Complex Reaction Mechanism

Complex Reaction MechanismComplex ReactionsComplex reaction mechanism involves multiple step and/or multiple intermediates in the reaction.Examples includeUnimolecular ReactionChain Reactions and Explosion reactionsCatalysis Biocatalysis

Unimolecular Reactions

These reactions are generally carried out with a bath/inert gas (M) for which [M] >> [A] ([A] = reactant).At low pressures, the rate law has partial orders with respect to A and M of 1 each, so an overall order of 2.At high pressures, first-order kinetics in [A] is observed.These observations are both1. A clue as to the mechanism, and2. A puzzle to be solved.Lindemann Mechanism

Lindemann Rate EquationUsing Steady State Approximation

Rate Expression for reactant as Bath gas

The rate laws for the Lindemann-Hinshelwood Mechanism are pressure dependent.High Pressure CaseLow Pressure Case

Solution for Practical Application

Example: cis-trans isomerization of 2-butene at 740K

Chain reactionsThe steps involved in a chain reaction may be non-cyclic or cyclic steps.

A chain reactions can further be treated as Linear chain reaction or Branched chain reaction

Example of Chain ReactionHalogenation of alkaneDecomposition of alkanesDecomposition of acetaldehydeOzone depletionSmog formationNuclear fission

Example of Branched Chain reactionCombustion

Water formation from H2 and O2

Nuclear Fission

Chain ReactionsClassifying steps in a chain reaction.

Initiation C2H6 (g) 2 CH3

Propagation StepsC2H6 + CH3 C2H5 + CH4

Branching Steps H2O + O 2 OHChain Reactions (Contd)Retardation StepHBr + H H2 + Br

Terminations Steps2 CH3CH2 CH3CH2CH2CH3

Inhibition StepsR + CH3 RCH3The H2 + Br2 ReactionThe overall rate for the reaction was established in 1906 by Bodenstein and Lind

The MechanismThe mechanism was proposed independently by Christiansen and Herzfeld and by Michael Polyani.MechanismRate Laws

Using the SSAUsing the SSA on the rates of formation of Br and H

Hydrogenation of EthaneThe Rice-Herzfeld MechanismMechanism

Rate Laws for the Rice-Herzfeld MechanismThe rate laws for the elementary reactions are as follows.

Decomposition of Acetaldehyde

Example.

Explosion ReactionAn explosion is a rapidly accelerating reaction arising from a rapid increase in reaction rate with increasing temperature

Inititation A R.

Propogation R. P + aR.

Termination R. Destruction

General Expression

k1 = rate constant for initiation stepk2 = rate constant for propagation stepkw = rate constant for termination on wallkg= rate constant for termination in gaseous phase

Occurence of explosion given by explosion region (regions of T,p).chain carriers are Branching: one chain carrier becomes two or more.

If T is low, rate constants are too small.

If p is low, the reaction rates, v, are too low due to infrequent collisions.

ExampleShow that an explosion happens when rate of chain branching exceeds that of chain termination.Answer:Focus only on the rate of prod of chain carrier as important. Rate of change of chain carrier:

Example Steady state approximation:

Then chain carrier production rate is

Examplewrite:

Then chain carrier production rate is

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