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 : Question on Mole fraction Mole fraction

    Mole fraction of any solution is equavalent to: (a) No. of moles of solute/ volume of solution in litter (b) no. of gram equivalent of solute/volume of solution in litters (c) no. of  moles of solute/ Mass of solvent in kg (d) no. of moles of any

  • Q : Cations Explain how dissolving the

    Explain how dissolving the Group IV carbonate precipitate with 6M CH3COOH, followed by the addition of extra acetic acid.

  • Q : Molality of glucose Help me to go

    Help me to go through this problem. Molecular weight of glucose is 180. A solution of glucose which contains 18 gms per litre is : (a) 2 molal (b) 1 molal (c) 0.1 molal (d)18 molal

  • Q : Units of Measurement Unit of

    Unit of measurement- These are also some systems for units:

         (1) C.G.S.

  • Q : Relative lowering of the vapour pressure

    Choose the right answer from following.The relative lowering of the vapour pressure is equal to the ratio between the number of: (a) Solute moleules and solvent molecules (b) Solute molecules and the total molecules in the solution (c) Solvent molecules and the tota

  • Q : Question on colligative property Choose

    Choose the right answer from following. Which of the following is a colligative property: (a) Osmotic pressure (b) Boiling point (c) Vapour pressure (d) Freezing point

  • Q : Q what is the basicity of primary

    what is the basicity of primary secondary and tertiary amines in chlorobenzene

  • Q : Mole fraction of benzene Choose the

    Choose the right answer from following. In a solution of 8.7g benzene C6H6 and 46.0 gm toluene ,(C6, H5, CH3) the mole fraction of benzene in this solution is: (a)1/6 (b)1/5 (c)1/2 (d)1/3

  • Q : Problem on Osmotic Pressure of solution

    The osmotic pressure of a 5% solution of cane sugar at 150oC  is (mol. wt. of cane sugar = 342)
    (a) 4 atm (b) 3.4 atm (c) 5.07 atm (d) 2.45 atm

    Answer: (c)
    Π = (5 x 0.0821 x 1000 x 423)/(342 x 100) = 5.07 atm

  • Q : Molarity of Barium hydroxide 25 ml of a

    25 ml of a solution of barium hydroxide on titration with 0.1 molar solution of the hydrochloric acid provide a litre value of 35 ml. The molarity of barium hydroxide solution will be: (i) 0.07 (ii) 0.14 (iii) 0.28 (iv) 0.35