Hein and Arena

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Oxidation-Reduction Chapter 17
Larry Emme
Chemeketa Community College
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Oxidation Number
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The oxidation number (oxidation state) of an atom
represents the number of electrons lost, gained,
or unequally shared by an atom.
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Oxidation numbers can be zero, positive, negative
or fractional.
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An oxidation number of zero means the atom has
the same number of electrons assigned to it as
there are in the free neutral atom.
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A positive oxidation number means the atom has
fewer electrons assigned to it than in the
neutral atom.
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A negative oxidation number means the atom has
more electrons assigned to it than in the neutral
atom.
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• The oxidation number of an atom that has gained
  or lost electrons to form an ion is the same as
  the positive or negative charge of the ion.

NaCl
Sodium has lost an electron.
Chlorine has gained an electron.
The charge on sodium is 1.
The oxidation number of sodium is 1.
The charge on chlorine is 1.
The oxidation number of chlorine is -1.
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• In covalently bonded substances, oxidation
  numbers are assigned by an arbitrary system based
  on relative electronegativities.

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For symmetrical covalent molecules each atom is
assigned an oxidation number of 0 because the
bonding pair of electrons is shared equally
between two like atoms of equal electronegativity.
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When the covalent bond is between two unlike
atoms, the bonding electrons are shared unequally
because the more electronegative element has a
greater attraction for them.
Oxidation Number -1
Oxidation Number 1
both shared electrons are assigned to chlorine
shared pair of electrons
unequal electron sharing
there is a partial transfer of an electron to
chlorine
after assignment chlorine has one more electron
than neutral chlorine
after assignment hydrogen has one less electron
than neutral chlorine
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Many elements have multiple oxidation numbers
N2 N2O NO N2O3 NO2 N2O5
N oxidation number 0 1 2 3 4 5 5
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Rules for Determining the Oxidation Number of an
Element Within a Compound

• Step 1 Write the oxidation number of each known
  atom below the atom in the formula.
• Step 2 Multiply each oxidation number by the
  number of atoms of that element in the compound.
• Step 3 Write an expression indicating the sum of
  all the oxidation numbers in the compound.
  Remember The sum of the oxidation numbers in a
  compound must equal zero.

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Determine the oxidation number for sulfur in
sulfuric acid.
H2SO4
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Determine the oxidation number for carbon in the
oxalate ion.
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Oxidation-Reduction
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Oxidation-reduction (redox) is a chemical process
in which the oxidation number of an element is
changed.
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Redox may involve the complete transfer of
electrons to form ionic bonds or a partial
transfer of electrons to form covalent bonds.
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• Oxidation occurs when the oxidation number of an
  element increases as a result of losing
  electrons. OIL

• Reduction occurs when the oxidation number of an
  element decreases as a result of gaining
  electrons. RIG
• In a redox reaction oxidation and reduction occur
  simultaneously.

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Leo the Lion says
GER
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• Oxidizing agent The substance that causes an
  increase in the oxidation state of another
  substance.

• The oxidizing agent is reduced in a redox
  reaction.

• Reducing agent The substance that causes a
  decrease in the oxidation state of another
  substance.
• The reducing agent is oxidized in a redox
  reaction.

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Balancing Oxidation-Reduction Equations
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The Loop Method
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Balance the equation Sn HNO3 ? SnO2
NO2H2O
Step 1 Assign oxidation numbers to each element
to identify the elements being oxidized and those
being reduced. Write the oxidation numbers below
each element to avoid confusing them with ionic
charge.
Sn HNO3 ? SnO2 NO2 H2O
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Balance the equation Sn HNO3 ? SnO2
NO2H2O
Step 2 Write two new equations, using only the
elements that change in oxidation number. Then
add electrons to bring the equations into
electrical balance.
Sno ? Sn4 4e-
4e-
4N5 1e- ? N4
1e-
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Balance the equation Sn HNO3 ? SnO2
NO2H2O
Step 3 Multiply the two equations by the smallest
whole numbers that will make the electrons lost
by oxidation equal to the number of electrons
gained by reduction.
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Balance the equation Sn HNO3 ? SnO2
NO2H2O
Step 4 Transfer the coefficient in front of each
substance in the balanced oxidation-reduction
equations to the corresponding substances in the
original equation.
Sn 4HNO3 ? SnO2 4NO2 H2O
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Balance the equation Sn HNO3 ? SnO2
NO2H2O
Step 5 Balance the remaining elements that are
not oxidized or reduced to give the final
balanced equation.
Sn 4HNO3 ? SnO2 4NO2 2H2O
2
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Balancing Practice
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Multiple Change Balancing
This ones hard!
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Answer to Multiple Change
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Activity Seriesof Metals
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activity series A listing of metallic elements
in descending order of reactivity.
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Sodium (Na) will displace any element below it
from one of its compounds.
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Mg(s) PbS(aq) ? MgS(aq) Pb(s)
Magnesium is above lead in the activity series.
Magnesium will displace lead from one of its
compounds.
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Ag(s) CuCl2(aq) ? no reaction
Silver is below copper in the activity series.
Silver will not displace copper from one of its
compounds.
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Electrolytic andVoltaic Cells
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electrolysis The process whereby electrical
energy is used to bring about a chemical change.
electrolytic cell An electrolysis apparatus in
which electrical energy from an outside source is
used to produce a chemical change.
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cathode The negative electrode.
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Electrolysis ofHydrochloric Acid
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In an electrolytic cell electrical energy from
the voltage source is used to bring about
nonspontaneous redox reactions.
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Hydronium ions migrate to the cathode and are
reduced.
H3O 1e- ? Ho H2O
Ho Ho ? H2
Cathode Reaction
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Chloride ions migrate to the anode and are
oxidized.
Cl-? Clo e-
Clo Clo? Cl2
Anode Reaction
17.3
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The hydrogen and chlorine produced when HCl is
electrolyzed have more potential energy than was
present in the hydrochloric acid before
electrolysis.
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The Zinc-Copper Voltaic Cell
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voltaic cell A cell that produces electrical
energy from a spontaneous chemical reaction.
(Also known as a galvanic cell).
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When a piece of zinc is put in a copper(II)
sulfate solution, the zinc quickly becomes coated
with metallic copper. This occurs because zinc
is above copper in the activity series.
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Zn(s) CuSO4(aq) ? ZnSO4(aq) Cu(s)
Zinc is above copper in the activity series.
Zinc will displace copper from one of its
compounds.
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If this reaction is carried out in a voltaic
cell, an electric current is produced.
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loss of electrons
gain of electrons
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LeClanche Cell

