Chain reaction.pptx

chain reaction

1. Chain reactions

2. A  chain reaction  is one where an intermediate formed in one step generates an intermediate in a subsequent step, then that intermediate generates another intermediate, and so on. The intermediates in a chain reaction are called  chain carriers . Chain Reaction: In   radical chain reactions , radicals (i.e. species with unpaired electrons) act as carriers. Other carriers include neutrons (in nuclear fission) and ions.  - the step where the chain carriers are initially formed. Example:   Cl atoms are formed by the dissociation of Cl 2  molecules either : through vigorous intermolecular collisions in a  thermolysis , i.e. a reaction initiated by heat, or, through absorption of a photon in a photolysis , i.e. a reaction stimulated by the absorption of a electromagnetic radiation .

3. The steps where chain carriers produced in the initiation step attack other reactant molecules, each attach results in the formation of a new carrier.  Steps which result the attach of a chain carrier on a reactant molecule produces more than one chain carrier. Example: Steps in which a chain carrier attack a product molecule formed earlier in the reaction, thus reducing the net rate of product formation Example :  The attack of a methyl radical on ethane: 

4. A step where carriers combine and end the chain. Example:  The attack of a methyl radical on ethane:  A step where carriers are removed by a process other then termination (i.e. not by having two carriers combining and ending the chain .) Possible ways for inhibition include reaction of the chain carriers with the walls of the reaction vessel or foreign species.  E.g.: 

5. The rate laws of chain reactions : The Rice-Herzfeld mechanism for the pyrolysis of ethanal in the absence of air. The pyrolysis (i.e. thermal decomposition) of ethanal (acetaldehyde, CH 3 CHO) is found to exhibit a simple rate law where: This reaction proceeds according the Rice-Herzfeld mechanism where:

6. According to steady state approximation, the net rate change of the intermediates may be set to zero, i.e.: Eq. ( i ) + Eq. (ii): Therefore, the steady state concentration of the methyl radical is:

7. According to eq.(b), the rate of formation of methane (i.e. the rate of the overall reaction ) is : Q. Write down the Rice-Herzfeld mechanism for the decomposition of acetaldehyde and show that the order of this reaction is three halves.