Chapter 21: Electrochemistry

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Chapter 21 Electrochemistry
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• Zn (s) Cu 2 (aq) ? Zn 2 (aq) Cu (s)
• Oxidation Zn (s) ? Zn 2 (aq)
• Reduction Cu 2 (aq) ? Cu (s)

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The metal that is HIGHER on the activity series
is more easily OXIDIZED
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Electrochemical Process

• Any conversion between electrical energy and
  chemical energy

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Electrochemical Process

• If a Redox reaction is to be used as a source of
  electrical energy, the 2 half reactions must be
  physically separated

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Electrochemical Cell

• Any device that converts chemical energy into
  electrical energy OR electrical energy into
  chemical energy
• Redox reactions occur in electrochemical cells

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Voltaic Cells

• Alessandro Volta, an Italian physicist invented
  the first electrochemical cell in 1800
• Voltaic cells are used to convert chemical energy
  into electrical energy

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Half Cell

• One part of the Voltaic cell where EITHER the
  oxidation OR reduction takes place

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Half Cell

• A metal rod is immersed in a solution of its ions

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Half Cell
Half Cell
Salt Bridge
Zn (s) ? Zn 2 (aq) 2e-
Cu 2 (aq) 2e- ? Cu(s)
Copper Electrode (cathode)
Zinc Electrode (anode)
Zinc Sulfate Solution
Copper Sulfate Solution
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Salt Bridge

• A tube containing a strong electrolyte, often
  potassium sulfate (K2SO4)
• Usually made of agar (a gelatinous substance)
  allows ions to pass from one cell to another but
  prevents the solutions from mixing completely ?
  2K and SO42-

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Electrodes

• A conductor in a circuit that carries electrons
  to or from a substance other than a metal

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Anode

• The electrode at which oxidation occurs

e- are produced ? labeled -
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Cathode

• The electrode at which reduction occurs

e- are consumed ? labeled
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Voltaic Cells

• Neither electrode is ACTUALLY charged
• All parts of the cell remain balanced in terms of
  charge at all times

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Voltaic Cells

• Electrons are produced at the zinc rod

Zn (s) ? Zn 2 (aq) 2e-
Anode
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Voltaic Cells

• The electrons leave the zinc anode and pass
  through the external circuit to the copper rod

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Voltaic Cells

• The electrons enter the copper rod and interact
  with the ions in solution

Cu 2 (aq) 2e- ? Cu(s)
Cathode
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Voltaic Cells

• To complete the circuit- both positive and
  negative ions move through the circuit via the
  salt bridge

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Voltaic Cells

• Electrons in the reaction MUST cancel
• Oxidation Zn (s) ? Zn 2 (aq) 2e-
• Reduction Cu 2 (aq) 2 e-? Cu (s)
• Zn (s) Cu 2 (aq) ? Zn 2 (aq) Cu (s)
• Zn(s) ZnSO4 (aq) CuSO4 (aq) Cu (s)

Salt Bridge
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Dry Cell

• A voltaic cell in which the electrolyte is a
  paste
• Ie. Flashlight Batteries

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Batteries

• A battery is a group of cells connected together

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Battery

• A 12-V car battery consists of 6 voltaic cells
  connected together
• Oxidation Pb (s) SO42- (aq) ? PbSO4 (s) 2e-
• Reduction PbO2 (s) 4H (aq) SO42- (aq) 2e-
  ?
• 
  PbSO4 (s) 2H2O (l)

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Electrical Potential

• Usually measured in volts (V)
• Cells ability to produce electricity
• Results from the a competition for electrons from
  the 2 half-cells

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Reduction Potential

• The tendency of a given half reaction to occur as
  a reduction

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Cell Potential

• The difference between the reduction potential of
  the 2 half-cells

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Standard Cell Potential (E0cell)

• E0cell E0red E0oxid
• Ion concentrations in the half-cells are 1M
• Any gasses are at 101 kPa
• Temperature is 25oC

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Standard Hydrogen Electrode

• Half-cell potentials cannot be measured
• Electrodes serve as a reference

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Standard Reduction Potentials
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Calculating Standard Cell Potentials

• If the cell potential for a given redox reaction
  is positive than the reaction is spontaneous
• If the cell potential for a given reaction is
  negative, than the reaction is nonspontaneous

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Example

• Determine the cell reaction, the standard cell
  potential, and the half-cell that that acts as
  the cathode for a voltaic cell composed of the
  following half-cells
• Cu 2 (aq) 2e- ? Cu (s) E0 Cu 2 0.34 V
• Al 3 (aq) 3e- ? Al (s) E0 Al 3 -1.66 V

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Example (cont)

• Cu 2 (aq) 2e- ? Cu (s) E0 Cu 2 0.34 V
• Al 3 (aq) 3e- ? Al (s) E0 Al 3 -1.66 V
• E0 Cu 2 0.34 V gt Al (s) E0 Al 3 -1.66 V
• Reduced Oxidized
• Cathode Anode

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Example (cont)

• Since Al is being oxidized
• Al 3 (aq) 3e- ? Al (s) E0 Al 3 -1.66 V
• Al (s) ? Al 3 (aq) 3e- E0 Al 3 1.66 V

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Example (cont)

• Cu 2 (aq) 2e- ? Cu (s) E0 Cu 2 0.34 V
• Al (s) ? Al 3 (aq) 3e- E0 Al 3 1.66 V
• 
  2.00V
• E0cell E0red E0oxid NOTE if you change the
  sign for the substance being oxidized you do not
  need this reaction

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Example (cont)

• Balance each half reaction
• (Cu 2 (aq) 2e- ? Cu (s))3
• 3Cu 2 (aq) 6e- ? 3Cu (s)
• (Al (s) ? Al 3 (aq) 3e-)2
• 2Al (s)? 2Al 3 (aq) 6e-

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Example (cont)

• 3Cu 2 (aq) 6e- ? 3Cu (s)
• 2Al (s)? 2Al 3 (aq) 6e-
• 3Cu 2 (aq) 2Al (s)? 3Cu (s) 2Al 3 (aq)
• E0cell 2.00V
• Note the E0cell is NOT multiplied by any number
  even though there are coefficients