Chemistry 1011

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Chemistry 1011

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

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18.1 Voltaic Cells

• YOU ARE EXPECTED TO BE ABLE TO
• Identify oxidation-reduction reactions (Review
  Chapter 4.3)
• Assign oxidation numbers to elements in molecules
  and ions (Review Chapter 4.3)
• Balance equations for oxidation-reduction (Redox)
  reactions (Review Chapter 4.3)
• Identify the anode and cathode in an
  electrochemical cell
• Construct a labelled diagram to show the
  structure of an electrochemical voltaic cell and
  describe its operation
• Represent a voltaic cell using the symbolism M1
  M12 M2 M22

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Oxidation-Reduction Reactions

• In simple terms, oxidation is the addition of
  oxygen to an element
• Mg(s) O2(g) ? 2MgO(s)
• Here the Mg is oxidized
• Mg ? Mg2 2e-
• In this oxidation process, the magnesium loses
  electrons
• In general, in an oxidation-reduction (Redox)
  reaction, one species loses electrons and the
  other gains electrons
• The species gaining electrons is reduced
• The species losing electrons is oxidized
  

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Oxidation-Reduction Reactions

• When zinc reacts with hydrochloric acid
• Zn(s) 2HCl(aq) ? ZnCl2(aq) H2(g)
• The zinc atoms lose electrons they are oxidized
• The H ions gain electrons they are reduced
• OXIDATION Zn ? Zn2 2e-
• REDUCTION 2H 2e- ? H2
• Oxidation and reduction take place together
• There is no net change in the total number of
  electrons

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Oxidation Number

• The oxidation number is an arbitrary number
  assigned to an element in a compound to indicate
  the charge that would be present if it were an
  ion
• The oxidation number of a pure element is 0
• Oxidation number of chlorine in Cl2 is 0
• Oxidation number of an element in a monatomic ion
  is equal to the charge on the ion
• Oxidation number of chlorine in NaCl is 1
• In compounds, the oxidation number of oxygen is
  usually 2
• In compounds, the oxidation number of hydrogen is
  usually 1

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Oxidation Number

• What is the oxidation number of Mn in MnO4- ?
• Charge due to 4O will be 8
• Ion has overall charge of 1
• Mn is assigned oxidation number of 7
• What is the oxidation number of N in NH3,
  N2, N2O, NO, NO2?
• -3, 0, 1 2 4

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Balancing Simple Half Reactions

• For simple ions the balanced half reaction must
  be balanced for mass and charge
• The numbers of atoms must be the same on both
  sides of the equation
• The net charge must be the same on both sides of
  the equation
• Zn ? Zn2 2e-
• Cl2 2e- ? 2Cl-

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Balancing More Complex Half Reactions

• Identify the element being oxidized or reduced
• Assign oxidation numbers to the element on both
  sides of the equation
• Identify change in oxidation number
• Add electrons to balance change in oxidation
  number
• Balance charge by adding H or OH-
• Balance hydrogen by adding H2O

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Balancing More Complex Half Reactions

• Balance MnO4-(aq) ? Mn2(aq) (acid
  solution)
• Oxidation number of Mn in MnO4-(aq) is 7
• Oxidation number of Mn in Mn2(aq) is 2
• 5 electrons must be added to the Left Hand Side
  of the equation to change oxidation number by 5
• MnO4-(aq) 5e- ? Mn2(aq)
• 8 H must be added to the LHS to balance for
  charge
• 8H(aq) MnO4-(aq) 5e- ? Mn2(aq)
• 4 H2O must be added to the RHS to balance
  completely
• 8H(aq) MnO4-(aq) 5e- ? Mn2(aq)
  4H2O(l)

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Balancing More Complex Half Reactions

• Balance SO42-(aq) ? SO2(g) (acid solution)
• Oxidation number of S in SO42-(aq) is ?
• Oxidation number of S in SO2(g) is ?
• ?? electrons must be added to the LHS? or RHS? of
  the equation to change oxidation number by ??
• SO42 -(aq) ?? ? SO2(g)
• ?? H must be added to the LHS? or RHS? to
  balance for charge
• ?? H2O must be added to the LHS? or RHS? to
  balance completely

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Voltaic Cells

• In a voltaic cell, chemical energy is converted
  to electrical energy
• In a voltaic cell, an oxidation-reduction (redox)
  reaction takes place
• The voltaic cell is constructed so that the
  electrons produced by the oxidation half reaction
  must travel round an external circuit to become
  available for the reduction half reaction

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Voltaic Cells

• When a strip of zinc is placed in a solution of
  copper ions, a redox reaction takes place
• Zn(s) Cu2(aq) ? Zn2(aq) Cu(s)
• Half reactions
• Zn ? Zn2 2e- oxidation
• Cu2(aq) 2e- ? Cu(s) reduction
• This reaction is spontaneous

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Designing the Voltaic Cell

• Dip zinc metal into zinc ions (zinc sulfate
  solution) in one beaker
• Dip copper metal into copper ions (copper sulfate
  solution) in a second beaker
• Link the two solutions with a salt bridge
• Link the two metal strips with a wire
• Electrons flow from the zinc to the copper
  through the wire
• Ions flow through the bridge

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The Zinc Copper Voltaic Cell

• Cell representation
• Zn Zn2 Cu2 Cu
• anode (oxidation) on left
• cathode (reduction) on right
• indicates salt bridge
• indicates phase boundary

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Voltaic Cell Operation

• The zinc strip is the ANODE
• (oxidation occurs - Zn ? Zn2 2e- )
• The copper strip is the CATHODE
• (reduction occurs - Cu2(aq) 2e- ? Cu(s) )
• The electrons flow around the external circuit
  from the anode to the cathode
• Zinc ions are produced in solution around the
  zinc strip
• Copper ions are removed from solution around the
  copper strip
• Ions migrate across the salt bridge to maintain
  electrical neutrality

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Other Metal Metal Ion Voltaic Cells

• Any two different metals can be used to produce
  an electric current
• Cu Cu 2 Ag Ag
• Zn Zn2 Fe2 Fe
• Al Al3 Cu2 Cu

Zn
Cu
LEMON
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The Zinc Hydrogen Voltaic Cell

• Zn(s) 2HCl(aq) ? ZnCl2(aq) H2(g)
• Zn(s) 2H(aq) ? Zn2(aq) H2(g)
• Half reactions
• Zn ? Zn2 2e- oxidation
• 2H(aq) 2e- ? H2(g) reduction
• Zn Zn2 H H2 Pt
• The hydrogen half cell consists of hydrogen gas
  bubbled over a platinum electrode, submerged in a
  solution of acid

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The Zinc Hydrogen Voltaic Cell
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Structure and Operation of Voltaic Cells A Summary

• A voltaic cell consists of two half cells joined
  by
• an external circuit through which electrons
  flow, and by
• a salt bridge through which ions move
• Each half cell consists of an electrode dipping
  into an aqueous solution
• If a metal participates in the cell reaction it
  will usually be the electrode otherwise an inert
  platinum electrode is used
• In one half cell oxidation occurs at the anode
  and reduction occurs at the cathode
• The overall cell reaction is the sumof the half
  reactions taking place at anode and cathode