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 radiations. These belong to the visible and ultraviolet regions of the electromagnetic spectrum (2000 to 8000 Å). The reactant molecules absorbs photons of light and get excited. These excited molecules then produce the reactions. A reaction which takes place by absorption of the visible and ultraviolet radiations is called a photochemical reaction. The branch of chemistry which deals with the study of photochemical reactions is called photochemistry.
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.
A bottle is filled with equimolar amounts of hydrogen and chlorine. It is tightly stoppered with a handball. When the lamp is turned on, a beam of light falls on the mixture through the bottom of the bottle. The reaction occurs with an explosion. The ball is expelled with high velocity so that it strikes the opposite wall of the lecture theatre.
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 themedium. 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
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).
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.
DETERMINATION OF ABSORBED INTENSITY
A photochemical reaction occurs by the absorption of photons of light by the molecules.Therefore, it is essential to determine the absorbed intensity of light for a study of the rate of reaction
An experimental arrangement for the purpose is illustrated in Fig. 30.3.
Light beam from a suitable source (tungsten filament or mercury vapour lamp) is rendered parallel by the lens. The beam then passes through a ‘filter’ or monochrometer which yields light of one wavelength only. The monochromatic light enters the reaction cell made of quartz. The part of light that is not absorbed strikes the detector. Thus the intensity of light is measured first with the empty cell and then the cell filled with the reaction sample. The first reading gives the incident intensity, I0, and the second gives the transmitted intensity, I. The difference, I0 – I = Ia, is the absorbed intensity.
The detector generally used for the measurement of intensity of transmitted light is :
(a) a thermopile (b) photoelectric cell (c) a chemical actinometer.
Thermopile:
It is made of a series of thermocouples in which unlike metals such as bismuth and silver are joined together. One end of the couple is blackened with lamp black and the other end is left as such. When the radiation strikes the black end it absorbs energy and is heated up. The temperature difference between the two ends causes a current to flow in the circuit as indicated by the galvanometer. The current is proportional to intensity of radiation. The thermopile is previously calibrated against a standard source of light.
Photoelectric Cell
A photoelectric cell can be conveniently used for measuring intensity of light. The light striking the active metal electrode (cesium, sodium or potassium) causes the emission of electrons. A current flows through the circuit which can be measured with an ammeter. The intensity of light is proportional to the current.
Chemical Actinometer:
A chemical actinometer uses a chemical reaction whose rate can be determined easily. One such simple device is Uranyl oxalate actinometer. It contains 0.05 M oxalic acid and 0.01 M uranyl sulphate in water. When it is exposed to radiation, oxalic acid is decomposed to CO2, CO and H2O.
The concentration of oxalic acid that remains can be found by titration with standard KMnO4
solution. The used up concentration of oxalic acid is a measure of the intensity of radiation.
LAWS OF PHOTOCHEMISTRY
There are two basic laws governing photochemical reactions :
(a) The Grothus-Draper law
(b) The Stark-Einstein law of Photochemical Equivalence
Grothus–Draper Law
When light falls on a cell containing a reaction mixture, some light is absorbed and the remaining light is transmitted. Obviously, it is the absorbed component of light that is capable of producing the reaction. The transmitted light is ineffective chemically. Early in the 19th century, Grothus and Draper studied a number of photochemical reactions and enunciated a generalisation. This is known as Grothus-Draper law and may be stated as follows : It is only the absorbed light radiations that are effective in producing a chemical reaction. However, it does not mean that the absorption of radiation must necessarily be followed by a chemical reaction. When the conditions are not favourable for the molecules to react, the light energy remains unused. It may be re-emitted as heat or light. The Grothus-Draper law is so simple and self-evident. But it is purely qualitative in nature. It gives no idea of the relation between the absorbed radiation and the molecules undergoing change.
Stark-Einstein Law of Photochemical Equivalence
Stark and Einstein (1905) studied the quantitative aspect of photochemical reactions by application
of Quantum theory of light. They noted that each molecule taking part in the reaction absorbs only
a single quantum or photon of light. The molecule that gains one photon-equivalent energy is
activated and enters into reaction. Stark and Einstein thus proposed a basic law of photochemistry
which is named after them. The Stark-Einstein law of photochemical equivalence may be stated as :
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.
The law of photochemical equivalence is illustrated in Fig. 30.6 where a molecule ‘A’ absorbs a
photon of radiation and gets activated. The activated molecule (A*) then decomposes to yield B. We could say the same thing in equational form as :
In practice, we use molar quantities. That is, one mole of A absorbs one mole of photons or one
einstein of energy, E. The value of E can be calculated by using the expression given
below:
-By Devesh Chaudhari
Founder of Pharma Life Science©
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