Chemistry 1011

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Chemistry 1011

• Introductory Chemistry II
• http//www.mi.mun.ca/pfisher/chemistry.html
• Password for final exams
• Midgley

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Chemistry 1011

• TOPIC
• Electrochemistry
• TEXT REFERENCE
• Masterton and Hurley Chapter 18

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18.2 Standard Voltages

• YOU ARE EXPECTED TO BE ABLE TO
• Define the standard electrode potential of a half
  cell
• Order species according to their ease of
  oxidation or reduction based on a table of
  standard reduction potentials
• Calculate the net cell voltage, Eo, of a
  combination of half cells from standard electrode
  potential data
• Determine whether a given redox reaction will be
  spontaneous or non-spontaneous

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Cell Voltage

• The force that pushes the electrons through the
  external circuit of a cell is known as the
• Potential difference, or
• Electromotive force (emf), or
• Voltage
• It is measured in volts
• The magnitude of the voltage depends on
• The nature of the redox reaction
• The concentrations of the ions in solution, (or
  pressures of any gases)

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Standard Voltage

• In order to compare the voltages of different
  cells, or to calculate the expected voltage of a
  given cell, measurements are taken under standard
  conditions
• Current flow is almost zero
• All ions and molecules in solution are at a
  concentration of 1.0 mol/L
• All gases are at a pressure of 1.0 atm

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The Zinc Hydrogen Voltaic Cell
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The Standard Voltage of the Zinc Hydrogen
Voltaic Cell

• Zn(s) 2H(aq) ? Zn2(aq) H2(g)
• 1.0 mol/L 1.0
  mol/L 1.0 atm
• Zn Zn2 H H2 Pt
• Cell voltage with no current flowing is 0.762V
• This is the standard voltage for this cell
• Zn(s) 2H(aq, 1.0M) ? Zn2(aq, 1.0M)
  H2(g, 1.0atm)
• Eo 0.762V

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Standard Half Reaction Voltages

• Each half reaction has a standard voltage
• Eoox (standard oxidation voltage)
• Eored (standard reduction voltage)
• Eo Eoox Eored
• Only Eo can be measured - the standard voltage of
  a half reaction cannot be measured directly

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Obtaining Values for Standard Half Reaction
Voltages

• Standard half reaction voltages are determined by
  arbitrarily assigning the value of zero to the
  standard reduction half reaction for hydrogen
  ions to give hydrogen gas
• 2H(aq,1.0M) 2e- ? H2(g,1.0atm)
• Eored (H ? H2) 0.000V
• Since Eo Eoox Eored
• Eoox (Zn ? Zn2) 0.762V

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Obtaining Values for Standard Half Reaction
Voltages

• Once one half reaction standard voltage is
  established, others can be deduced
• For Zn(s) Cu2(aq) ? Zn2(aq)
  Cu(s) the standard cell voltage is
  1.101V
• Zn(s) ? Zn2(aq,1.0M) 2e- Eoox (Zn
  ? Zn2) 0.762V
• Cu2(aq,1.0M) 2e- ? Cu(s) Eored (Cu2? Cu)
  ??V
• Since Eo Eoox Eored
• 1.101V 0.762V Eored
• Eored 0.339V

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Standard Reduction Potentials

• Standard half cell voltages are found in tables
  of standard potentials
• These are the values for reduction half reactions
  based upon the convention that
• 2H(aq,1.0M) 2e- ? H2(g,1.0atm)
• Eored (H ? H2) 0.000V
• Standard reduction potential Eored

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Standard Reduction Potentials

• Oxidizing Agent Reducing Agent Eored (V)
• Li(aq) e- ? Li(s) -3.040
• Na(aq) e- ? Na(s) -2.714
• Zn2(aq) 2e- ? Zn(s) -0.762
• Ni2(aq) 2e- ? Ni(s) -0.236
• 2H(aq) 2e- ? H2(g) 0.000
• Cu2(aq) 2e- ? Cu(s) 0.339
• Ag(aq) e- ? Ag(s) 0.799
• NO3-(aq) 4H(aq) 3e- ? NO(g)
  2H2O 0.964
• MnO4-(aq) 8H(aq) 5e- ? Mn2(aq) 4H2O
  1.512
• F2(g) 2e- ? 2F-(aq) 2.889

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Standard Reduction Potentials

• Elements above hydrogen in the table of standard
  reduction potentials will react with a solution
  of hydrogen ions to produce hydrogen gas
• M(s) 2H(aq) ? M2(aq) H2(g)
• M2(aq) 2e- ? M(s) Eored negative
• M(s) ? M2(aq) 2e- Eoox positive
• 2H(aq) 2e- ? H2(g) Eored 0.000V
• M M2 H H2 Pt Eocell positive
• Elements below hydrogen in the table of standard
  reduction potentials will NOT react with a
  solution of hydrogen ions to produce hydrogen gas

