Unit 10 Oxidation Reduction Reactions Redox

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Unit 10 Oxidation Reduction Reactions(Redox)
Electrochemistry Chapter 20, Read pp657 673.

• Oxidation Reduction Rxn A chemical reaction
  in which Electrons are transferred from one atom
  to another.
• everyday uses batteries, extraction of
  metals from ores, preventing corrosion.
• Metal/nonmetal rxn, a simple Redox Rxn.
• ex. Na(s) ½Cl2(g) ? NaCl(s)

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B. Oxidation and Reduction

• Oxidation Loss of one or more electrons (the
  element is said to be oxidized).
• Reduction Gain of one or more electrons (the
  element is said to be reduced).
• LEO the lion goes GER

Where there is a reduction, there must be an
oxidation, so the overall reaction is called
REDOX.
xidation
ain
eduction
ose
lectron
lectron
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C. Oxidation Numbers (States)

• How can we tell, when looking at a reaction, if
  electrons were transferred?
• For ionic bonds, charges change, so assign
  charges and look for changes.
• For covalent bonds there are no charges to look
  at, so we assign make believe charges called
  oxidation numbers, to keep track of electrons.
• Assign either charges or oxidation numbers to all
  elements on each side of a reaction and look for
  changes if any change, then the reaction is REDOX

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D. Assigning Oxidation Numbers (States)

• Assigning Oxidation Numbers (Ox. No.) for
  single elements
• 1. Ox. No. for an element in its free state is
  zero.
• ex. Cu has 29 ps and 29 es so total
  charge 0.
• Zn
• H2 S8
• 2. Ox no. of a single atom ion is equal to the
  charge on the ion.
• ex. Cu2 H1
  O-2

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Assigning Ox. No. (States) for Elements in the
Combined State ( compounds or polyatomic ions)

• 3. If an element has only one possible charge
  listed for it in the Periodic Table of the
  Elements, that is its ox. no. when in the
  combined state.
• Group 1 metals are always 1 in cmpds or ions.
• Group 2 metals are always 2 in cmpds or ions.
• Group 3 elements are always 3 in cmpds or ions.
• (exception Tl can be 1 or 3)
• Other common elements F -1, Ag 1, Zn 2
• ex. What is the oxidation number of Na in
  NaCl?

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• 4. Some elements with more than one charge have
  one most common ox no. that should be known.
• Oxygen is usually assigned the ox no. 2.
• exceptions Peroxides O is 1 ex. H2O2,
  Na2O2.
• and O is 2 in OF2.
• Hydrogen is usually assigned the ox no. 1.
• exceptions When bonded to Group 1 or 2
  metals
• (called hydrides) ex.
  NaH, MgH2.
• 5. All other elements can have multiple ox nos.
  The correct ox no. for a specific cmpd. can be
  calculated as follows

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• The sum of the ox no. of all atoms of all
  elements in a compound must equal zero!
• The sum of the ox no. of all atoms of all
  elements in a polyatomic ion equals the charge on
  the ion!
• ex. Find the oxidation states for all elements
  in the following species (compounds or ions).
• a. Cl2 b. Cl- c. MgCl2 d.
  SO3
• e. H2SO3 f. SO32 g. MgH2 h.
  MgO2
• i. Al2(SO4)3

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E. Oxidation Numbers and Redox Rxns.

• Oxidation Numbers (or STATES) have two uses when
  analyzing a chemical equation
• To tell if an equation is a Redox Rxn look at
  ox nos. for all elements in the equation, if any
  change from reactant to product, its Redox!
• ex. Which of the following equations is a redox
  rxn?
• (1) HCl(aq) NaOH(aq) ?NaCl(aq) H2O(l)
• (2) AgNO3(aq) NaCl(aq) ? NaNO3(aq) AgCl(s)
• (3) CaCO3(s)?CaO(s) CO2(g)
• (4) 2HCl(aq) Zn(s) ? ZnCl2(aq) H2(g)

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2. To tell which substance is oxidized and which
substance is reduced.

• When a substance is oxidized, it loses electrons.
  Should its ox no. increase or decrease?
• When a substance undergoes oxidation, its
  oxidation number increases!
• When a substance undergoes reduction, its
  oxidation number decreases!
• ex. For the reaction 2HCl Zn ?ZnCl2 H2,
  which substance is oxidized and which is reduced?
• H 1 ? 0 Reduction Zn 0?2
  Oxidation so H1 is reduced and Zn0 is oxidized.
• Notice Reactants are identified as oxidized or
  reduced!

