⏱️General Chemistry II Unit 1 – Chemical Kinetics: Rates and Mechanisms

Key Concepts

• Chemical kinetics studies the rates of chemical reactions and the factors influencing them
• Reaction rate measures the change in concentration of reactants or products per unit time
• Rate law expresses the relationship between reaction rate and reactant concentrations
• Reaction order determines how the concentration of a reactant affects the rate of a reaction
• Molecularity refers to the number of reactant particles that participate in an elementary step
• Activation energy is the minimum energy required for reactants to overcome and initiate a reaction
• Catalysts lower the activation energy of a reaction without being consumed in the process
• Reaction mechanisms describe the step-by-step sequence of elementary reactions leading to the overall reaction

Rate Laws and Reaction Orders

• Rate law is an equation that relates the reaction rate to the concentrations of reactants
• General form of a rate law: $Rate = k[A]^m[B]^n$, where $k$ is the rate constant, $[A]$ and $[B]$ are reactant concentrations, and $m$ and $n$ are reaction orders
• Reaction order is the power to which the concentration of a reactant is raised in the rate law
  • Zero-order reaction: rate is independent of reactant concentration (photochemical reactions)
  • First-order reaction: rate is directly proportional to the concentration of one reactant (radioactive decay)
  • Second-order reaction: rate is proportional to the square of the concentration of one reactant or the product of the concentrations of two reactants (dimerization)
• Overall reaction order is the sum of the individual reaction orders for each reactant
• Rate constant ($k$) is specific to a reaction at a given temperature and has units that depend on the overall reaction order
• Integrated rate laws describe the concentration of reactants or products as a function of time for different reaction orders

Factors Affecting Reaction Rates

• Temperature: increasing temperature typically increases reaction rate by providing more kinetic energy for collisions
  • Arrhenius equation: $k = Ae^{-E_a/RT}$, where $A$ is the pre-exponential factor, $E_a$ is the activation energy, $R$ is the gas constant, and $T$ is the absolute temperature
• Concentration: increasing reactant concentrations generally increases reaction rate by providing more particles for collisions
• Pressure: increasing pressure for gaseous reactants increases reaction rate by reducing the volume and increasing the frequency of collisions
• Surface area: increasing the surface area of solid reactants increases reaction rate by exposing more particles for collisions (grinding, crushing)
• Catalysts: substances that lower the activation energy of a reaction without being consumed, increasing the reaction rate
  • Homogeneous catalysts are in the same phase as the reactants (enzymes in biochemical reactions)
  • Heterogeneous catalysts are in a different phase from the reactants (solid catalysts in gas-phase reactions)

Reaction Mechanisms

• Reaction mechanism is a step-by-step description of the elementary reactions that lead to the overall reaction
• Elementary reaction is a single step in a reaction mechanism involving one or more reactant molecules
• Molecularity refers to the number of reactant particles that participate in an elementary reaction
  • Unimolecular: one reactant molecule (isomerization)
  • Bimolecular: two reactant molecules (substitution)
  • Termolecular: three reactant molecules (rare due to low probability of simultaneous collision)
• Rate-determining step is the slowest step in a reaction mechanism, which determines the overall rate of the reaction
• Intermediate is a species formed during a reaction mechanism that is consumed in a subsequent step and does not appear in the overall balanced equation
• Catalyst participates in the reaction mechanism but is regenerated at the end of the catalytic cycle

Activation Energy and Catalysts

• Activation energy ($E_a$) is the minimum energy required for reactants to overcome and initiate a reaction
• Transition state is a high-energy, unstable arrangement of atoms formed during the reaction process
• Activated complex is a transient species at the peak of the energy barrier between reactants and products
• Catalysts lower the activation energy by providing an alternative reaction pathway with a lower energy barrier
  • Catalysts increase the rate of both the forward and reverse reactions without affecting the equilibrium constant
  • Enzymes are biological catalysts that are highly specific to their substrates and operate under mild conditions
• Homogeneous catalysts are in the same phase as the reactants (acid-base catalysis in liquid phase)
• Heterogeneous catalysts are in a different phase from the reactants (solid catalysts in gas-phase reactions)
  • Adsorption of reactants onto the catalyst surface is a key step in heterogeneous catalysis
  • Active sites are specific locations on the catalyst surface where the reaction occurs

Experimental Methods in Kinetics

• Spectroscopic methods: measure the concentration of reactants or products over time using UV-visible, infrared, or Raman spectroscopy
• Titrimetric methods: determine the concentration of reactants or products by titration with a standard solution
• Calorimetric methods: measure the heat evolved or absorbed during a reaction using a calorimeter
• Pressure measurements: monitor the change in pressure for gas-phase reactions using a pressure sensor
• Stopped-flow techniques: rapidly mix reactants and measure the concentration of species at very short time intervals
• Flash photolysis: initiate a reaction using a short, intense pulse of light and monitor the concentration of intermediates
• Isotopic labeling: use isotopically labeled reactants to trace the reaction pathway and identify intermediates
• Computational methods: use quantum chemical calculations and molecular dynamics simulations to model reaction mechanisms and predict rate constants

Real-World Applications

• Atmospheric chemistry: understanding the kinetics of reactions involved in the formation and depletion of ozone, smog, and greenhouse gases
• Combustion: optimizing the rate and efficiency of fuel combustion in engines and power plants
• Catalysis: designing and improving catalysts for industrial processes (Haber-Bosch process for ammonia synthesis, catalytic converters in automobiles)
• Biochemistry: studying the kinetics of enzyme-catalyzed reactions and the regulation of metabolic pathways
• Pharmacology: determining the rate of drug absorption, distribution, metabolism, and excretion in the body
• Materials science: controlling the rate of polymerization reactions and the properties of resulting polymers
• Food science: understanding the kinetics of food spoilage reactions and the effect of preservatives on shelf life
• Environmental remediation: designing treatment processes for the degradation of pollutants and contaminants in water and soil

Common Pitfalls and Study Tips

• Ensure you understand the difference between reaction rate, rate law, and reaction order
• Practice writing rate laws for different types of reactions and determining the overall reaction order
• Remember that the rate constant ($k$) is temperature-dependent and has units that depend on the overall reaction order
• Be able to identify the rate-determining step in a reaction mechanism and explain its significance
• Understand the role of catalysts in lowering the activation energy and increasing the reaction rate
• Practice solving problems using the integrated rate laws for different reaction orders
• Familiarize yourself with the experimental methods used to study reaction kinetics and their applications
• Pay attention to the units of concentration, time, and rate constants in kinetics problems
• Use concept maps or flowcharts to visualize the relationships between different kinetics concepts
• Practice explaining the key concepts and their applications in your own words to reinforce your understanding