Chemistry Walkthrough: Unit 5 Part 2

Overview

This lecture covers the final part of Unit 5, focusing on Collision Theory and Catalysis. These topics explain reaction rates and how they can be influenced or enhanced.

Collision Theory

• Purpose: Explains why reactions occur at certain rates.
• Criteria for Reaction:
  1. Collision: Reactants must collide.
  2. Sufficient Energy: Collision must have enough energy.
  3. Correct Orientation: Collision must occur at the right angle.
• Analogy: Like hitting a baseball at the correct angle for a home run.

Arrhenius Equation

• Equation: ( k = Ae^{-\frac{E_a}{RT}} )
  • k: Reaction rate constant.
  • A: Frequency factor (how often collisions occur in the right orientation).
  • E: Base of the natural logarithm (~2.7).
  • Ea: Activation energy.
  • R: Gas constant (8.31 J/K·mol).
  • T: Temperature in Kelvin.
• Concepts:
  • Higher Ea requires more energy, reducing successful collisions.
  • Higher T increases average collision energy, increasing successful collisions.

Reaction Coordinate Diagrams

• Axes: Energy vs. Reaction Progress.
• Components:
  • Start: Reactants.
  • End: Products.
  • Activation Energy (Ea): Energy required to reach the transition state.
  • ∆E or ∆H: Change in energy from reactants to products.
• Types of Reactions:
  • Exothermic: ∆E is negative (energy released).
  • Endothermic: ∆E is positive (energy absorbed).

Catalysis

• Catalyst: Substance that increases reaction rate without being consumed.
• Example Reaction:
  • H2O2: Reactant.
  • I-: Catalyst (enters and exits unchanged).
  • Intermediate: Produced and consumed during the reaction.

Types of Catalysts

• Enzymes: Biological catalysts (e.g., DNA polymerase).
• Homogeneous Catalysts: Same phase as reactants (e.g., all aqueous).
• Heterogeneous Catalysts: Different phase (e.g., solid catalyst in gaseous reaction).

Spectrophotometry

• Process: Measures light absorption by a solution.
  • I0: Initial light intensity.
  • I1: Transmitted light intensity.
  • Transmittance: ( \frac{I1}{I0} )
  • Absorbance (A): (-\log \left( \frac{I1}{I0} \right) )
• Beer-Lambert Law:
  • Formula: ( A = \varepsilon bc )
  • Variables:
    • A: Absorbance.
    • b: Path length.
    • c: Concentration.
• Main Takeaway: Absorbance is proportional to concentration; can be treated as a proxy for concentration in experiments.

Conclusion

This session concludes the coverage of Unit 5 topics relevant to the AP Chemistry curriculum, focusing on understanding collision theory, catalytic processes, and spectrophotometry.

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These notes serve as a comprehensive summary for reviewing key concepts and equations from this lecture.