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F. Oxidizing and Reducing Agents

• An Agent causes something to occur.
• Oxidizing agent causes another substance to lose
  electrons, it must be the substance gaining
  electrons!
• Oxidizing Agent is the substance Reduced!
• Reducing Agent is the substance Oxidized!
• ex. In the rxn Zn(s) Cu2(aq)?Zn2(aq)
  Cu(s), which substance is the reducing agent?
• (1) Zn (2) Cu2 (3) Zn2
  (4) Cu

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G. Half Reactions

• Overall equations dont show electrons. Half
  reactions show only the oxidation or only the
  reduction with the electrons written in!
• Oxidation Loss of Electrons electrons are
  written as a product M ?M e

0 1 1
Note Total charge on each side of half rxn must
be equal.
Conservation of Charge!
2. Reduction Gain of Electrons electrons
are written as a reactant X 2e ? X2
Redox rule
Red electrons on left
Ox electrons on right
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• Write out the half rxns for the following
  equations
• ex. Pb(s) Ag(aq) ? Pb2(aq) Ag(s)

oxidation
Pb0 ? Pb2
2e
Note electrons go on side with highest ox no.
reduction
Ag ? Ag0
e
1 -2 0 1 2
4 -2
ex. H2S(g) O2(g) ? H2O(l) SO2(g)
oxidation
S2 ?S4
6e
reduction
2e
O0 ? O2
Remember the Redox Rule!
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H. Balancing Redox Reactions

• Since Redox rxns involve transfer of electrons,
  the charge must be balanced on both sides of a
  rxn in order to have an equal transfer
  Conservation of Charge! Adjust coefficients to
  make charge equal.
• ex. Balance the following redox rxn
• Pb(s) Ag(aq) ? Pb2(aq)
  Ag(s)

2 2
Pb0 ? Pb2
2e
Ag ? Ag0
e
2( )
2Ag 2e ? 2Ag0
Multiply to make electron exchange equal.
Place these coefficients into the original
equation.
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II. Electrochemistry
A. Electrochemical cell - Produces electricity
from a spontaneous redox reaction that is
divided. Electrons are moved from one atom to
another through a wire
1. Parts of the Electrochemical Cell
a. Half Cell the reaction is set up in separate
containers as oxidation and reduction half cells.
Zn
Cu
Zn2
Cu2
ex. Danielle Cell A piece of Zn in Zn2
solution and Cu in Cu2 solution make up the two
half cells
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b. Conducting Wire since zinc is a more
reactive metal than copper (see Table J), it will
lose electrons (oxidation) and Cu2 ions will
gain them (reduction). The electrons travel
through a wire from the oxidation half reaction
to the reduction half reaction making an electric
current!
2e-
2e-
2e-
2e-
electrons?
2e-
Zn
2e-
Cu
Cu2
2
Oxidation
Reduction Zn ?Zn2 2e-
Cu2 2e- ? Cu
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c. Salt Bridge a tube containing a salt
solution (such as KCl) that is used to allow ion
migration.
2e-
2e-
2e-
Zn
Zn
Cu
anions
e- e-
2e-
2e-
cations
Cu2
Zn2
As Zn2 goes into solution, 2Cl- come out of the
salt bridge
As Cu2 comes out of solution, 2K come out of
the salt bridge
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d. Anode and Cathode

• An anode is the metal electrode where oxidation
  takes place. In the Danielle cell, Zinc is the
  anode
• Zn(s) ? Zn2(aq) 2 e- Since the metal is
  losing electrons, it becomes a positive ion and
  dissolves in water. The anode will decrease in
  mass!
• A cathode is the metal electrode where reduction
  takes place. Copper is the cathode Cu2 2e-?Cu
• Since ions are gaining electrons and making
  solid metal, the cathode will increase in mass!

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2. Spontaneous Reactions and Chart J

• The further you move up Table J , the more
  easily the metal is oxidized (more reactive
  metals lose electrons easier).

• The further you move down, the more easily
  they are reduced.

ex. At which electrode will oxidation take
place? Higher metal in table J is oxidized, so
Al is oxidized (anode).

• A reaction will be spontaneous if the pure
  metal is on top, and the ion is below.

ex. Will Zn react spontaneously with Co2?
Since Zn is above Co the reaction will go.
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B. Electrolytic Cell uses an external
electric power source to cause a nonspontaneous
redox reaction to occur.
Ex. 2NaCl(l) electricity 2Na(s) Cl2(g)
fused salt
e-


e-
NaCl(l)
Cl2
Cl-
Na
Na
Cl-
Cl Na
Reduction Na e ?Na occurs at Cathode
still.
Oxidation 2Cl ?Cl2 2e occurs at Anode still.
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Electroplating
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Comparing Types of Electrochemical Cells