Chemical reactions Chemical reactions are the heart of chemistry. People havealways known that they exist. The Ancient Greeks were the firststo speculate on the composition of matter. They thought that itwas possible that individual particles made up matter. Later, in the Seventeenth Century, a German chemist namedGeorg Ernst Stahl was the first to postulate on chemicalreaction, specifically, combustion. He said that a substancecalled phlogiston escaped into the air from all substances duringcombustion. He explained that a burning candle would go out if acandle snuffer was put over it because the air inside the snufferbecame saturated with phlogiston. According to his ideas, woodis made up of phlogiston and ash, because only ash is left aftercombustion. His ideas soon came upon some contradiction. Whenmetal is burned, its ash has a greater mass than the originalsubstance. Stahl tried to cover himself by saying thatphlogiston will take away from a substance's mass or that it hada negative mass, which contradicted his original theories. In the Eighteenth Century Antoine-Laurent Lavoisier, inFrance, discovered an important detail in the understanding ofthe chemical reaction combustion, oxigine (oxygen). He said thatcombustion was a chemical reaction involving oxygen and anothercombustible substance, such as wood. John Dalton, in the early Nineteenth Century, discovered theatom. It gave way to the idea that a chemical reaction wasactually the rearrangement of groups of atoms called molecules.Dalton also said that the appearance and disappearance ofproperties meant that the atomic composition dictated theappearance of different properties. He also came up with ideathat a molecule of one substance is exactly the same as any othermolecule of the same substance. People like Joseph-Lois Gay-Lussac added to Dalton'sconcepts with the postulate that the volumes of gasses that reactwith each other are related (14 grams of nitrogen reacted withexactly three grams of hydrogen, eight grams of oxygen reacted toexactly one gram of hydrogen, etc.) Amedeo Avogadro also added to the understanding of chemicalreactions. He said that all gasses at the same pressure, volumeand temperature contain the same number of particles. This ideatook a long time to be accepted. His ideas lead to thesubscripts used in the formulas for gasses. From the work of these and many other chemists, we now havea mostly complete knowledge of chemical reactions. There are nowmany classification systems to classify the different types ofreactions. These include decomposition, polymerization, chainreactions, substitute reactions, elimination reactions, additionreactions, ionic reactions, and oxidation-reduction reactions. Decomposition reactions are reactions in which a substancebreaks into smaller parts. As an example, ammonium carbonatewill decompose into ammonia, carbon dioxide, and water.Polymerization reactions are reactions in which simplersubstances combine to form a complex substance. The thing thatmakes this reaction unusual is that the final product is composedof hundreds of the simpler reagent (a substance that contributesto a chemical reaction) species. One example is thepolymerization of terephthalic acid with ethylene glycol to formthe polymer called Dacron, a fibre, or Mylar, in sheet form:nH2OC(C6H4)CO2H + nHOCH2CH2OH -> [...OC(C6H4)CO2CH2CH2O...]n + 2nH2Oin which n is a large number of moles. A chain reaction is aseries of smaller reactions in which the previous reaction formsa reagent for the next reaction. The synthesis of hydrogenbromide is a good example: H2 + Br2 -> 2HBrThis is a simple equation that doesn't properly prove thereaction. It is very complex and starts with this: Br2 -> 2Br The next three reactions are related and should be groupedtogether. A substation reaction is a reaction in which asubstance loses one or more atoms and replaces them with the samenumber of atoms of another element from another substance. Hereis the example of chloroform that reacts with antimonytriflouride: CHCl3 + SbF3 -> CHClF2An elimination reaction is a reaction in which a compound isbroken into smaller parts when heated. Here is an example whenthe same substance is heated and goes through another reaction: 2CHClF2 -> C2F4 + 2HClAn addition reaction is a reaction in which atoms are added to amolecule. If the added atoms are hydrogens, then the reaction iscalled a hydrogenization reaction. If Oleic acid ishydrogenized, this what you get: C18H34O2 + H2 -> C18H36O2 Another reaction is called an ionic reaction. It occursbetween two ions and can happen very quickly. For example, whensilver nitrate and sodium chloride are mixed you get silverchloride: AgNO3 + NaCl -> AgCl + NaNO3 The last type of reaction is called oxidation-reduction.These are reactions that involve a change in oxidation number.It is a reaction if the oxidation number goes up. It is areduction reaction if the oxidation number goes down. It is now known that there are three types of chemicalreactions. They are classified into three types: exoergic(exothermic), endoergic (endothermic), and aergic (athermic). Inthese cases, energy is supplied, but the different types ofreactions initiate the energy differently. Exoergic, or exothermic, reactions release energy duringthe reaction. Combustion is one of the major reactions that dothis. The burning of wood, or any other fuel, gives off heat,and the burning of glucose in our bodies gives off both energyand heat. Endoergic, or endothermic, reactions absorb energy duringthe reaction. The melting of an ice cube is an example of anendothermic reaction. Aergic, or athermic, reactions neither give off nor absorbenergy. There are very few cases in which this happens. There are some things that must be considered in a chemicalreaction. Kinetics is one of these things. Kinetics decides The

