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 : Osmotic Pressure The O.P. (Osmotic

    The O.P. (Osmotic Pressure) of equimolar solution of Urea, BaCl2 and AlCl3, will be in the order:(a) AlCl3 > BaCl2 > Urea  (b) BaCl2 > AlCl3 > Urea  (c) Urea > BaCl2<

  • Q : Solution problem What is the correct

    What is the correct answer. To made a solution of concentration of 0.03 g/ml of AgNO3, what quantity of AgNO3 must be added in 60 ml of solution: (a) 1.8  (b) 0.8  (c) 0.18  (d) None of these

  • Q : Explain polyhalogen compounds with

    Carbon compounds containing more than one halogen atom are called polyhalogen compounds. Most of these compounds are valuable in industry and agriculture. Some important polyhalogen compounds are described as follows:

    Q : Surface Tension Vapour Pressure The

    The vapor pressure of small liquid drops depends on the drop size. Although the surface properties of a liquid are different from those of the bulk liquid, the special surface properties can be ignored except in a few situations. One is the case in which a liquid is dispersed into fine dr

  • Q : Molar mass lculwhat is the equation for

    lculwhat is the equation for caating molar mass of non volatile solute

  • Q : Question related to molarity Help me to

    Help me to go through this problem. Molarity of a solution containing 1g NaOH in 250ml of solution: (a) 0.1M (b) 1M (c) 0.01M (d) 0.001M

  • Q : What is Elevation in boiling point? The

    The boiling of a liquid may be defused by the temperature at which its vapour pressure which is equal to atmospheric pressure. The effect of addition in a non-volatile solute on the boiling point shown and its solution containing non-volatile solute with tempe

  • Q : Question based on lowest vapour pressure

    Give me answer of this question. Among the following substances the lowest vapour pressure is exerted by: (a) Water (b) Mercury (c) Kerosene (d) Rectified spirit

  • Q : Describe characteristics of halides and

    Halides characteristics

  • 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

©TutorsGlobe All rights reserved 2022-2023.