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Standard Voltages for Voltaic Cells

• The table of standard reduction potentials gives
  standard voltages for reduction half reactions
• Standard voltages for oxidation half reactions
  are obtained by reversing these reactions and
  changing the sign of the Eored value
• If Zn2(aq) 2e- ? Zn(s) Eored
  -0.762
• Then Zn(s) ? Zn2(aq) 2e-
  Eoox 0.762

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Computing Standard Cell Potential

• The standard voltage of a cell is the sum of the
  standard potentials for the two half reactions
• For the cell
• Zn Zn2 Cu2 Cu
• Zn(s) ? Zn2(aq) 2e- Eoox
  0.762V
• Cu2(aq) 2e- ? Cu(s) Eored
  0.339V
• Zn(s) Cu2(aq) ? Zn2(aq) Cu(s)
• Eocell Eoox Eored 0.762 0.339 1.101V

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Oxidizing Agents

• An oxidizing agent is a species that can gain
  electrons
• The strongest oxidizing agents are the species
  that gain electrons most readily
• They have the largest positive Eored values
• Oxidizing strength increases moving down the left
  column of the table of standard reduction
  potentials
• Oxidizing agents in the table of standard
  reduction potentials can oxidize any species above

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Reducing Agents

• A reducing agent is a species that readily loses
  electrons
• The strongest reducing agents are the species
  that lose electrons most readily
• They have the largest negative Eored values (The
  largest positive Eoox values)
• Reducing strength increases moving up the right
  column of the table of standard reduction
  potentials
• Reducing agents in the table of standard
  reduction potentials can reduce any species below

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Strong Reducing and Oxidizing Agents

• Reducing agent causes another species to be
  reduced - it is oxidized Li(s) ? Li(aq)
  e- Eoox 3.040V
• Oxidizing agent causes another species to be
  oxidized - it is reduced
• F2(g) 2e- ? 2F-(aq) Eored 2.889V

Table of Standard Reduction Potentials R
strongest reducing agent O strongest oxidizing
agent
R
O
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Spontaneity of Redox Reactions

• In order for a redox reaction to occur
  spontaneously, the calculated cell potential MUST
  BE POSITIVE
• Questions
• Will copper metal be oxidized to Cu2 ions by
  dilute hydrochloric acid?
• Will copper metal be oxidized to Cu2 ions by
  dilute nitric acid?

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Reaction of Copper with Dilute Hydrochloric Acid??

• Possible oxidation half reaction
• Cu(s) ? Cu2(aq) 2e- Eoox -0.339V
• Possible reduction half reaction (H and Cl- ions
  are present - Cl- ions cannot be reduced)
• 2H(aq) 2e- ? H2(g) Eored 0.000
• Net possible reaction
• Cu(s) 2H(aq) ? Cu2(aq) H2(g)
• Net calculated cell voltage
• Eocell Eoox Eored - 0.339 0.000 -
  0.339 V
• Reaction will not be spontaneous i.e no reaction

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Reaction of Copper with Dilute Nitric Acid??

• Possible oxidation half reaction
• Cu(s) ? Cu2(aq) 2e- Eoox -0.339V
• Possible reduction half reactions (H and NO3-
  ions are present)
• 2H(aq) 2e- ? H2(g) Eored 0.000V
• NO3-(aq) 4H(aq) 3e- ? NO(g) 2H2O
  Eored 0.964V
• Net spontaneous reaction (Add multiples of the
  two half reactions so that same electrons (6) in
  each half)
• 3Cu(s) 2NO3-(aq) 8H(aq) ? 3Cu2(aq)
  2NO(g) 4H2O
• Net calculated cell voltage
• Eocell Eoox Eored - 0.339 0.964
  0.629 V
• Reaction will be spontaneous i.e reaction takes
  place

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Voltaic Cells with Inert Electrodes

• Half cells will frequently be constructed with
  inert electrodes (often carbon or platinum)
• The Hydrogen half cell is one example H H2
  Pt
• A cell with two inert electrodes might be
• Pt Fe2(aq) Fe3(aq) Cl -(aq) Cl2(g)
  Pt

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The Leclanché Cell

• The Leclanché cell is the ordinary commercial
  flashlight battery
• Zn Zn2 MnO2 Mn2O3 C
• Anode half reaction
• Zn(s) ? Zn2(aq) 2e- Eoox
  0.762V
• Cathode half reaction (complex)
• 2MnO2(s) 2NH4(aq) 2e- ? Mn2O3(s) 2NH3(aq)
  H2O Eored 0.7 V
• Net cell voltage Eocell 1.5V

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