AP Chemistry Unit 5: Kinetics (Topics 5.1 - 5.3)

Unit Overview

• Unit 5 Title: Kinetics
• Topics Covered:
  • 5.1: Reaction Rates
  • 5.2: Introduction to Rate Law
  • 5.3: Concentration Changes Over Time
• Supplementary Materials: Links provided for course and exam description (CED) and accompanying packet.

Topic 5.1: Reaction Rates

• Definition: Rate at which reactants convert to products.
• Influences on Reaction Rate:
  • Concentrations of reactants
  • Temperature
  • Surface area
  • Catalysts
  • Environmental factors
• Experiment Example:
  • Decomposition of N2O5 in a flask.
  • Data indicates concentration changes of N2O5, NO2, and O2 over time.
  • Stoichiometry influences rates:
    • 2 moles N2O5 consumed for 4 moles NO2 and 1 mole O2 produced.
• Key Concepts:
  • Concentration changes are proportional to stoichiometric coefficients.
  • Rate comparisons using stoichiometry.

Topic 5.2: Introduction to Rate Law

• Key Knowledge:
  • Rate expressed as function of reactant concentrations.
  • Orders of reaction determined experimentally.
  • Rate constant (k) is temperature dependent.
• Experiment Example:
  • NO and H2 reaction kinetics.
  • Rate orders: First order with respect to H2, second order with respect to NO.
  • Method: Comparing initial rates to determine order.
• Rate Law Formulation:
  • Rate = k [NO]^2 [H2]
• Units of Rate Constant: Depend on overall order.

Topic 5.3: Concentration Changes Over Time

• Graphical Analysis:
  • First-order: Natural log of concentration vs. time is linear.
  • Second-order: Reciprocal of concentration vs. time is linear.
• Half-life Concepts:
  • Constant for first-order reactions.
  • Relationship: Half-life = 0.693/k.
• Calculus in Kinetics: Used to derive integrated rate laws for different orders.

Summary of Reaction Orders

• Zero Order: Rate = k, straight line concentration vs. time.
• First Order: Natural log concentration vs. time is linear. Half-life constant.
• Second Order: 1/[A] vs. time is linear.

Practical Experimentation

• Plotting and Analyzing Data:
  • Use graphs to determine reaction order.
  • Calculation of rate constants from slopes.
• Experimental Modifications:
  • Adjusting concentrations, temperatures to control reaction rate.

Additional Topics and Examples

• Collision Theory: Successful collisions lead to product formation.
• Experimental Variables: Concentration, temperature, surface area.
• Use of Spectrophotometry: To track reaction progress via absorbance changes.

Review and Application

• Calculations: Rate constant, reaction order, half-life.
• Graph Interpretation: Determining order and constants from plotted data.

Key Equations

• First-order Rate Law: ln[A]_t = -kt + ln[A]_0
• Second-order Rate Law: 1/[A]_t = kt + 1/[A]_0
• Zeroth-order Rate Law: [A]_t = -kt + [A]_0
• Half-life (First-order): t_1/2 = 0.693/k

Tips for Success

• Relate Changes in Reactants to Rate Laws: Use stoichiometry for relationships.
• Identify Order via Experiment: Use data tables and graphs.
• Understand Graphical Methods for Analysis: Know which plots show linearity for each order.
• Review Calculus Applications: Understand derivations of rate laws.