Utilizes relationship between chemical potential energy

1
Electrochemistry

• Utilizes relationship between chemical potential
  energy electrical energy

2
Redox Reactions

• Need battery to start car
• Prevent corrosion
• Bleach is an oxidizing agent
• Na, Al, Cl prepared or purified by redox
  reactions
• Breathing
• O2 ? H2O and CO2

3
Redox Reactions

• Synthesis
• Decomposition
• Single Replacement
• Double Replacement only is not redox

Often Redox
Always Redox
4
Predicting Redox Reactions

• Use Table J to predict if a given redox reaction
  will occur.
• Any metal will donate its electrons to the ion of
  any metal below it.
• Any nonmetal will steal electrons from the ion of
  any nonmetal below it.

Memory Jogger
5
Predicting Single Replacement Redox Reactions

• Element Compound ? New Element New
  Compound
• If the element is above the swapable ion, the
  reaction is spontaneous.
• If the element is below the swapable ion, the
  reaction is not spontaneous.

Memory Jogger
6
Predicting Redox Reactions

• A BX ? B AX
• A B are metals. If metal A is above metal B
  in Table J, the reaction is spontaneous.
• X AY ? Y AX
• X Y are nonmetals. If nonmetal X is above
  nonmetal Y in Table J, the reaction is
  spontaneous.

Memory Jogger
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Which are spontaneous?
Yes

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

Yes
No
Yes
No
No
Yes
8
Started with Zn(NO3)2 Cu and AgNO3 Cu. Which
beaker had the Zn ions which had the Ag ions?
9
Overview of Electrochemistry

• TWO kinds of cells (kind of opposites)
• Galvanic or Voltaic (NYS Electrochemical)
• Use a spontaneous reaction to produce a flow of
  electrons (electricity). Exothermic.
• Electrolytic
• Use a flow of electrons (electricity) to force a
  nonspontaneous reaction to occur. Endothermic.

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Vocabulary

• Redox
• Half-reaction
• Oxidation
• Reduction
• Cell
• Half-Cell
• Electrode

• Anode
• Cathode
• Galvanic
• Voltaic
• Electrochemical
• Electrolytic
• Salt bridge

11
Electrochemical Cells

• Use a spontaneous single replacement redox
  reaction to produce a flow of electrons.
• Electrons flow from oxidized substance to reduced
  substance.
• Called Galvanic cells, voltaic cells, or
  electrochemical cells (NYS)

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Electrochemical Cells

• Redox reaction is arranged so the electrons are
  forced to flow through a wire.
• When the electrons travel through a wire, we can
  make them do work, like light a bulb or ring a
  buzzer.
• So the oxidation reduction reactions have to be
  separated physically.

OJ clock
13
Al / CuCl2 Lab

• Was a redox reaction.
• Did NOT force electrons to travel through a wire.
  Got NO useful work out of system.
• Have to be clever in how we arrange things.

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2Al 3Cu2 ? 2Al3 3Cu
Got no useful work because half-reactions werent
separated.
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Half-Cell

• Where each of the half-reactions takes place.
• Need 2 half-cells to have a complete redox
  reaction.
• Need to be connected by a wire for the electrons
  to flow through.
• Need to be connected by a salt bridge to maintain
  electrical neutrality.

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Schematic of Galvanic Cell
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Parts of a Voltaic Cell

• 2 half-cells oxidation reduction
• Each half-cell consists of a container of an
  aqueous solution an electrode or surface at
  which the electron transfer takes place.
• Wire connecting electrodes.
• Salt bridge connects solutions.

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How much work can you get out of this reaction?

• You can measure the voltage by making the
  electrons travel through a voltmeter.
• The galvanic cell is a battery. Of course, its
  not a very easy battery to transport or use in
  real-life applications.

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Electrode
Surface at which oxidation or reduction
half-reaction occurs. Anode Cathode
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An Ox Ate a Fat Red Cat

• Anode Oxidation
• The anode location for the oxidation
  half-reaction.
• Reduction Cathode
• The cathode location for the reduction
  half-reaction.

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Anode / Cathode

• How do you know which electrode is which?
• Use Table J to predict which electrode is the
  anode and which electrode is the cathode.

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Anode

• Anode Oxidation Electron Donor
• The anode is the metal thats higher in Table J.

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Cathode

• Cathode Reduction Electron Acceptor
• The cathode is the metal thats lower in Table J.

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Zn is above Cu, Zn is anode
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Notation for Cells
Zn?Zn2??Cu2?Cu
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Direction of Electron Flow(wire)

• Anode to Cathode

Direction of Positive Ion Flow (salt bridge)
Anode to Cathode
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Positive Negative Electrode

• Negative electrode is where electrons originate
  here its the Zn electrode.
• Positive electrode is electrode that attracts
  electrons here its the Cu electrode.

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Aqueous Solution

• Solution containing ions of the same element as
  the electrode.
• Cu electrode solution may be Cu(NO3)3 or CuSO4.
• Zn electrode solution may be Zn(NO3)2 or ZnSO4.

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Salt Bridge

• Allows for migration of ions between half-cells.
  
• Necessary to maintain electrical neutrality.
• Reaction will not proceed without salt bridge.

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A(s) BX(aq) ? B(s) AX(aq)

• Single replacement rxn occurs during operation of
  galvanic cell.
• One electrode will gain mass (B) and one
  electrode will dissolve (A).
• The concentration of metal ions will increase in
  one solution (making AX) decrease in one
  solution (using up BX).

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Half-Reactions

• Zn ? Zn2 2e-
• Cu2 2e- ? Cu

_________________________
Zn Cu2 ? Zn2 Cu
Which electrode is dissolving? Which species is
getting more concentrated?
Zn
Zn2
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Zn Cu2 ? Zn2 Cu
Cu

• Which electrode is gaining mass?
• Which species is getting more dilute?

Cu2
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When the reaction reaches equilibrium

• The voltage goes to 0.

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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, direction of electron flow
  in wire, direction of positive ion flow in salt
  bridge, positive electrode, negative electrode.
• Negative electrode is where electrons originate.
  Positive electrode attracts electrons.

35
Electron flow ?
wire
Pb cathode
Positive ion flow ?
Al anode
Salt bridge
-
?
Pb2 NO3-1
Al3 NO3-1
36
What are half-reactions?

• Al ? Al3 3e-
• Pb2 2e- ? Pb

Al metal is the electrode its dissolving. Al3
ions go into the solution.
Pb2 ions are in the solution. They pick up 2
electrons at the surface of the Pb electrode
plate out.
37
Overall Rxn

• 2(Al ? Al3 3e-)
• 3(Pb2 2e- ? Pb)

_____________________________
2Al 3Pb2 ? 2Al3 3Pb
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2Al 3Pb2 ? 2Al3 3Pb
Al

• Which electrode is losing mass?
• Which electrode is gaining mass?
• Whats happening to the Al3?
• Whats happening to the Pb2?

Pb
Increasing
Decreasing
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Application Batteries
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Dry Cell
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Mercury battery
42
Application Corrosion
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Corrosion Prevention
44
Whats wrong with this picture?
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