• The LeClanche Cell was described by Georges
  LeClanche (1839-1882) in 1867. The two electrodes
  are carbon and zinc, with a sal ammoniac
  electrolyte. The carbon electrode is mixed with
  manganese peroxide. This battery was used mainly
  for intermittent service, such as ringing
  electric bells.

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Dry Cell battery
Anode Zn ? Zn2 2 e Zn2 2 NH3 ?
Zn(NH3)22 Removal of NH3
Uses Portable radios, toys, flashlights. Advanta
ges Inexpensive, safe, many sizes. Disadvantages
High current drain, NH3 builds up causing
voltage drop, short shelf life.
Cathode 2 NH4 2 e ? 2 NH3 H2 H2 MnO2
? MnO H2O Removal of H2
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Alkaline Battery
Uses Same as dry cell. Advantages No voltage
drop, longer shelf life. Disadvantage
Expensive
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Super-iron batteryNew type of Alkaline Battery
Uses Same as dry cell. Advantages works well
in high-drain-rate electronics. Disadvantages
Expensive.
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Lead Storage Battery
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Mercury Battery
Used in calculators, watches, hearing aids,
cameras, and devices where small size is needed.
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Lithium Battery
Because it is light in weight, and has a large
voltage (3.4 V per cell) these are used in
pacemakers, cell phones, laptops, camcorders.
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Hydrogen Fuel Cell
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Honda FCX Clarity Fuel Cell Vehicle
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Downs cell for sodium production
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Copper Electrolysis
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Electrorefining of copper metal
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Zinc strips help protect the iron hull of an oil
tanker from oxidization. This strip is attached
to the hulls interior surface.
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Tarnish on silverware is a coating of silver
sulfide (Ag2S). Tarnish begins when silver atoms
come into contact with hydrogen sulfide (H2S) in
the air. The silver ions and sulfide ions combine
to form blackish silver sulfide. Aluminum atoms
can help restore the silver to its shiny self.
Directions at http//faculty.chemeketa.edu/lemme/
CH20122/handouts/Removing Silver Tarnish.pdf
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Corrosion
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Corrosion Prevention
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Dental Voltaic Cell
Al ? Al3 3e
O2 4 H 4e ? 2 H2O
The short circuit between the Al foil and the
filling produces a current that is sensed by
the nerve of the tooth.
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• In 1936, while excavating ruins of a
  2000-year-old village near Baghdad, workers
  discovered mysterious small vase. A 6-inch-high
  pot of bright yellow clay dating back two
  millennia contained a cylinder of sheet-copper 5
  inches by 1.5 inches. The edge of the copper
  cylinder was soldered with a 60-40 lead-tin alloy
  comparable to today's solder. The bottom of the
  cylinder was capped with a crimped-in copper disk
  and sealed with bitumen or asphalt. Another
  insulating layer of asphalt sealed the top and
  also held in place an iron rod suspended into the
  center of the copper cylinder. The rod showed
  evidence of having been corroded with an acidic
  agent.

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The ancient battery in the Baghdad Museum
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The jar was found in Khujut Rabu just outside
Baghdad and is composed of a clay jar with a
stopper made of asphalt. Sticking through the
asphalt is an iron rod surrounded by a copper
cylinder. When filled with vinegar or any
other electrolytic solution - the jar produces
about 1.1 volts.
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Atlantis Light-Bulb? From Egypt Hieroglyphics
Light Blubs
Filament
Cord
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http//www.world-mysteries.com/sar_lights_fd1.htm
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The End