INTRODUCTION
Ordinary reactions occur by absorption of heat energy
from outside. The reacting molecules are energised and
molecular collisions become effective. These bring about the
reaction. The reactions which are caused by heat and in
absence of light are called thermal or dark reactions.
On the other hand, some reactions proceed by
absorption of light radiation. These belong to the visible and
ultraviolet regions of the electromagnetic spectrum (2000 to
8000 Å). The reactant molecules absorb photons of light and
get excited. These excited molecules then produce the
reactions.
PHOTOCHEMISTRY
A reaction which takes place by absorption of visible and
ultraviolet radiation is called a photochemical reaction.
The branch of chemistry which deals with the study of
photochemical reactions.
DEMONSTRATION OF A PHOTOCHEMICAL REACTION
A mixture of hydrogen and chlorine
remains unchanged with lapse of
time. But when exposed to light,
the reaction occurs with a loud explosion.
LIGHT ABSORPTION
When light is passed through a medium, a
part of it is absorbed. It is this absorbed
portion of light which causes photochemical
reactions. Let a beam of monochromatic
light pass through a thickness dx of the
medium. The intensity of radiation reduces
from I and I-dI.
The intensity of radiation can be defined as the number of
photons that pass across a unit area in unit time.
Let us denote the number of incident photons by N and the
number absorbed in thickness dx by dN. The fraction of
photons absorbed is then dN/N which is proportional to
thickness dx. That is,
where b is proportionality constant called absorption
coefficient. Let us set I = I0 at x = 0 and integrate. This gives:
→(1)
Lambert first derived equation (1) and it is known as Lambert
Law. Beer extended this relation to solutions of compounds in
transparent solvents. The equation (1) then takes the form (2).
→(2)
where C = molar concentration; ∈is a constant characteristic
of the solute called the molar absorption coefficient. The
relation (2) is known as the Lambert-Beer Law. This law forms
the basis of spectrophotometric methods of chemical analysis
LAWS OF PHOTOCHEMISTRY
There are two basic laws governing photochemical reactions:
(a) The Grotthus-Draper law
(b) The Stark-Einstein law of Photochemical Equivalence
Grotthus–Draper Law
-It is only the absorbed
light radiations that are
effective in producing a
chemical reaction
Stark-Einstein Law of
Photochemical
Equivalence
In a photochemical reaction, each molecule of the reacting
substance absorbs a single photon of radiation causing the
reaction and is activated to form the products.
PRIMARY AND SECONDARY REACTIONS
The overall photochemical reaction may consist of:
(a) a primary reaction
(b) secondary reactions
A primary reaction proceeds by absorption of radiation.
A secondary reaction is a thermal reaction which occurs
subsequent to the primary reaction.
For example, the decomposition of HBr occurs as follows:
QUANTUMYIELD (OR QUANTUM EFFICIENCY)
The number of molecules reacted or formed per photon of
light absorbed is termed Quantum yield.
- For a reaction that obeys strictly the Einstein law, one
molecule decomposes per photon, the quantum yield φ = 1.
- When two or more molecules are decomposed per photon,
φ > 1 and the reaction has a high quantum yield.
- If the number of molecules decomposed is less than one per
photon, the reaction has a low quantum yield.
CAUSE OF HIGH QUANTUM YIELD
When one photon decomposes or forms more than one
molecule, the quantum yield φ > 1 and is said to be high. The
chief reasons for high quantum yield are:
(a) Reactions subsequent to the Primary reaction.
One photon absorbed in a primary reaction dissociates one
molecule of the reactant. But the excited atoms that result
may start a subsequent secondary reaction in which a further
molecule is decomposed.
(b) A reaction chain forms many molecules per photon.
When there are two or more reactants, a molecule of one of
them absorbs a photon and dissociates (primary reaction).
The excited atom that is produced starts a secondary
reaction chain.
Examples of high quantum yield
Decomposition of Hl
The decomposition of hydrogen iodide is brought about by
the absorption of light of less than 4000 Å. In the primary
reaction, a molecule of hydrogen iodide absorbs a photon
and dissociates to produce H and I. This is followed by
secondary steps as shown below:
CAUSES OF LOW QUANTUM YIELD
(a) Deactivation of reacting molecules.
The excited molecules in the primary process may be
deactivated before they get opportunity to react. This is
caused by collisions with some inert molecules or by
fluorescence.
(b) Occurrence of reverse of primary reaction.
Here the primary reaction generally yields a polymer. The
product then undergoes a thermal reaction giving back the
reactant molecules.
(c) Recombination of dissociated fragments.
In a primary process the reactant molecules may dissociate
to give smaller fragments. These fragments can recombine to
give back the
reactant.
CALCULATION OF QUANTUM YIELD
By definition, the quantum yield, φ, of a photochemical
reaction is expressed as :
or
