance, which enhances anion stability.
- Trick: Draw resonance structures to visualize how electron delocalization increases acidity. More resonance structure = stronger acid due to enhanced stability of the conjugate base.

Ka: Represents the strength of acid dissociation; larger Ka indicates a stronger acid.
pKa: The negative logarithm of Ka, which serves as a more manageable number for comparing acid strengths (smaller pKa means stronger acid).

Definition of pH:
pH = - ext{log}[ ext{H}_3 ext{O}^+]
Conversion of pH to hydronium concentration:
[ ext{H}_3 ext{O}^+] = 10^{- ext{pH}}
Percent Ionization: For strong acids, the hydronium concentration approximates the original acid concentration; for weak acids, perform calculations to find the percent ionization.

Average vs. Instantaneous Rates:
- Average Rate: Change in concentration over time:
  ext{Average Rate} = - rac{ ext{Δ}[A]}{ ext{Δ}t}
- Instantaneous Rate: The slope of a tangent to the curve at a point in time.
- Trick: Use a graph to visualize changes; the steeper the slope, the faster the reaction.

Determine Reaction Order: Analyze initial rates and create plots.
- Plotting Tips: First-order reactions will yield straight lines when plotting natural logs of concentration against time. Recognize graph shapes for zero, first, and second order reactions for quick identification.

Equilibrium Constant (K):
K = rac{[ ext{products}]}{[ ext{reactants}]}
Understanding Q (Reaction Quotient): Provides the ratio of concentrations at any point in a reaction, revealing shifts in equilibrium.

Shifting Equilibrium: Factors influencing shifts include concentrations, pressure, and temperature.
- Pressure Changes: Increasing pressure favors the side of the reaction with fewer moles of gas.
- Temperature Changes: For exothermic reactions, increasing temperature shifts equilibrium left; for endothermic, it shifts right.

Review acid-base strength trends alongside corresponding production rates in reactions.
Master calculations related to Ka, pKa, pH, and percent ionization. Practice sample problems to get comfortable.
Understand major points about equilibrium: be prepared to answer scenario-based questions on how systems respond to changes in conditions.
Kinetics: Reinforce your understanding by analyzing provided data sets; be ready to derive reaction orders from experimental graphs.