Oxidation and Reduction Reactions and Electrochemistry

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Oxidation and Reduction Reactions and
Electrochemistry

• The Ubiquitous Electron

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• Redox and Iron in your Body

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Types of Reactions

• Ions or molecules react w/ no apparent change in
  electronic structure (ex. Double displacement)
• Ions or atoms undergo changes of electronic
  structure, the way e- transfer or the way atoms
  share e- changes.

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Oxidation- Reduction Reaction Definition

• Chemical change that occurs when electrons are
  transferred between reactants
• All oxidation reactions are accompanied by
  reduction reactions
• Important in the corrosion of metals, sources
  of energy, life processes

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Oxidation

• Part of the redox rxn in which electrons are
  removed or apparently removed from an atom (loss
  of electrons ? atom gets more positively charged)

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• Movie

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Reduction

• Part of the redox rxn in which electrons are
  added or apparently added to an atom (gain of
  electrons? atoms get more negatively charged)

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• Movie

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OIL RIG

• Oxidation Is Losing
• Reduction Is Gaining

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LEO the lion goes GER

• Loss of Electrons in Oxidation
• Gain of Electrons in Reduction

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• Ionization or Solvation the process of
  surrounding solute particles with solvent
  particles to form a solution
• Video
• like dissolves like

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Net Ionic Equations

• When reactions take place in water chemists write
  the equation in ionic form (particles ionize
  break into their ions in water)
• Chemists only write down the ions that take part
  in the reaction
• Spectator ions- ions that arent involved in the
  reaction (chemists dont write these)
• Makes rxn easier to balance

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• Cu NO3-1 ? Cu2 NO

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• Show chemistry connections video 736 minutes
  into video, found in redox folder

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Rules for Assigning Oxidation Numbers

• Use oxidation numbers (charges on atoms) to
  determine which atom underwent reduction and
  which atom underwent oxidation

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Rules

• The oxidation number for any free element is 0
  (zero). Also any diatomic molecule is 0 (zero)
• H2, O2, I2, Cl2, F2, N2, Br2
• Fe 0 charge
• O2 0 charge

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• The oxidation number of any monoatomic ion is
  equal to the charge written on the ion.
• Na 1 1
• Cl-1 -1

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• Oxidation number of hydrogen in most of its
  compounds is 1 (except for LiH then H is 1)
• 1
• Ex. HCl

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• Oxidation of oxygen in most of its compounds is
  2.(except peroxides -1)
• -2
• Ex. H2O

-1 Ex. H2O2
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• Sum of the oxidation numbers of all of the atoms
  must equal the apparent charge of that particle.

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• Ex. H2SO4
• -zero charge
• 1 ? -2
• H2SO4
• 2 6-80
• S 6

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• Ex. NO3 1
• ? -2(3) -1
• 5 (-6) -1
• N 5

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• Group 1? 1
• Group 2 ? 2
• Aluminum Boron ? 3
• Group 17 ? -1

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• Ex. KMnO4
• K
• Mn
• O

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7
-2
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• Page 174 67, 69

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• Identifying redox, chemistry connections 1129
  minutes in

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Identifying Redox Reactions

• First, figure out the oxidation numbers of all
  elements in the reaction
• If oxidation number changes as you move from
  reactants to products it is REDOX.

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This is REDOX, Mg- loss e- (oxidation), H gained
e-(reduction)
This is NOT REDOX
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• P 618 in modern chem- 2, 15

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Oxidizing Reducing Agents

• Think of these agents as causers of redox rxns
• Look at reactants
• Some substances are better oxidizing or reducing
  agents

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• Reducing Agents substance that donates the
  electron (contains the atoms that are oxidized-
  or loss the e-)
• Causes the reduction to occur
• Oxidizing Agent substance that gains the e-
  (contains the atoms that are reduced or gains e-)
• Causes oxidation to occur

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• Ex.
• 4Al 3O2 ? 2Al2O3

0 0 3 -2
Al- lost e- , oxidized -reducing agent O- gained
e-, reduced -O2 is the oxidizing agent
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Balancing Redox Reactions --Half Reaction Method

