Coconote
AI notes
AI voice & video notes
Export note
Try for free
Understanding Reaction Mechanisms and Catalysts
Nov 15, 2024
Lecture on Reaction Mechanisms, Catalysts, and Reaction Coordinate Diagrams
Introduction
Focus on reaction mechanisms, catalysts, and reaction coordinate diagrams.
Learn to derive rate laws from reaction mechanisms.
Discuss characteristics of catalysts.
Explore reaction coordinate diagrams to understand activation energy.
Reaction Mechanisms
Definition
: Sequence of elementary steps that cannot be broken down further.
Most reactions have multiple steps.
Rate Law
: Determined by the slowest step (rate-determining step).
Elementary Reactions
Three classes:
Unimolecular
: One reactant molecule.
Bimolecular
: Two reactant molecules.
Termolecular
: Three reactant molecules (rare).
No reactions with more than three molecules colliding at once.
Mechanisms
Composed of elementary steps.
Can be summed to determine the overall reaction.
Catalysts and Intermediates
:
Intermediate
: Produced and consumed within the reaction.
Catalyst
: Present at the start and end, but not consumed.
Rate Laws from Mechanisms
Elementary Steps
: Coefficients of reactants can be used to determine rate law.
Slow Step
: Determines overall reaction rate.
Rate Law Scenarios
:
Slow step first: Rate law of the first step is the overall rate law.
Slow step second: More complex derivation needed, involving equilibrium expressions.
Example Problem
Two-step mechanism: O3 breaking down and reacting with another O3.
Rate Law Derivation
:
Start with slow step's rate law.
Avoid intermediates in overall rate law by substituting equivalent expressions from equilibrium steps.
Catalysts
Function
: Speed up reactions by lowering activation energy (Eₐ) through alternative pathways.
Not consumed during the reaction.
Do not shift equilibrium; only affect the rate.
Reaction Coordinate Diagrams
Components
:
Y-axis: Energy.
X-axis: Reaction progress.
Shows energy changes from reactants to products.
Activation Energy (Eₐ)
: Energy required to reach the transition state.
Transition State
: Peak of the energy barrier.
Types of Reactions
Endothermic
: Products higher in energy than reactants (ΔH positive).
Exothermic
: Products lower in energy (ΔH negative).
Multi-step Reactions
Indicated by multiple peaks/hills in diagrams.
Intermediates represented by valleys between peaks.
Slow step identified by the highest activation energy.
Summary
Catalysts provide alternative mechanisms with lower activation energies.
Reaction coordinate diagrams help visualize energy changes and identify kinetic properties.
Final Notes
Explore more on Chad's Prep for practice and further study materials.
Reaction kinetics problems available in the General Chemistry Master Course.
📄
Full transcript