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Photochemical Reactions – Chemistry Optional Notes – For W.B.C.S. Examination.
ফোটো রাসায়নিক রাসায়নিক বিক্রিয়া – রসায়ন নোট – WBCS পরীক্ষা।
Chemistry is one such optional offered in the W.B.C.S. Examination. It is one of the optional subjects that is preferred by the aspirants graduated with the subject. Chemistry optional is a scoring subject if aspirants have sound knowledge in it. Similar to other optional papers, chemistry also has two papers. The paper I of chemistry optional deals with Inorganic Chemistry and Physical Chemistry. Paper-II completely deals with Organic Chemistry.Following previous years question papers are a must when it comes to optional subjects.Chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules’ absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules.Continue Reading Photochemical Reactions – Chemistry Optional Notes – For W.B.C.S. Examination.
These new chemical species can fall apart, change to new structures, combine with each other or other molecules, or transfer electrons, hydrogen atoms, protons, or their electronic excitation energy to other molecules. Excited states are stronger acids and stronger reductants than the original ground states.
It is this last property that is crucial in the most important of all photochemical processes, photosynthesis, upon which almost all life on Earth depends. Through photosynthesis, plants convert the energy of sunlight into stored chemical energy by forming carbohydrates from atmospheric carbon dioxide and water and releasing molecular oxygen as a byproduct. Both carbohydrates and oxygen are needed to sustain animal life. Many other processes in nature are photochemical. The ability to see the world starts with a photochemical reaction in the eye, in which retinal, a molecule in the photoreceptor cell rhodopsin, isomerizes (or changes shape) about a double bond after absorbing light. Vitamin D, essential for normal bone and teeth development and kidney function, is formed in the skin of animals after exposure of the chemical 7-dehydrocholesterol to sunlight. Ozone protects Earth’s surface from intense, deep ultraviolet (UV) irradiation, which is damaging to DNA and is formed in the stratosphere by a photochemical dissociation (separation) of molecular oxygen (O2) into individual oxygen atoms, followed by subsequent reaction of those oxygen atoms with molecular oxygen to produce ozone (O3). UV radiation that does get through the ozone layer photochemically damages DNA, which in turn introduces mutations on its replication that can lead to skin cancer.Also read , How Do I Start W.B.C.S. Preparation For 2021 ?
Photochemical reactions and the properties of excited states are also critical in many commercial processes and devices. Photography and xerography are both based upon photochemical processes, while the manufacture of semiconductor chips or the preparation of masks for printing newspapers relies on UV light to destroy molecules in selected regions of polymer masks.
The use of photochemistry by humans began in the late Bronze Age by 1500 BCE when Canaanite peoples settled the eastern coastline of the Mediterranean. They prepared a purple fast dye (now called 6,6’-dibromoindigotin) from a local mollusk, using a photochemical reaction, and its use was later mentioned in Iron Age documents that described earlier times, such as the epics of Homer and the Pentateuch. In fact, the word Canaan may mean “reddish purple.” This dye, known as Tyrian purple, was later used to colour the cloaks of the Roman Caesars.
In the simplest photochemical process, excited states can emit light in the form of fluorescence or phosphorescence. In 1565, while investigating a Mexican wood that relieved the excruciating pain of urinary stones, Spanish physician Nicolás Monardes made an aqueous (water-based) extract of the wood, which glowed blue when exposed to sunlight. In 1853 English physicist George Stokes noticed that a quinine solution exposed to a lightning flash gave off a brief blue glow, which he called fluorescence. Stokes realized that lightning gave off energy in the form of UV light. The quinine molecules absorbed this energy and then reemitted it as less-energetic blue radiation. (Tonic water also glows blue because of quinine, which is added to provide a bitter taste.)
In the 16th century Florentine sculptor Benvenuto Cellini recognized that a diamond exposed to sunlight and then placed into the shade gave off a blue glow that lasted for many seconds. This process is called phosphorescence and is distinguished from fluorescence by the length of time it persists. Synthetic inorganic phosphors were prepared in 1603 by cobbler-alchemist Vincenzo Cascariolo of Bologna by reducing the natural mineral barium sulfate with charcoal to synthesize barium sulfide. Exposure to sunlight caused the phosphor to emit a long-lived yellow glow, and it was sufficiently regarded that many traveled to Bologna to collect the mineral (called Bologna stones) and make their own phosphor. Subsequent work by Italian astronomer Niccolò Zucchi in 1652 demonstrated that the phosphorescence is emitted at longer wavelengths than needed to excite the phosphor; for instance, blue phosphorescence follows UV excitation in diamonds. In addition, in 1728 Italian physicist Francesco Zanotti showed that phosphorescence keeps the same colour even when the colour of the excitation radiation is altered to increasing energy. These same properties are also true of fluorescence.
The modern era of organic photochemistry began in 1866, when Russian chemist Carl Julius von Fritzche discovered that a concentrated anthracene solution exposed to UV radiation would fall from the solution as a precipitate. This precipitation happens because the anthracene molecules join together in pairs, or dimers, which are no longer soluble.
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