--%>
Assignment Help - homework help

What is Flash Photolysis Reactions. Explain with examples.

An example illustrates the type of mechanism that can be written to explain the development of flash photolysis reactions.

Often, as the reactions in the ozone layer of the earth's atmosphere, we are interested in the kinetic behavior of species that are not available as on the shelf chemicals. The rate constants of reactions involving such species as H atoms, O atoms, and OH and HO2 radicals must be known if the dynamics of complex reaction mixtures are to be understood. Rate constant data can sometimes be obtained by generating such species by a high intensity short duration flash of light and then following their subsequent reaction. This approach is known as flash photolysis.

A typical flash used in such studies can generate about 1 mol of produces in the several microsecond duration of the flash. The amount of one or more of the products formed directly or indirectly, is usually determined by measuring the absorption of light at a suitable wavelength. As an illustration of this technique, studies of the formation and reaction of the perhydroxyl radical,H2O are described.

The two reactions are: 

H2O = hv  72_First order reactions1.png OH + H

OH + H2  72_First order reactions1.png H2O + H 

The formation of HO2 radicals by this flash photolysis route and studies of the kinetics of subsequent reactions has been reported†. The reaction system contained small amounts of H2Oand O2 and enough H2 to produce a total pressure of 1 atm. The formation of H atoms from the photolysis at water and their attachment to O2 to form HO2 occurred rapidly. Within about 10μs the intermediates, H and OH, were largely consumed. The kinetics of the subsequent reactions ofH2O radicals could then be studied.

The optional transmission at 210 nm, where HO2 absorbs, can be used to follow the decrease of this species. If the reaction that removes HO2 is second order in HO2, the H2O concentration will vary with the time according to:

1/c = 1/ c0 + kt

That the reaction is indeed order is confirmed by the linear relation for 1/ {log (I0 /i)}, which is proportional to 1/c, versus t, as shown in this order and the net product of the reaction, H2O2, suggest that the decay of H2O occurs by the reaction:

HO2 + HO2  72_First order reactions1.png H2O2 + O2

The reaction following the initial photolysis process can be changed by changing the reagents in the reaction system.

The principle reaction sequent system then is:

OH + HO2  72_First order reactions1.png H2O + O2

Principal reactions used for the simulation curves:

Reaction Rate constant
OH + H2 72_First order reactions1.png H2O + H 4 × 104 L mol-1 s-1
H + O2 H2 72_First order reactions1.png HO2 + H2 2 × 1010 L2 mol-2 s-1
H + O2 + Ar 72_First order reactions1.png HO2 + Ar 6 × 109 L2 mol-2 s-1
H + O2 + H2O 72_First order reactions1.png HO2 + H2O 1.4 × 1011 L2 mol-2 s-1
HO2 + HO2 72_First order reactions1.png H2O2 6 × 109 L mol-1 s-1
OH + HO2 72_First order reactions1.png H2O + O3 1.2 × 1011 L mol-1 s-1

   Related Questions in Chemistry

  • Q : Reducible Representations The number of

    The number of times each irreducible representation occurs in a reducible representation can be calculated.Consider the C2v point group as described or Appendix C. you can see that (1) sum of

    		•
  • Q : Avogadros hypothesis how avogadros

    how avogadros hypothesis used to deduce the atomicity of elementry gases ?

    		•
  • Q : Quantum Mechanical Operators The

    The quantum mechanical methods, illustrated previously by the Schrödinger equation, are extended by the use of operators. Or, w

    		•
  • Q : Problem on colligative properties

    Choose the right answer from following. The magnitude of colligative properties in all colloidal dispersions is : (a) Lowerthan solution (b)Higher than solution(c) Both (d) None

    		•
  • Q : Kinds of insulators Describe all the

    Describe all the kinds of insulators which are present?

    		•
  • Q : Infrared Adsorption The adsorption of

    The adsorption of infrared radiation by diatomic molecules increases the vibrational energy fo molecules and gives information about the force constant for the "spring" of the molecule.;The molecular motion that has the next larger energy level spacing aft

    		•
  • Q : Latent heat of vaporization Normal

    Normal butane (C4H10) is stored as a compressed liquid at 90°C and 1400 kPa. In order to use the butane in a low-pressure gas-phase process, it is throttled to 150 kPa and passed through a vaporizer. The butane emerges from the vaporizer as a

    		•
  • Q : Define Bond Energies - Bond Charges

    Energy changes in some chemical reactions can be used to deduce the energies of chemical bonds. Our understanding of the molecular basis of thermodynamic properties is extended when we ask why the enthalpy change for a reaction is what it is. We deduce,

    		•
  • Q : Question on molality Provide solution

    Provide solution of this question. Which of the following concentration factor is affected by change in temperature : (a)Molarity (b) Molality (c)Mole fraction (d)Weight fraction

    		•
  • Q : What do you mean by the term tripod

    What do you mean by the term tripod? Also state its uses?

    		•