6.4 Comparing collision theory and transition state theory
3 min read•july 22, 2024
Chemical reactions happen when molecules collide or form an activated complex. focuses on the frequency and energy of these collisions, while examines the formation of an activated complex at the reaction's energy peak.
Both theories explain how affects reaction rates using exponential equations. They help us understand factors like , , and catalysts. theory provides a more detailed picture of reaction mechanisms and molecular structure's role.
Collision Theory and Transition State Theory
Collision vs transition state theory
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Collision theory explains reactions occur when reactant molecules collide with enough energy (activation energy, Ea) and proper orientation
Collision frequency and energy distribution of collisions determine reaction rate
Arrhenius equation relates (k) to temperature (T): k=Ae−Ea/RT, where A is pre-exponential factor, R is gas constant
Transition state theory states reactants form an activated complex (transition state) at highest energy point along
Activated complex is in equilibrium with reactants
Reaction rate depends on concentration of activated complex
Eyring equation: k=hkBTe−ΔG‡/RT, where kB is Boltzmann constant, h is Planck's constant, ΔG‡ is Gibbs free energy of activation
Both theories consider (activation energy) reactants must overcome and relate rate constant to temperature using exponential functions
Collision theory focuses on collision frequency and energy distribution, while transition state theory emphasizes formation and concentration of activated complex
Explains role of molecular orientation in successful collisions
Collision theory weaknesses:
Does not consider detailed molecular structure of reactants and products
Assumes all collisions with sufficient energy lead to successful reactions
Does not provide complete picture of reaction mechanism
Transition state theory strengths:
Provides more detailed description of reaction mechanism
Considers molecular structure and geometry of activated complex
Allows calculation of rate constants using statistical mechanics
Explains role of entropy in formation of activated complex
Transition state theory weaknesses:
Assumes activated complex is in equilibrium with reactants
Does not account for possibility of non-equilibrium effects
Requires knowledge of molecular structure and vibrational frequencies of activated complex, which can be difficult to obtain experimentally
Application in reaction kinetics
Determining effect of temperature on reaction rate
Use Arrhenius equation (collision theory) or Eyring equation (transition state theory) to calculate change in rate constant with temperature
Comparing reaction rates for different reactions
Use activation energy and pre-exponential factor (collision theory) or Gibbs free energy of activation (transition state theory) to compare rates of different reactions
Investigating effect of catalysts on reaction rates
Both theories explain role of catalysts in lowering activation energy and increasing rate constant
Transition state theory provides insights into specific interactions between catalyst and reactants in activated complex
Analyzing effect of molecular structure on reaction rates
Transition state theory better suited for understanding how molecular structure and geometry influence formation and stability of activated complex, and consequently, reaction rate