speed of the reaction and what is happening on a molecular level.There are a few things that decide the course and speed of thereaction. The first thing is the reactants. Different reactants reactat different speeds. Even the position of the reactants willaffect the reaction rate. The next thing is the catalyst that contributes a neededsubstance to the reaction. It Is part of the energyconsiderations. The catalyst is an outside substance that isincluded in the reaction, but is not consumed during the reactionlike the reactants are. They cannot make impossible reactionsoccur, they only contribute to the reaction to increase thereaction rate. There are also such things as negative catalysts,or inhibitors. Inhibitors retard the reaction rate. This isalso a way to control reactions. A good example in nature of acatalyst is in a firefly. The reaction that releases the lightis complex. Lucifern, which the firefly makes naturally, isoxidized in the presence of luciferase, another natural enzyme,which acts as a catalyst in the reaction. Thus, the reactionmakes an excited form of luciferase, which soon returns to itsoriginal state. Energy as light is released when the lucifrasereturns to its normal state. The insect can easily control thisreaction with an inhibitor it naturally makes. Another contributor in this consideration is entropy. It isthe measure of energy not available for work in the reaction thatbecomes energy moved to disorder. Entropy is simply ameasurement of unusable energy in a closed thermodynamic system. An acid and base reaction is another thing to consider.Acids and bases react very readily to each other. When an acidand a base react, they form water and a salt. Acids and bases neutralize each other and form a salt as abyproduct. This reaction reaches what is called equilibria,(When a substance is completely neutral in charge and acidity). One example of how acids and bases react is the reaction ofcalcium hydroxide and phosphoric acid to produce calciumphosphate and water: 3Ca(OH)2 + 2H3PO4 -> Ca3(PO4)2 + 6H2O The last detail is the reaction conditions. Thetemperature, humidity, and barometric pressure will affect thereaction. Even a slight change in any one of these could changethe reaction. There are many branches of Chemistry that use chemicalreactions, infact, almost all of them. Here are some examples. Photochemistry is one branch of chemistry that deals withchemical reactions. It has to do with the radiant energy of allkinds formed during chemical reactions. Photochemists willexperiment with chemical reactions. They will perform reactionsnormally only possible at high temperatures in room temperatureunder ultra-violet radiation. The reaction rate can becontrolled for observation by varying the intensity of theradiation. X-rays and gamma rays are commonly used in theseprocedures. The most important photochemical reaction isphotosynthesis. Carbon-dioxide and water combine with chlorophyllas a catalyst to give off oxygen. Photochemical reactions arecaused by photons that are given off by the light source. Thereactant molecules absorb the photons and get excited. They areat such an excited state, they can decompose, ionize, cause areaction with other molecules, or give off heat. Another science that uses chemical reactions isBiochemistry. They use them to produce products that a personeither can't produce or cannot do as well as they should. Thebest example of this the production of insulin. It was firstproduced in very tiny beads until someone realized that the bodydoes in a very similar way. The person was Robert B. Merrifeild.He was the first to urge scientists to study living systems forthe answers to problems that could be solved with synthesizingchemical reactions in the body. This was actually the first steptoward the development of bionics. Scientists today are still toying with chemical reactions.They are trying to control them with lasers. Scientists aretrying to use lasers to prod a chemical reaction that could goone way or another, the way they want it to. They want to directthe molecules in one direction. The control of photons to excitemolecules and cause reactions has been elusive. Recently,though, chemist Robert J. Gordon at the University of Illinoisachieved "coherent phase control of hydrogen disulfide moleculesby firing ultraviolet lasers of different wavelengths at them." Laser chemistry looks promising and is a way that chemistry isstill being expanded. Again, chemical reactions are the mainpart of a branch of chemistry. Here again, scientists are playing with chemical reactions.In April of 1995, a chemist named Peter Schultz and a physicistnamed Paul McEuen of the University of California at Berklyannounced that they could control chemical reactions molecule bymolecule. "The key to the technique is to put a dab of platinumon the microscopic tip of an atomic force microscope. (The tipof such a microscope is a tiny cantilever that rides like aphonograph needle just above the surface of a sample and reactsto forces exerted by the electrons beneath it.)" The Platinumacts like a catalyst, stimulating a reaction between tworeactants, just stimulating a reaction one molecule at a time.The molecules are stimulated in a pattern giving the wantedresults. This discovery opens doors for nanoengineering andmaterial sciences. It gives a good view of what happens, onemolecule at a time. Chemical reactions are a large part of chemistry. Thispaper is an overveiw of that extensive subject. It gives a goodidea about the history of chemical reactions as well as thefuture. Hopefully, there will be no end to the expansion ofchemistry and our knowledge. Since Scientists are stillexperimenting, chemical reactions will always be a part ofchemistry. 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