There are two types of Chemical
Changes:
Chemical Reactions
“REDOX”
1. Reactions in which ions are being
rearranged with no change in their
oxidation states. METATHESIS
2. Reactions in which electrons are
being transferred from atom(s) to
atom(s). REDOX
Metathesis Reactions
Reaction Types
Metathesis
Double
Displacement
Neutralization
•
Redox
Precipitation
Single
Displacement
Ion Exchange
Ions are combined to form new compounds
Combustion
Synthesis
Decomposit
Electron Transfer
•
Electrons are lost from one atom, or group of atoms, and gained by
another atom, or group of atoms, resulting in new atomic interactions
forming new compounds
•
Metathesis Reactions
1. Precipitation reactions
are metathesis
reactions in which an
insoluble compound is
formed from two
aqueous solutions.
– The solid precipitates
out of the solution
much like rain or
snow precipitates out
of the air.
Metathesis reactions occur when two ionic
aqueous solutions are mixed and the ions
switch partners.
AX + BY → AY + BX
Metathesis reactions remove ions from
solution in two ways:
1. form an insoluble solid.
2. form predominantly unionized molecules
like H2O(l) and CO2(g).
Ion removal is the driving force of metathesis
reactions.
Metathesis Reactions
2. Acid-Base (neutralization) Reactions
– Formation of the nonelectrolyte H2O
acid + base → salt + water
•A salt is another
name for an ionic
compound
formed from the
cation of a base
and an anion
from an acid
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Metathesis Reactions
3. Double Displacement reactions
– Are reactions where two aqueous
solutions result in the formation of nonelectrolyte compounds other than water.
Example:
2 HCl(aq) + Na2SO3( aq) → 2 NaCl( aq ) + H2O(l ) + SO2(g )
Types of Redox Rxn’s
Redox Reactions
Reactions in which one or more
electrons are transferred
2Na(s) + Cl2(g) 2NaCl(s)
Species loosing
electrons is said to be
oxidized and the species
gaining electrons is said
to be reduced
Combustion of Methane
CH4 + O2 CO2 + H2O
• Carbon looses electrons and oxygen
• Synthesis
gains electrons
• Decomposition
• Single Replacement
Electrons are
transferred
in most cases
• Combustion
•Notice that in the reaction, there are no
ions present as either reactants or
products, however, electrons are still
transferred
•An understanding of oxidation states will
show the transfer of electrons
Oxidation Numbers
• In order to keep
track of what
loses electrons
and what gains
them, we assign
oxidation
numbers.
Oxidation States
(Oxidation Numbers)
• A system of book-keeping for
electrons in molecules or ions
• The imaginary charges
atoms would have in molecules
and real charges in ionic
compounds
2
• Oxidation states provide a way to
track electrons in chemical reactions
• Especially useful for reactions
containing covalent compounds which
have no charges
* Note
Assigning Oxidation Numbers
1. The oxidation state of
an atom in an element is
zero
Charges are written:
Fe2+ , Cl1-
• Na(s), O2(s), Hg(l)
Oxidation states are written:
Fe+2 , Cl-1
2. The oxidation state of a
monatomic ion is the same
as its charge
• Na+, Cl-, N-3, Ca+2
• Alkali metals, alkaline earth
metals, Al, Ga, Ge.
4. Oxygen is always -2 in
its compounds
H2O, CO2
Exceptions: Peroxides
(containing O22-) in which
oxygen is -1
3. Fluorine is always -1
in its compounds
HF ,
H+1 F-1 ,
PF3
P+3 F-1
Notice the oxidation states are written
+1, whereas a charge would be written
1+
5.Hydrogen is +1 when
bonded to a non-metal
and -1 when bonded to
a metal
H2O, HCl, HN3
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6. Other oxidation states are
calculated from algebraic
sum of known states
Transitions, metalloids, and
usually carbon, silicon, and boron.
The sum of the oxidation numbers in
a neutral compound is 0; The sum of
the oxidation numbers in a polyatomic
ion is the charge on the ion.
1. Identify the oxidation states for each
atom in each of the following
compounds/ions:
• SF6
• PbS
• Al3+
• N2
• HClO
• NaNO2
• CO2
• CaCO3
• AsH2
• HNO2
• K2Cr2O7
• P4S2
• NO3
-
• NH4
Identify the oxidation state for each of
the following compounds in the reaction:
CH4 + O2 CO2 + H2O
Carbon was oxidized and oxygen
was reduced
+
Oxidation
• CN, CN-
Notice, carbon has changed from a
–4 to a + 4 oxidation state and oxygen
has changed from 0 to –2 and –2
Recognizing a Redox
Reaction
Reaction Type
7. For non-described atoms in
compounds, the atom with the
greatest electronegativity is
assigned a negative oxidation
state equal to its imaginary
charge in an ionic compound
Oxidation and Reduction
Reduction
In terms of oxygen
gain
loss
In terms of hydrogen
loss
gain
In terms of electrons
loss
gain
• A species is oxidized when it loses
electrons.
Here, zinc loses two electrons to go from
neutral zinc metal to the Zn2+ ion.
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Oxidation and Reduction
• A species is reduced
when it gains electrons.
• What is reduced is the oxidizing agent.
Here, each of the H+
gains an electron and
they combine to form H2.
H+ oxidizes Zn by taking electrons from it.
• What is oxidized is the reducing agent.
Zn reduces H+ by giving it electrons.
Common Oxidizing and
Reducing Agents
Metals
(Cu) are
reducing
agents
Oxidation and Reduction
Metals
(Na, K,
Mg, Fe)
are
reducing
agents
HNO3 is an
oxidizing
agent
Cu + HNO3 -->
2 K + 2 H2O -->
-->
-->
Cu2+ + NO2
2 KOH + H2
2. Identify the species being reduced and oxidized in each of
the following reactions. Then label the oxidizing agent
and the reducing agent in the reaction:
a. 2C(s) + 3F2(g) → 2CF3(s)
b. S8 (s) + 8O2 (g) → 8SO2(s)
c. 2H2O → 2H2 (g) + O2 (g)
d. Mg(s) + O2(g) → MgO(s)
e. 2KClO3 (s) → 2KCl (s) + 3O2 (g)
f. Ca (s) + O2 (g) → CaO(s)
g. Cu(s) + AgNO3(aq) → Cu(NO3)2(aq) + Ag(s)
h. Na(s) + H2S(aq) → Na2S(aq) + H2(g)
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