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.
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In the Eighteenth Century Antoine-Laurent Lavoisier, in France, discovered an important detail in the understanding of the chemical reaction combustion, oxygшn (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 the 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 oxidation-reduction.
These are reactions that involve a change in oxidation number. It is a reaction if the oxidation number goes up. It is a reduction reaction if the oxidation number goes down.
It is now known that there are three types of chemical reactions. They are classified into three types: exoergic (exothermic), endoergic (endothermic), and aergic (athermic). In these cases, energy is supplied, but the different types of reactions initiate the energy differently.
Exoergic, or exothermic, reactions release energy during the reaction. Combustion is one of the major reactions that do this. The burning of wood, or any other fuel, gives off heat, and the burning of glucose in our bodies gives off both energy and heat.
Endoergic, or endothermic, reactions absorb energy during the reaction. The melting of an ice cube is an example of an endothermic reaction.
Aergic, or athermic, reactions neither give off nor absorb energy. There are very few cases in which this happens.
There are some things that must be considered in a chemical reaction. 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 the reaction.
The first thing is the reactants. Different reactants react at different speeds. Even the position of the reactants will affect the reaction rate.
The next thing is the catalyst that contributes a needed substance to the reaction. It Is part of the energy considerations. The catalyst is an outside substance that is included in the reaction, but is not consumed during the reaction like the reactants are. They cannot make impossible reactions occur, they only contribute to the reaction to increase the reaction rate. There are also such things as negative catalysts, or inhibitors. Inhibitors retard the reaction rate. This is also a way to control reactions. A good example in nature of a catalyst is in a firefly. The reaction that releases the light is complex. Lucifern, which the firefly makes naturally, is oxidized in the presence of luciferase, another natural enzyme, which acts as a catalyst in the reaction. Thus, the reaction makes an excited form of luciferase, which soon returns to its original state. Energy as light is released when the lucifrase returns to its normal state. The insect can easily control this reaction with an inhibitor it naturally makes.
Another contributor in this consideration is entropy. It is the measure of energy not available for work in the reaction that becomes energy moved to disorder. Entropy is simply a measurement 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 acid and a base react, they form water and a salt.
Acids and bases neutralize each other and form a salt as a byproduct. 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 of calcium hydroxide and phosphoric acid to produce calcium phosphate and water:
3Ca(OH)2 + 2H3PO4 -> Ca3(PO4)2 + 6H2O
The last detail is the reaction conditions. The temperature, humidity, and barometric pressure will affect the reaction. Even a slight change in any one of these could change the reaction.
There are many branches of Chemistry that use chemical reactions, infact, almost all of them. Here are some examples.
Photochemistry is one branch of chemistry that deals with chemical reactions. It has to do with the radiant energy of all kinds formed during chemical reactions. Photochemists will experiment with chemical reactions. They will perform reactions normally only possible at high temperatures in room temperature under ultra-violet radiation. The reaction rate can be controlled for observation by varying the intensity of the radiation. X-rays and gamma rays are commonly used in these procedures. The most important photochemical reaction is photosynthesis. Carbon-dioxide and water combine with chlorophyll as a catalyst to give off oxygen. Photochemical reactions are caused by photons that are given off by the light source. The reactant molecules absorb the photons and get excited. They are at such an excited state, they can decompose, ionize, cause a reaction with other molecules, or give off heat.
Another science that uses chemical reactions is Biochemistry. They use them to produce products that a person either can’t produce or cannot do as well as they should. The best example of this the production of insulin. It was first produced in very tiny beads until someone realized that the body does in a very similar way. The person was Robert B. Merrifeild. He was the first to urge scientists to study living systems for the answers to problems that could be solved with synthesizing chemical reactions in the body. This was actually the first step toward the development of bionics.
Scientists today are still toying with chemical reactions. They are trying to control them with lasers. Scientists are trying to use lasers to prod a chemical reaction that could go one way or another, the way they want it to. They want to direct the molecules in one direction. The control of photons to excite molecules and cause reactions has been elusive. Recently, though, chemist Robert J. Gordon at the University of Illinois achieved “coherent phase control of hydrogen disulfide molecules by firing ultraviolet lasers of different wavelengths at them.” Laser chemistry looks promising and is a way that chemistry is still being expanded. Again, chemical reactions are the main part of a branch of chemistry.
Here again, scientists are playing with chemical reactions. In April of 1995, a chemist named Peter Schultz and a physicist named Paul McEuen of the University of California at Berkly announced that they could control chemical reactions molecule by molecule. “The key to the technique is to put a dab of platinum on the microscopic tip of an atomic force microscope. (The tip of such a microscope is a tiny cantilever that rides like a phonograph needle just above the surface of a sample and reacts to forces exerted by the electrons beneath it.)” The Platinum acts like a catalyst, stimulating a reaction between two reactants, just stimulating a reaction one molecule at a time. The molecules are stimulated in a pattern giving the wanted results. This discovery opens doors for nanoengineering and material sciences. It gives a good view of what happens, one molecule at a time.
Chemical reactions are a large part of chemistry. This paper is an overview of that extensive subject. It gives a good idea about the history of chemical reactions as well as the future. Hopefully, there will be no end to the expansion of chemistry and our knowledge. Since Scientists are still experimenting, chemical reactions will always be a part of chemistry.
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“Dances With molecules,” Science News, Vol. 147, May 27, 1995
Eastman, Richard H., General Chemistry: Experimental and Theory, Holt, Rhinehart, and Winston Inc., 1970
“One Molecule at a Time”, Discover, January 1996
Pauling, Linus and Peter, Chemistry, W. H. Freeman and Co., 1975
“Reactions, Chemical,” Encyclopedia Americana, 1982, Vol. 23
“Reactions, Chemical,” Academic American Encyclopedia, 1991, Vol. 16
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