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 H2O+ 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 × 10L 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 : Solubility of a gas The solubility of a

    The solubility of a gas in water depends on: (a) Nature of the gas (b) Temperature (c) Pressure of the gas (d) All of the above.

    Can someone help me in finding out the right answer.

  • Q : The three facts on the evaporation

    Describe briefly the three facts on the evaporation?

  • Q : Problem on bubble point The following

    The following mixture of hydrocarbons is obtained as one stream in a petroleum refinery.

    449_dew point.jpg

    Q : Mass percent Help me to go through this

    Help me to go through this problem. 10 grams of a solute is dissolved in 90 grams of a solvent. Its mass percent in solution is : (a) 0.01 (b) 11.1 (c)10 (d) 9

  • Q : Decanormal and decinormal solution

    Provide solution of this question.10N/and 1/10N solution is called: (a) Decinormal and decanormal solution (b) Normal and decinormal solution (c) Normal and decanormal solution (d) Decanormal and decinormal solution

  • Q : Various cons of eating the organic foods

    Describe the various cons of eating the organic foods? Briefly illustrate it.

  • Q : Question based on vapour pressure While

    While a substance is dissolved in a solvent, the vapour pressure of the solvent is decreased. This results in: (a) An increase in the boiling point of the solution (b) A decrease in the boiling point of solvent (c) The solution having a higher freezing point than

  • Q : BASIC CHARACTER OF AMINES IN GAS PHASE,

    IN GAS PHASE, BASICITIES OF THE AMINES IS JUST OPPOSITE TO BASICITY OF AMINES IN AQEUOUS PHASE .. EXPLAIN

  • Q : Molarity of HCl solution 20 ml of HCL

    20 ml of HCL solution needs 19.85 ml of 0.01M NaOH solution for complete neutralization. Morality of the HCL solution is:  (i) 0.0099 (ii) 0.099 (iii) 0.99 (iv) 9.9

    Choose the right answer from above.

  • Q : Problem on mole fraction of glucose

    Provide solution of this question. While 1.80gm glucose dissolve in 90 of H2O , the mole fraction of glucose is: (a) 0.00399 (b) 0.00199 (c) 0.0199 (d) 0.998