• Half Reaction equation that shows just the
  oxidation or reduction part of the rxn.
• In balancing we balance each of the half rxns
  first, then add them together reduce

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Steps

• Place oxidation s on everything after it is in
  the net ionic form.
• ID the oxidation ½ rxn and the reduction ½ rxn
• Write out the ½ rxns.
• Balance the atoms by placing coefficients in
  front of the atoms ? except for H and O
• Ex. Cl2 ? Cl-1 become Cl2 ? 2Cl-

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• 5. Place the of electrons lost on the product
  side of oxidation ½ rxn, place of electrons
  gained on reactant side of reduction ½ rxn
• 6. To balance hydrogens and oxygens
• Acidic soln add H H2O
• Basic soln add OH- H2O

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• Balance the charges ( e- lost must equal e-
  gained) by using a least common multiple (
  multiply the whole ½ rxn)
• Add two ½ rxns together and reduce if necessary.

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• Chemistry connections- balancing with blood
  alcohol tests (2100-2600)

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Electrochemistry

• Movie

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• Because redox reactions involve electron
  transfer, the release or absorption of energy can
  occur in the form of electrical energy rather
  than heat
• Electrochemistry is the branch of chemistry that
  deals w/ electricity related applications of
  redox reactions

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Electrochemical Process

• Conversion of chemical to electrical energy
• Ex. Flashlight batteries, biological systems,
  electroplating
• If the substance that is oxidized is separated
  from the substance that is reduced you get an
  energy transfer of electrical energy instead of
  heat

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• Electrons can be transferred from one side to the
  other through a connecting wire
• Electric current moves in a circuit (while the
  electrons are being balanced by the movement of
  ions in solution)

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Part of a Cell

• Electrodes
• Conductor in a circuit that carries electrons
  from one substance to another
• Anodes electrode where oxidation occurs, anions
  (-) are attracted to this when they are oxidized
  by losing electrons (the positive electrode)
• Cathode electrode where reduction occurs,
  cations () are attracted to this when they are
  reduced by gaining electrons (negative electrode)

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• Salt Bridge
• Porous partition that separates the 2 half
  reactions
• Contains a conducting solution that allows the
  passage of ions from one compartment to the other
  w/ out mixing the solutions in the half reactions

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• Half Cell
• Part of the voltaic cell in which either
  oxidation or reduction occurs
• The two half cells together make a complete
  electrochemical cell

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• Ex. Oxidation half cell
• Zn ? Zn2 2 e-
• (zinc rod in zinc sulfate)
• Reduction half cell
• Cu2 2e- ? Cu
• (copper rod in copper sulfate)

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• Complete Cell Notation
• Anode electrode anode solution cathode
  solution cathode electrode
• (the double line represents the salt bridge)
• Ex. Zn (s) Zn 2 (aq) Cu2 (aq) Cu
  (s)

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e-
e-
e-
e-
Anode-positive electrode, oxid. occurs
Cathode-neg. electrode, red. occurs
Salt bridge
Zn rod
Cu rod
ZnSO4
CuSO4
Zn(s) ZnSO4(aq)CuSO4 (aq) Cu (s)
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• Fuel Cell

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Type of Cells

• Dry Cell voltaic cell in which the electrolyte
  (conducting solution) is a paste
• Generates direct current by converting chemical
  to electrical energy by a spontaneous redox
  reaction
• Also called galvanic cells or voltaic cells
• Ex. Batteries (zinc-carbon, alkaline, mercury)
• Ex. Flashlight battery (zinc-carbon)
• Zinc container (anode) filled w/ a moist paste
  (salt paste) made of MnO2, ZnCl2, NH4Cl and water
  w/ a graphite rod (cathode) embedded into it

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• Alkaline batteries (do not have a carbon rod
  cathode which allows them to be smaller- uses a
  graphite/ MnO2 mix)
• Mercury (cathode is HgO/carbon mix)

