Notes

Outline

Chemical Kinetics Part II-Reaction Theories

Concentration vs Time realtionship for Second Order System Inverse conc vs time is linear 1 / [A]t – 1 / [A]0 = kt Derivation Problem

T-139 Graphical Representation of Second Order System

T-14 Derivation of Second Order System

Graphical Interpretation of Kinetic Rate Order of a Component If conc vs time is linear then zero order kinetics If log conc vs time is linear then first order kinetics If inverse of conc vs time is linear then second order kinetics

Temperature Effect on Reaction Rate Arrenhius Equation ln (k1 / k2) = Ea / R [ 1 / T2 – 1 / T1]                       where  k1 = rate constant at T1; k2 = rate constant at T2; Ea = Activation Energy of Reaction; R = Universal Gas Law Constant = 8.31 x 10 –3 Kj / mol K; T1 = Initial Kelvin Temp; T2 = Final Kelvin Temp Activation Energy-Minimum energy that molecules must possess which leads to product formation Derivation and Problem

T-140 Derivation of Arrenhius Equation

Theories on Reaction Rates Collisional Theory of Reaction Rates Mechanistic Theory

Collisional Theory of Reaction Rates (molecular view) Reactant molecules must collide effectively in order to produce product molecules(ie: must be effective contact)

T-143 Effective vs Ineffective Collisions

Factors Increasing Effective Collisions Increase in the total number of colliding molecules (concentration) Increasing the concentartion increases the number of molecules present for collision and therefore the number of effective collisions is enhanced Increase the energy of Impact upon collision (Temperature) Increasing the energy of impact means increasing the kinetic energy of colliding molecules which is only done by increasing temperature

Factors Increasing Effective Collisions(cont) Increase the proper orientation of the colliding molecules (Catalyst) Catalyst positions the molecules upon its molecular surface thus improving the molecular orientation

Mechanistic Theory A Chemical Process represented macroscopically as a net reaction can be viewed  molecularly as a sequence of elementary reactions or steps This sequence of elementary steps constitutes the reaction mechanism Reaction model (theory) of how a reaction proceeds from a molecular view that prtoceeds to product formation

What do Reaction Mechanisms Do? Explain product formation of a reaction Predicts results under different set of conditions

T-141   Energy Profile For A Reaction (one step concerted)

Molecularity Molecularity-number of reactant molecules involved in an elementary step of a reaction mechanism Unimolecular step- an elementary reaction involving only one reactant molecule CH3NC --à CH3CN Bimolecular step-an elementary step involving two molecules of reactant A  +   B  -à C Termolecular step- an elementary reaction involving three molecules colliding simultaneously (very rare) A  +  B  +  C -à D

Validity of Reaction Mechanism Based on Net Reaction A + B -à C   +   D     Net Reaction Proposed mechanism 1. A   +   B --à  I1 2. I1 --à  C   +  I2 3. I2 --à  D All steps must add up and simplify to the net reaction

Transition State Theory Transition States(Activated Complex)- extremely short lived species which show the breaking and forming of bonds. Every elementary step has a transition state Species of highest energy state for an elementary step

Intermediates Intermediate- “isolatable” species which is lower in energy than the transition state of the step Some elementary steps do not possess an intermediate (only a transition state) Intermediates are produced in one step and then consummed in a later step usually the next step.

Types of Reaction Mechanisms Concerted (one step) Contains no intermediate only a transition state Multi-stepped Contains 2 or more elementarty steps each of which may or may not have an intermediate. All elementary steps have a transition state

Rate Law Expression Of An Elementary Step Theoretical Rate Law based on Reaction Mechanism(Theory) aA  +  bB -à cC     elementary reaction Rate Law Expression R = k [A]a [B]b In the Theoretical Rate Law Expression the coefficients become the rate orders

Theoretical Rate Law Expression For Reaction Process The slowest step in the mechanism is the rate determining step. The rate determining step is used to write the theoretical rate law expression for the reaction.

Effect of a Catalyst On A Reaction Catalyst- substance that increases the rate of product formation without being ultimately changed Catalysts affect a reaction by: Altering the Reaction mechanism Lowering the Energy of Activation Improving the orientation of colliding molecules upon impact Types of Catalysts Heterogeneous (metallic catalysts for Hydrogenation) Homogeneous (enzymes, protonic acids, and bases)

T-144   Energy Profile With and Without a Catalyst

T-145 Work of a Heterogeneous Catalyst

Enzymes Enzyme-Biochemical Catalyst consisting of a protein part (apoenzyme) and a non-protein part (co-enzyme) “Lock and Key” Mechanistic Theory Reactant molecules (substrate) locks into a compatibly shaped cavity on the surface of the enzyme molecule(active site) producing the Substrate Enzyme  Complex This properly orients the  substrate molecule for most effective collision converting substrate to product Product diengages from the active site freeing it to accept another substrate molecule

Factors Affecting Enzymatic Activity Temperature Acidity Active site blocking

T-146  Lock and Key Model for Enzyme Activity