Transcript Oxidation-Reduction Reactions

Oxidation-Reduction Reactions
REDOX Reactions
Oxidation State
• Oxidation numbers are very similar
to charge.
• There are some different rules for
assigning them.
Rules for Oxidation State
• 1. The oxidation number for any
uncombined element is zero
• Ex. The oxidation state for Na is
zero.
Rule 2
• The oxidation number for a
monatomic ion equals the charge
on the ion.
• Ex. Cl-1 has an oxidation of -1.
Rule 3
• The more electronegative element
in a binary compound is assigned
the number equal to the charge it
would have if it were an ion.
• Ex. The oxidation number of O in
NO is -2 because oxygen is more
electronegative than nitrogen.
Rule 4
• The oxidation number of fluorine in
a compound is always -1.
• Ex. The oxidation number of F in
LiF is -1.
Rule 5
• Oxygen has an oxidation number of
-2 unless it is combined with
fluorine, when it is +2, or it is in a
peroxide such as H2O2, when it is 1.
• Ex. The oxidation of O in NO2 is -2.
Rule 6
• The oxidation state of hydrogen in
most of its compounds is +1 unless
it is combined with a metal, in
which case it is -1.
• Ex. The oxidation of H in LiH is -1.
The oxidation of H in HCl is +1.
Rule 7
• In compounds, Group 1 and 2
elements and aluminum have
oxidation numbers of +1, +2, and
+3, respectively.
• Ex. The oxidation number of Ca in
CaCO3 is +2.
Rule 8
• The sum of the oxidation numbers
of all atoms in a neutral compound
is 0.
• Ex. The oxidation number of each
atom in CaCO3 can be found by
knowing the rules above and
knowing the compound is neutral
• Ca = +2 (rule 7)
• O = -2 x 3 atoms = -6 (rule 5)
• C = +4 (rule 8—balances out the
charge)
Rule 9
• The sum of the oxidation numbers
of all atoms in a polyatomic ion
equals the charge of the ion.
• Ex. The oxidation of each element
in H2PO42- can be determined by
knowing the overall charge is -2
• H = +1 x 2 atoms = +2 (rule 6)
• O = -2 x 4 atoms = -8 (rule 5)
• P = +4 (rule 9)
Types of Reactions
5 types plus a new one
Type of Reactions
Chemical reactions are
classified into five general
types
Combination
Decomposition
Single Replacement
Double Replacement
Combustion
Combination (Synthesis)
Two or more elements or simple
compounds combine to form
(synthesize) one product
•
A + B
AB
•
2Mg + O2  2MgO
•
2Na + Cl2  2NaCl
•
SO3 + H2O

H2SO4
Decomposition
One substance is broken down (split) into
two or more simpler substances.
•
AB 
A + B
•
2HgO
2Hg + O2
•
2KClO32KCl + 3 O2
Learning Check R1
Classify the following reactions as
1) combination or
2) decomposition:
___A. H2 + Br2 2HBr
___B. Al2(CO3)3 Al2O3 + 3CO2
___C. 4 Al + 3C  Al4C3
Solution R1
Classify the following reactions as
1) combination or 2)
decomposition:
_1_A. H2 + Br2  2HBr
_2_B. Al2(CO3)3  Al2O3 + 3CO2
_1_C. 4 Al + 3C 
Al4C3
Single Replacement
One element takes the place of an
element in a reacting
compound.
A + BC  AB + C
Zn + 2HCl  ZnCl2 + H2
Fe +CuSO4 FeSO4 + Cu
Learning Check R2
Classify the following reactions as
1) single replacement
2) double replacement
__A. 2Al + 3H2SO4  Al2(SO4)3 + 3H2
__B. Na2SO4 + 2AgNO  Ag2SO4 + 2NaNO3
__C. 3C + Fe2O3 
2Fe + 3CO
Solution R2
Classify the following reactions as
1) single replacement
2) double replacement
1_A. 2Al + 3H2SO4
 Al2(SO4)3 +
3H2
2_B. Na2SO4 + 2AgNO3
 Ag2SO4 +
2NaNO3
1_C. 3C + Fe

2Fe + 3CO
Combustion
A reaction in which a compound (often
carbon) reacts with oxygen
C + O2
CH4 + 2O2
 CO2

