Electrochemical cells: utilize relationship between chemical potential energy

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• Electrochemical cells utilize relationship
  between chemical potential energy electrical
  energy

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Everyday Redox Reactions

• battery to start car
• prevent corrosion of metals
• cleaning with bleach (oxidizing agent)
• Na, Al, Cl prepared or purified by redox
  reactions
• breathing
• O2 ? H2O and CO2

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Redox Reactions

• synthesis rxns
• decomposition rxns
• SR rxns
• DR rxns are NOT redox rxns!

all are redox rxns
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Predicting Redox Reactions

• Table J used to predict if given redox reaction
  will occur
• single metal donates electrons to ions of
  metals below itself (oxidaizes)
• single non-metal takes electrons from ion of
  non-metals below itself (reduces)

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Predicting Redox Reactions

• A BX ? B AX
• If metal A above metal B (Table J)
  rxn is spontaneous
• If metal A below metal B
  rxn is NOT spontaneous

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X AY ? Y AX

• If non-metal X above non-metal Y (Table J) rxn
  is spontaneous
• If non-metal X below non-metal Y rxn
  is NOT spontaneous

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Spontaneous or not?

• Li AlCl3 ?
• Cs CuCl2 ?
• I2 NaCl ?
• Cl2 KBr ?
• Fe CaBr2 ?
• Mg Sr(NO3)2 ?
• F2 MgCl2 ?

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• placed Cu(s) in beakers
• one contains Zn(NO3)2(aq) other contains
  AgNO3(aq)
• Which beaker contains which ions?
• contains Zn ions?
• contains Ag ions?

A
B
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Overview of Electrochemistry

• TWO kinds of Electrochemical
  Cells

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• galvanic or Voltaic cells
  
• (Regents electrochemical cells)
• use spontaneous rxn to produce flow of electrons
  (electricity)
• exothermic

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• electrolytic cells
• use flow of electrons (electricity) to force
  non-spontaneous rxn to occur
• endothermic

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Vocabulary

• galvanic cell
• Voltaic cell
• electrochemical cell

Board of Regents considers all 3 as
electrochemical cells - not exactly accurate
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galvanic cells/Voltaic cells

• (NYS electrochemical cells)
• use spontaneous SR redox rxn
• produce flow of electrons
• electrons flow from oxidized substance to reduced
  substance

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Cell Set-up

• components arranged so e- forced to flow through
  wire
• when e- travel through a wire, can make them do
  work light a bulb, ring a buzzer
• oxidation reduction reactions must be separated
  physically

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Parts of a galvanic/Voltaic Cell

• 2 half-cells
• - one for oxidation rxn
• - one for reduction rxn
• each ½ cell consists of
• container with aq soln
• /- ions
• electrode
• surface where e- transfer takes place
• wire connects 2 electrodes
• salt bridge connects 2 solutions

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How much work can you get out of this reaction?

• can measure voltage by allowing electrons to
  travel through voltmeter
• galvanic cell is a battery
• not easy battery to transport or use in real-life
  applications

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electrode surfaces at which oxidation or
reduction half-reaction occur
anode oxidation surface decreases
in mass cathode reduction
surface increases in mass
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An Ox ate a Red Cat

• Anode Oxidation
• anode location for oxidation half-rxn
• Reduction Cathode
• cathode location for reduction half-rxn

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Anode / Cathode

• how know which electrode is which?
• Table J
• use to predict which electrode is anode and
  which electrode is cathode

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Anode

• Anode Oxidation Electron Donor
• anode composed of metal HIGHER on Table J

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Cathode

• Cathode Reduction Electron Acceptor
• cathode composed of metal LOWER on Table J

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Zn above Cu Zn is anode Cu is cathode cathode
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Direction of e- Flow (through wire)
Anode ? Cathode
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Direction of () Ion Flow (salt bridge)
Anode ? Cathode
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negative electrode (anode)

• e- originate here Zn electrode in this picture
• Zn electrode decreases in mass
• Zn2 ions increase in concen
• aq solns contain ions of same element as
  electrode

?
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positive electrode (cathode)

• e- attracted here Cu electrode in this picture
• Cu electrode increases in mass
• Cu2 ions decrease in concen
• aq solns contain ions of same element as
  electrode

?
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Salt Bridge

• allows migration of ions between half-cells
• necessary to maintain electrical neutrality
• reaction can not proceed without salt bridge

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Half-Reactions

• ox Zn ? Zn2 2e-
• red Cu2 2e- ? Cu

• Which electrode is dissolving?
• Which electrode is gaining mass?
• Which species is increasing its concen?
• Which species is getting more dilute?

Cu
Cu2
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When the reaction reaches equilibrium

• voltage is 0!
• electrons no longer flow

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Construct Galvanic Cell with Al Pb

• Use Table J to identify anode cathode
• Draw Cell put in electrodes solutions
• Label
• anode
• cathode
• positive electrode
• negative electrode,
• direction of electron flow in wire
• direction of positive ion flow in salt bridge
• remember
• negative electrode where electrons originate,
  positive electrode attracts electrons

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Oxidation Al ? Al3 3e-
Reduction Pb2 2e- ? Pb
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Overall Rxn

• (Al ? Al3 3e-)
• (Pb2 2e- ? Pb)

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2Al 3Pb2 6e-? 2Al3 3Pb 6e-
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Application Batteries
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Dry Cell Battery
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Mercury Battery
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Corrosion
to resist corrosion coat one metal with
stronger more durable metal
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Whats wrong with this picture?