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Chemical Reactions
As
you drive your car, energy is released as gasoline combines explosively with
oxygen gas to give carbon dioxide and water vapor. This is an example of a
common, but exceedingly important, chemical change or chemical reaction. During
a cmplex series of reactions within the cells of your body, glucose and other
carbohydrates in food are consumed (metabolized) as they react with oxygen to
produce carbon dioxide and water vapor that are exhaled as you breathe. Both of
these examples demonstrate that some substances disappear and other substances
are produced during chemical reactions.
The
two reactions are similiar in several ways. In both cases, a compound
containing carbon reacts with oxygen to produce carbon dioxide and water.
Whether inside the human body, or in an automobile engine, or in the open air,
substance react to produce new and different substance. Once we understand what
is happening, we find that reactions are predictable.
In
each chemical reaction no atoms are created or destroyed; they are rearranged
to form different substances. In each cases, matter is conserved. There is no
change in total mass.
In
order to be able to write chemical equations for reactions like the ones
described, you must first be quite familiar with chemical formulas and what
they represent.
Chemical Reactions and Chemical
Equations
Chemical
reactions occur when substances undego fundamental changes in identity; one or
more substances are used up while one or more substances are formed.
Examples include the burning of gasoline
and of glucose. The substances present at the beginning of a reaction, the
starting materials, are called reactants. The substances produced by the
reaction are called products.
Chemical
equations are used to represent, symbolically, what is taking place during the
reaction. The reaction of glucose (a sugar) with oxygen gas during metabolism
to produce carbon dioxide and water can
be written as a chemical equation in words or in symbols (chemical formulas).
Glucose
+ oxygen → carbon dioxide +
water
C6H12O6
+ 6 O2(g) → 6 CO2(g) + 6 H2O(g)
The
reactants, or starting materials, are shown on the left side of the equation
and are separated by a plus sign (+). The products are shown on the right side
of the equation. Reactants and products are separated by an arrow (→), which is
read yields, or produces. The reaction of glucose with oxygen yields carbon
dioxide and water. The chemical equation can be written in words, but the
equation-written in chemical formulas-tell us much more.
The
chemical reactions described thus far were chosen because they pertain to
familiar processes. We will also be writing chemical equations for reactions
that are much less familiar, but no less important.
Special
symbols are often used in chemical equations to give specific information about
the substances involved, or the conditions for the reaction. Substances that
exist as a gas at the time of the reaction may be designted by a (g)
immediately after a formula, as was shown with the gases in the equation for
the combustion of glucose. The symbols (s)
and (l) may be used to identify solids and liquids, respectively. A
substance that is dissvolved in water, to make what we call an aqueous
sollution, may be identified by the symbol (aq) in the equation.
Sometimes
a substance is added to speed up a reaction that otherwise would take place at
a slow rate, or not at all. This substance, called a catalyst, is shown above
the arrow; it is not used up during the reactions. Enzymes are special catalyst
that are manufactured and used by the human body for each and every reaction
that occurs during metabolism.
Classifying Reactions
After
becoming familiar with balancing chemical equations, we should look more
closely at several different types of reactions and how they can be classified.
Chemists classify reactions by several approaches, but most reactions can be
placed into one or more of the following five categories.
1. Combustions
Reactions. During cobustion, compounds containing carbon, hydrogen, and
sometimes oxygen burn in air (consuming oxygen) to produce carbon dioxide and water.
2. Combination
(Synthesis) Reactions. When one elemen reacts or combines with another elemen
to produce a compound, we can say that a new substance is synthesized.
Reactions of this type are classified as combination or synthesis reactions.
They can be represented in a general way as follows.
A + B → AB
3. Decomposition
Reactions. A decomposition reaction is one in which a single compound,
symbolized as AB, is broken down into two or more simple substances. This type
of reaction can be represented as follows.
AB → A + B
4. Single-Replacement
Reactions. In single–replacement reactions, an element, symbolized as A, reacts
with a compound, BC, to take the place of one of the components of the
compound. This type of reaction can be represented by the following general
equations.
A + BC → AC + B (when A
is a metal)
A + BC → BA + C (when A
is nonmetal)
Many of the reactions
that fit into one of the first four categories involve on oxidation-reduction
process that will be described later in this chapter.
5. Double-Replacement
Reactions. In double-replacement reactions, two compounds, AB and CD, can be
thought of as “exchanging partners” to produce two different compounds, AD anf
CB.
AB + CD → AD + CB
The positive ion, A, in
the first compound combines with the negative ion, D, in the second
compound while the positive ion, C, of
the second compound combines with the negative on, B, in the first compound.
Combustion
When a substance
containing carbon and hydrogen (a hydrocarbon) undergoes complete combusition,
or burning, oxygen is consumed as carbon dioxide and water are produced. The general
unbalanced equation becomes
Hydrocarbon + O2 → CO2
+ H2O (not balanced)
Demonstrated the
balancing of the equation for the combusition of methane, CH4, a simple
hydrocarbon. The quickest way to get this done is to
·
Balance the carbon atoms first
·
Balance the hydrogen atoms second
·
Balance the oxygen atoms last
Synthesis (Combination)
Reactions
The
production of a single compound from the reaction of two or more substance can
be called a combination reaction or a synthesis reaction. Reactions of this
type have the general
A
+ B → AB
The
synthesis of ammonia gas, NH3(g) from N2(g) and H2(g) is an important
industrial process. The ammonia can be used directly as a fertilizer or to produce
other chemicals used in fetilizers, in explosives, and by industry to make
other chemicals.
N2(g)
+ 3 H2(g) → 2 NH3(g)
Decomposition Reactions
When
a single compound breaks down into two or more simpler substances, the reaction
involves decomposition, as indicated by the general equation.
AB
→ A + B
The
synthesis reaction of H2(g) with O2(g) to produce water releases energy. The
reserve reaction-decomposition-must take up energy.
The
energy to carry out the reaction must be supplied continually from a battery or
another source of direct electric current(dc). The process is called
electrolysis (Greek of separating by electricity). When the energy source is
turned off, the reaction ceases.
Single-Replacement
Reactions of Metals
When
a piece of copper wire is submerged in a solution of silver nitrate, a chemical
reaction occurs. Shiny, needlelike crystals of silver form on the copper wire.
The reaction is
Cu(s)
+ 2 AgNO3(aq) → Cu(NO3)2(aq) + 2 Ag(s)
(balanced)
In
this reaction, the copper is oxidized to Cu 2+ ions and has replaced the Ag+
ions of silver AgNO3. The Ag- ions are reduced to solid silver. Reactions like
these fit the single-replacement equation having the form
A
+ BC → AC + B
Double-Replacement
Reactions
Double-replacement
or methathesis reactions take the form
AB
+ CD → AD + CB
In
other words, ions in compounds AB and CD switch partners. This type of reaction
takes place in aqueous solution when at least one of the products is
·
An insoluble or nearly insoluble solid,
called a percipitate
·
A stable covalent compound, including
water and common gases
The driving force
behind these reactions andmany others is the formation of a stable product.