C3H8 + 5O2 
CO2 + 2H2O
3CO2 + 4H2O
C6H12O6 + 6O2 6CO2 + 6H2O
Learning Check R3
Balance the combustion equation
___C5H12 + ___O2
___CO2 + ___H2O
Solution R3
Balance the combustion equation
1 C5H12 + 8 O2
5 CO2 + 6 H2O
Oxidation-Reduction Reactions
REDOX Rxns
Oxidation and Reduction
 Reactions that involve a loss or gain of
electrons
 Occurs in many of the 4 types of reactions
and combustion
 Important in food metabolism, batteries,
rusting of metals
Requirements for Oxidization-Reduction
 Electrons are transferred
 Two processes occur
Oxidation = Loss of electrons (LEO)
Zn
Zn2+ + 2e-
Reduction = Gain of electrons (GER)
Cu2+ + 2e-
Cu
Balanced Red-Ox Equations
Combine the oxidation and reduction reactions to
make
Loss of electrons = Gain of electrons
Zn + Cu2+ + 2e-
Zn2+ + 2e- + Cu
Zn + Cu2+
Zn2+ + Cu
Gain/Loss of Hydrogen
In organic and biological
reactions
oxidation = Loss of H
reduction = Gain of H
Half-Reaction
• The part of a reaction involving
oxidation or reduction alone.
• The overall reaction is the sum of
the two half reactions
• Ex.
• 3Cu  3Cu2++ 6 e• 2NO3-1 + 6 e- + 8H+ 2NO + 4 H2O
• 3Cu + 2NO3-1 + 8 H+ 3Cu2+ + 2NO + 4H2O
Learning Check R3
Identify the following as an 1) oxidation or a
reduction process:
__A.
Sn
Sn4+ + 4e-
__B.
Fe3+ + 1e-
Fe2+
__C.
Cl2 + 2e-
2Cl-
Solution R3
Identify the following as an 1) oxidation or a
reduction process:
1_ A.
Sn
Sn4+ + 4e-
2_ B.
Fe3+ + 1e-
Fe2+
2_ C.
Cl2 + 2e-
2Cl-
Learning Check R4
In light-sensitive sunglasses, UV
light initiates an oxidationreduction reaction
Ag+ + ClAg + Cl
A. Which reactant is oxidized
1) Ag+
2) Cl3) Ag
B. Which reactant is reduced?
1) Ag+
2) Cl3) Cl
Solution R4
In light-sensitive sunglasses, UV light
initiates an oxidation-reduction reaction
Ag+ + ClAg + Cl
A. Which reactant is oxidized
2) ClClCl + eB. Which reactant is reduced?
1) Ag+
Ag+ + eAg
Oxidizing and Reducing Agents
Reducing Agents
• A substance that has the potential
to cause another substance to be
reduced.
• Lose electrons (gain higher charge)
Oxidizing Agent
• Substance that has the potential to
cause another substance to be
oxidized
• Gains electrons (become more
negative)
Summary Chart of Terminology
Term
Change in
oxidation
number
Change in
electron
population
Oxidation
Increases
Loss of
electrons
Reduction
Decreases
Gain of
electrons
Oxidizing
agent
Decreases
Gains
electrons
Reducing
agent
Increases
Loses
electrons
Autooxidation
• A process in which a substance acts
as both an oxidizing agent and a
reducing agent
Electrochemistry
Electrodes, half cells, anodes, cathodes,
voltaic and electrolytic cells
Electrochemistry
• The branch of chemistry that deals
with electricity-related applications
of oxidation-reduction
A bright idea!
• As redox rxns occur, electrons are
transferred which also produces
heat.
• But if the substances are separated
by a porous barrier then they are
able to transfer electricity instead
of heat
Electrode
•Conductor used to
establish electrical
contact with a
nonmetallic part of a
circuit such as an
electrolyte
Half-cell
• A single electrode immersed in a
solution of its ions.
Anode
• One half cell is the anode which is
the electrode where oxidation
takes place.
Cathode
• Other half cell where reduction
takes place.
Electrochemical cell
• A system of electrodes and
electrolytes in which either
chemical reactions produce
electrical energy of an electric
current produces chemical change.
Voltaic Cells
•If the redox rxn in an electrochemical cell
occurs spontaneously and produces electrical
energy then it is a voltaic cell.
Types of Dry Cell Batteries
Zinc-Carbon Dry Cells
Alkaline Batteries
Mercury batteries
Zinc-Carbon Dry Cells
•Ex. Flashlight batteries
•Have Zn container which is the anode
•Filled with moist paste of MnO2, graphite,
and NH4Cl
Alkaline Batteries
•Ex. Batteries found
in small, portable
devices such as CD
players
•Very similar to the
Zn-C but it does not
contain the C which
allows them to be
smaller
•Uses paste of Zn
and KOH instead of
a solid metal anode.
Mercury Batteries
•Ex. Calculator batteries
•The anode half rxn is the same as the
alkaline but the cathode involves HgO
Rechargeable cells
•Combination of voltaic and electrolytic cells
•Acts as a voltaic cell during use and as an
electrolytic cell during charging.
•Ex. Car battery