Published on July 24, 2014
Photosensitisation: Photosensitisation Certain reactions are known which are not sensitive to light. These reactions can be made sensitive by adding a small amount of foreign material, which can absorb light and stimulate the reaction without itself taking part in the reaction. Such an added material is known as photosensitizer and the phenomenon as photosensitisation. Photosensitised reactions are spontaneous involving an increase in free energy of the system. Role played by a Photosesitizer: Role played by a Photosesitizer The formation of a photosensitizer is to absorb light, become excited and then pass on this energy to one of the reactants. As a result of these the reactants activate by the sensitizer for reaction, without itself taking part in the reaction. Thus, the photosensitiser acts as a carrier of energy. Reactions sensitized by Hg atom : Reactions sensitized by Hg atom When a mixture of Hg vapour and H 2 gas is illuminated by light of wavelength 2537 0 A, the dissociation of molecular hydrogen into atomic hydrogen takes place. The mechanism which is now suggested is the excitation of Hg- atom and then to transfer energy to H 2 - molecule which will dissociate to form atoms. PowerPoint Presentation: When reaction is carried out between H 2 and O 2 in presence of Hg- vapour under the influence of light radiation, the reaction leads to the formation of H 2 O 2 and H 2 O. The accepted mechanism of the above is that the first stage is the formation of hydrogen atoms by collision between excited Hg- atom and hydrogen molecule. PowerPoint Presentation: The above reaction may be followed by reactions such as: Now H 2 O 2 may either be isolated as such or further decomposed to form H 2 O and O 2 PowerPoint Presentation: The reaction between H 2 and CO is photosensitized by Hg- atoms. The main products are formaldehyde and glyoxal in similar amounts. The value of this reaction is nearly 2. The accepted mechanism can be shown below: PowerPoint Presentation: Hg- vapour photosensitises the decomposition of NH 3 in the presence of light of = 2537 0 A. = 7 and the rate of this photosensitised reaction is about 200 times greater than the photolysis of NH 3 (in the absence of Hg- vapour. The possible mechanism is: PowerPoint Presentation: Other examples of Hg- photosensitised reactions are: Decomposition of PH 3 Ozonalysis of Oxygen Decomposition of acetone Decomposition of water vapour Decomposition of ethyl alcohol Chemiluminescence : Chemiluminescence It is the phenomenon of emission of visible light as a result of chemical change at a temperature at which a black body normally does not emit visible light. From the above definition, it is evident that chemiluminescence is the reverse of photochemical reaction in which light absorption causes chemical action. Example: - Phosphorus which produces a greenish-yellow glow. It is due to the oxidation of ‘P’ vapour by atmospheric oxygen. P 2 O 5 also produces glow which is oxidized to P 2 O 5 . PowerPoint Presentation: When a solution of SrCl 2 is added to dilute H 2 SO 4 in dark, a feeble glow is produced along with the precipitate of SrSO 4 Evans observed that a solution of p-phenyl bromomagnesium (Grignard reagent) in ether produces greenish-blue glow consisting of a single broad band. PowerPoint Presentation: When alkali metal vapours react with halogens or with mercuric halides, luminescence occurs which consists of the spectrum of the alkali metal. The fact behind the mechanism based on Na- metal contains monoatomic and diatomic molecules. It shows the following mechanism: PowerPoint Presentation: When the surface of mercury is allowed to contact with a stream of atomic hydrogen, a blue fluorescence appears. The spectrum of this blue glow consists of the resonance line 2537 0 A and a band of spectral lines from 4520 to 3250 0 A due to HgH (mercuric hydride). The possible mechanism has been shown below: Theoretical Explanation of Chemiluminescence: Theoretical Explanation of Chemiluminescence It can be explained on the basis of quantum theory. Certain chemical reactions result in the formation of products which are in electronically excited state. When the excited products return to their ground states, they do so by emitting extra energy in the form of a visible light radiations. Thus, by this process, the chemical energy is converted into light energy. Sensitized Chemiluminescence: Sensitized Chemiluminescence Kautsky reported in 1925 that oxidation of unsaturated silicon hydride by KMnO 4 is accompanied by a luminescent glow. If the same oxidation is carried out in the presence of certain dyestuffs such as rhodamine-B, a strong red fluorescence characteristic of the dye is observed. This type of change is known as sensitized chemiluminescence. Explanation: - During the oxidation of SiH 2 , the energy is given out and is subsequently transferred to the dyestuff, raising it to an excited state. When it returns to the ground state, a fluorescence is observed.