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• Lead storage batteries
• Group of cells that are connected together
• Can be recharged (use in a car)
• Ex. 12 V battery- 6 voltaic cells connected
  together
• Each cell contains 2 lead electrodes or grids
• Anode- grid packed w/ spongy lead
• Cathode grid packed w/ PbO2
• Immersed in 5M H2SO4
• Recharging occurs whenever the car is running
• Doesnt last forever- byproduct PbSO4 falls from
  electrodes and collects on bottom (loses too much
  lead)

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• Fuel Cells
• A voltaic cell in which the reactants are being
  continuously supplied and the product are being
  continuously removed
• A fuel substance undergoes oxidation, from which
  electrical energy is obtained continuously
• No recharging, no pollution
• Ex. H-O cell submarines, military vehicles,
  Apollo

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Electrical Potential

• In a voltaic cell, the oxidizing agent at the
  cathode pulls the electrons through the wire away
  from the reducing agent at the anode
• The pull on the electrons is called the
  electric potential
• Electrical potential is measured in volts (V)

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Electrode potential

• The potential difference measure across the
  complete voltaic cell is easily measured
• It equals the sum of the electrode potentials for
  each of the two half-reactions
• The individual electrode potential for a
  half-reaction cannot be measured directly, but it
  can be measured by connecting to a standard
  half-cell as a reference (we use a Hydrogen
  electrode that is in a 1.0M acidic solution at 1
  atm and 25 C)

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Standard Reduction Potentials (p. 796-book)

• Electrode potentials are always written as
  reductions
• The more negative the voltage? oxidation
  (stronger reducing agent)
• The more positive the voltage? reduction
  (stronger oxidizing agent)

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Standard Cell Potential (E cell)

• Use this formula
• Ecell Ereduction - Eoxidation
• or
• Ecell Ecathode - Eanode
• A spontaneous reaction will have positive value
  for E cell

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Zn (s) Zn 2 (aq) Cu2 (aq) Cu (s)

• Oxidation Zn2 2 e- ? Zn
• EZn 2 -.76 V
• Reduction Cu 2 2e- ? Cu
• ECu2 .34V
• Ecell Ereduction - Eoxidation
• .34V - (-.76V)
• 1.10V
• Cu2 Zn ? Cu Zn2

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Zn (s) Zn 2 (aq) Fe2 (aq) Fe (s)
(anode) (cathode)

• Oxidation Zn2 2 e- ? Zn
• EZn 2 -.76 V
• Reduction Fe 2 2e- ? Fe
• EFe2 -.44V
• Ecell Ereduction - Eoxidation
• -.44V - (-.76V)
• .32V
• Fe2 Zn ? Fe Zn2

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Practice

• Mn Mn 2 Br2 Br-
• H2C2O4 CO2 MnO4-1 Mn2
• Ni Ni 2 Hg22 Hg
• Cu Cu2 Ag1 Ag
• Pb Pb 2 Cl2 Cl-

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• Mn Mn 2 Br2 Br-
• Ecell 1.07-(-1.18) 2.25 V
• Br2 Mn ? Mn2 2Br-
• H2C2O4 CO2 MnO4-1 Mn2
• E cell 1.51- (-.49) 2.00 V
• 2 MnO4- 6 H 5 H2C2O4 ? 2Mn2 8H2O 10
  CO2
• Ni Ni 2 Hg22 Hg
• 1.04 V
• Ni Hg2 2 ? Ni 2 2Hg
• Cu Cu2 Ag1 Ag
• .46 V
• Cu 2 Ag ? Cu2 2 Ag
• Pb Pb 2 Cl2 Cl-
• 1.49V
• Pb Cl2 ? Pb2 2Cl-

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• Video
• How its made nails
• Corrision Pics

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Redox/ Electrochemistry Quest(anode song)

• Redox
• oxidation s
• ID if redox or not
• Oxidizing or reducing agent (strengths)
• Balancing- set up ½ rxns
• Balancing oxygens/hydrogens
• Acids (add H and H20)
• Bases (add OH- and H20)

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• Electrochem
• What is an electrochemical cell
• Example
• Parts anode, cathode, salt bridge, what each
  part does)
• Standard cell potential (getting the voltage and
  write equation)