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5.S: Chemical Reactions (Summary)
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The processes that occur during a chemical change can be represented using a
chemical equation
. In a chemical equation, the chemical formulas for the substance or substances that
undergo
the chemical reaction (the
reactants
) and the formulas for the new substance or substances that are formed (the
products
) are both shown, and are linked by an
arrow
. The arrow in a chemical equation has the properties of an “equals sign” in mathematics, and because of this,
in a chemical equation, there must be the same number and types of atoms on each side of the arrow.
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A chemical equation in which the same number and types of atoms appear on each side of the arrow is called balanced. In order to balance an equation, insert
coefficients
in front of the appropriate reactants or products until there are the same number and types of atoms on both sides of the arrow.
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In a
synthesis reaction'
, elements or compounds undergo reaction and combine to form a
single new substance
.
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In a
decomposition'
reaction, a single compound will break down to form two or more new substances. The substances formed can be elements, compounds, or a mixture of both.
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In a
single-replacement'
(single-displacement)
reaction,
an element and a compound will react so that their elements are switched. As a general rule,
metals
will replace
metals
in compounds and
non-metals
will typically replace
non-metals
.
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A
double-replacement'
(double-displacement) reaction, two ionic compounds in aqueous solution switch anions and form two new compounds. In order for a chemical reaction to occur, one of the new compounds that is formed must be
insoluble
in water, forming a solid
precipitate
or a gas.
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The
oxidation number'
of an element represents the total number of electrons which have been
removed
(a positive oxidation state) or
added
(a negative oxidation state) to get the element into its present state. The term
oxidation
describes the
loss
of electrons by an element and an
increase
in oxidation state; the term
reduction
describes the
gain
of electrons and a
decrease
in oxidation state.
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A chemical reaction in which oxidation numbers undergo a change is called a
redox reaction
. In a redox reaction, the element that “
loses electrons
” is said to be
oxidized
and will have
an increase in its oxidation number
. The element that “
gains electrons
” in a redox reaction is said to be
reduced
and will have a
decrease in its oxidation number
.
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In a simple
synthesis
reaction
involving reaction of elements, the product will be a compound containing both elements. Write the product considering the common charges that the elements adopt as ions or the number of bonds that the elements typically form in molecules.
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In a simple
single-replacement
reaction, (in general)
metals
(including
carbon
and
hydrogen
) will tend to replace
metals
and that
non-metals
will replace
non-metals
.
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In a
double-replacement
reaction, the anions and cations simply of the two compounds simply exchange. In order for a reaction to occur, one of the products must
precipitate
, otherwise no chemical reaction has occurred. Changes in oxidation numbers do not occur in double-replacement reactions.
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Solubility trends can be predicted using a simple set of rules shown in Table 5.5; you should review there rules, remembering to apply them
in order
.
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The energy required to initiate a chemical reaction is called the
activation energy
. The greater the activation energy, the slower, or less favorable a reaction will be. The magnitude of the activation energy is directly linked to the
rate
of a chemical reaction.