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 oxidation-reduction.
These