Unit 5 Notes

Warm-Up Problems

  • Problem Statement: Three towns located at corners A, B, and C of a triangle. Car travels:

    • From A to B at 30 mph

    • From B to C at 40 mph

    • From C back to A at 60 mph

  • Question: What is the average speed of the whole trip?

Unit Overview

Unit 5: Kinetics

  • AP Exam Weighting: 7-9%

Unit 5 Topics

Topics Covered

  • 5.1 – Reaction Rates

  • 5.2 – Introduction to Rate Law

  • 5.3 – Concentration Changes over Time

  • 5.4 – Elementary Reactions

  • 5.5 – Collision Model

  • 5.6 – Reaction Energy Profile

  • 5.7 – Introduction to Reaction Mechanisms

  • 5.8 – Reaction Mechanism and Rate Law

  • 5.9 – Pre-equilibrium Approximation

  • 5.10 – Multistep Reaction Energy Profile

  • 5.11 – Catalysis

Kinetics Overview

  • Definition: Study of how reactions happen. Derived from the Greek word "kinesis" (motion).

Key Concepts

Collision Theory (5.5)

  • Core Principle: Reactions occur via collisions between particles.

  • Factors Affecting Collisions:

    • Kinetic energy of particles (KE = ½ mv²)

    • Orientation and geometry of the collision

Maxwell-Boltzmann Distribution Curves

  • Purpose: Show the number of particles having a certain amount of kinetic energy at a specific temperature (e.g., T = 310 K).

Activation Energy (Ea)

  • Definition: Minimum energy required for reactants to collide successfully and allow the reaction to proceed.

Reaction Rates (5.1)

Definition

  • Reaction Rate: How quickly a reaction occurs, defined as the change in concentration over time.

  • Equation:

    • Average Rate = Δ[Reactants]/Δt

    • Units: mol L⁻¹ s⁻¹

Concentration Notation

  • Concentration in molarity is represented with brackets, e.g., [NO2] means concentration of NO2 in mol/L.

  • Change in concentration indicated by the delta symbol, e.g., Δ[NO2] means change in concentration of NO2.

Important Characteristics

Reaction Dynamics

  • As the reaction proceeds, average rate decreases due to a lower concentration of reactants leading to fewer collisions.

  • Instantaneous Rate: Tangent slope at any point on the concentration vs. time curve reflects the reaction rate at that moment.

Stoichiometry of Reaction Rates

  • In reactions with differing stoichiometric coefficients:

    • Rate = −(1/a)Δ[A]/Δt = −(1/b)Δ[B]/Δt = (1/c)Δ[C]/Δt

Reaction Rate and Concentration Dependency

  • Reaction rates generally depend on the concentration of reactants. Determined experimentally through several methods:

    • Logically using calculated rates

    • Algebraically through concentration ratios

    • Graphically through slope analysis

Reaction Mechanisms (Unit 5.8)

Fundamental Aspects

  • Mechanism Definition: Sequence of elementary steps leading from reactants to products.

  • Types of Elementary Reactions:

    • Unimolecular: Involves one particle

    • Bimolecular: Involves two particles

    • Termolecular: Involves three particles (rare)

Intermediates and Catalysts

  • Intermediates: Short-lived products in early steps, not part of the overall equation.

  • Catalysts: Substances that speed up reactions without being consumed; may work by lowering activation energy or providing an alternate pathway.

Catalysts

Types

  • Homogeneous Catalysts: Same phase as reactants; no physical boundary.

  • Heterogeneous Catalysts: Different phase from reactants; may provide surface for reactions via adsorption.

Functionality

  • Mechanism Involvement: Reacts with one reactant to form an intermediate, which then reacts to produce the final product, regenerating the catalyst.

Summary

  • Review of the kinetics study encompasses various elements: reaction rates, collision theory, rate laws, energy profiles, and mechanisms, which all play integral roles in understanding chemical reactions and their dynamics.