Chemical Reactions Essays (1782 words) - Chemistry,

Chemical Reactions Chemical reactions are the heart of chemistry. People have always known that they exist. The Ancient Greeks were the firsts to speculate on the composition of matter. They thought that it was possible that individual particles made up matter. Later, in the Seventeenth Century, a German chemist named Georg Ernst Stahl was the first to postulate on chemical reaction, specifically, combustion. He said that a substance called phlogiston escaped into the air from all substances during combustion. He explained that a burning candle would go out if a candle snuffer was put over it because the air inside the snuffer became saturated with phlogiston. According to his ideas, wood is made up of phlogiston and ash, because only ash is left after combustion. His ideas soon came upon some contradiction. When metal is burned, its ash has a greater mass than the original substance. Stahl tried to cover himself by saying that phlogiston will take away from a substance's mass or that it had a negative mass, which contradicted his original theories. In the Eighteenth Century Antoine-Laurent Lavoisier, in France, discovered an important detail in the understanding of the chemical reaction combustion, oxigine (oxygen). He said that combustion was a chemical reaction involving oxygen and another combustible substance, such as wood. John Dalton, in the early Nineteenth Century, discovered the atom. It gave way to the idea that a chemical reaction was actually the rearrangement of groups of atoms called molecules. Dalton also said that the appearance and disappearance of properties meant that the atomic composition dictated the appearance of different properties. He also came up with idea that a molecule of one substance is exactly the same as any other molecule of the same substance. People like Joseph-Lois Gay-Lussac added to Dalton's concepts with the postulate that the volumes of gasses that react with each other are related (14 grams of nitrogen reacted with exactly three grams of hydrogen, eight grams of oxygen reacted to exactly one gram of hydrogen, etc.) Amedeo Avogadro also added to the understanding of chemical reactions. He said that all gasses at the same pressure, volume and temperature contain the same number of particles. This idea took a long time to be accepted. His ideas lead to the subscripts used in the formulas for gasses. From the work of these and many other chemists, we now have a mostly complete knowledge of chemical reactions. There are now many classification systems to classify the different types of reactions. These include decomposition, polymerization, chain reactions, substitute reactions, elimination reactions, addition reactions, ionic reactions, and oxidation-reduction reactions. Decomposition reactions are reactions in which a substance breaks into smaller parts. As an example, ammonium carbonate will decompose into ammonia, carbon dioxide, and water. Polymerization reactions are reactions in which simpler substances combine to form a complex substance. The thing that makes this reaction unusual is that the final product is composed of hundreds of the simpler reagent (a substance that contributes to a chemical reaction) species. One example is the polymerization of terephthalic acid with ethylene glycol to form the polymer called Dacron, a fibre, or Mylar, in sheet form: nH2OC(C6H4)CO2H + nHOCH2CH2OH -* [...OC(C6H4)CO2CH2CH2O...]n + 2nH2O in which n is a large number of moles. A chain reaction is a series of smaller reactions in which the previous reaction forms a reagent for the next reaction. The synthesis of hydrogen bromide is a good example: H2 + Br2 -* 2HBr This is a simple equation that doesn't properly prove the reaction. It is very complex and starts with this: Br2 -* 2Br The next three reactions are related and should be grouped together. A substation reaction is a reaction in which a substance loses one or more atoms and replaces them with the same number of atoms of another element from another substance. Here is the example of chloroform that reacts with antimony triflouride: CHCl3 + SbF3 -* CHClF2 An elimination reaction is a reaction in which a compound is broken into smaller parts when heated. Here is an example when the same substance is heated and goes through another reaction: 2CHClF2 -* C2F4 + 2HCl An addition reaction is a reaction in which atoms are added to a molecule. If the added atoms are hydrogens, then the reaction is called a hydrogenization reaction. If Oleic acid is hydrogenized, this what you get: C18H34O2 + H2 -* C18H36O2 Another reaction is called an ionic reaction. It occurs between two ions and can happen very quickly. For example, when silver nitrate and sodium chloride are mixed you get silver chloride: AgNO3 + NaCl -* AgCl + NaNO3 The last type of reaction is called