unit 14: electrochemlpchem: wz. unit 14: electrochemistry
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LPChem: Wz
Electrochemistry Oxidation-Reduction
Unit 14: The Chemistry-Electricity Connection
Unit 14: Electrochem
LPC
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: Wz
Electrochemistry
Chemistry and ElectricityGalvanic Cell: a spontaneous chemical reaction that causes electric current to be generated.
Electrolytic Cell: a nonspontaneous chemical reaction caused by the application of electric current.
Unit 14: Electrochemistry
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Electrochemistry
An electrochemical reaction involves the transfer of electrons: 2 Na + Cl2 2 NaCl
2 Na0 2 Na1+
Cl2 0 2 Cl1-
Unit 14: Electrochemistry
+ 2 e-
oxidation2 e- +
reduction
The electrons produced
here……Are used up here.
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Electrochemistry
An electrochemical reaction involves the transfer of electrons:
2 Na0 2 Na1+
Cl2 0 2 Cl1-
Unit 14: Electrochemistry
+ 2 e-
Loss of Electrons is Oxidation
2 e- +
Gain of Electrons is Reduction
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Electrochemistry
An electrochemical reaction involves the transfer of electrons:
2 Na0 2 Na1+
Cl2 0 2 Cl1-
Unit 14: Electrochemistry
+ 2 e-
Oxidation IsLoss (of electrons)
2 e- +
ReductionIsGain (of electrons)
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Electrochemistry
An electrochemical reaction involves the transfer of electrons:
2 Na0 2 Na1+
Cl2 0 2 Cl1-
Redox Reaction (a complete reaction)
Unit 14: Electrochemistry
+ 2 e-2 e-
+
oxidation (half-reaction)reduction (half-reaction)
2 Na0
++ 2 e-
+ Cl20 2 NaCl + 2 e-
Electrochemistry
Transfer of electrons:
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Unit 14: Electrochemistry
2 Na0
+ Cl20 2 NaCl
Redox Reaction
+1 -10 0
oxidation
reduction
Elements in their
“elemental state” have a oxidation # of zero.Elements in compounds
have oxidation #s based on periodic table location.
Electrochemistry
Transfer of electrons:
2 Na1+ + 2 Cl1- 2 NaCl
Not Redox.
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Unit 14: Electrochemistry
2 Na0
+ Cl20 2 NaCl
Redox Reaction
+1 -10 0
oxidation
reduction
+1 +1-1 -1
Ions have an oxidation number equal to
their charge.
+1 = +1
-1 = -1
Electrochemistry
Zn + CuSO4 ZnSO4 + CuWhat element was oxidized?
What element was reduced?
Which element LOST electrons?
Which element GAINED electrons?Unit 14: Electrochemistry
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-2+2
-2+2
Zn (0 +2)
Cu (+2 0)
Cu (+2 0)
Zn (0 +2)
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Galvanic Cells:
Zn + CuSO4 ZnSO4 + CuHow do I make this reaction into a battery?Physically separate the half-reactions
Zn Zn2+ + 2e-
2e- + Cu2+ CuUnit 14: Electrochemistry
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Galvanic Cells:
Physically separate the half-reactions
Zn Zn2+ + 2e-
2e- + Cu2+ Cu
Unit 14: Electrochemistry
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Galvanic Cells:
When the half-reactions are separate, electrons produced by the oxidation must travel through a wire to the reduction.
Unit 14: Electrochemistry
Zn Zn2+ + 2e- 2e- + Cu2+ Cu
e-
e- e-
e-
Galvanic Cells:
The salt bridge completes the circuit by allowing ions to transfer.This is necessary to equalize the charge (so the
beakers don’t end up charged).
Unit 14: Electrochemistry
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Zn Zn2+ + 2e- 2e- + Cu2+ Cu
KCl
Cl- K+
Galvanic Cells:
Galvanic Cells are typically drawn in alphabetical order:(This makes the electrons move left to right through the wire.)
Unit 14: Electrochemistry
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Zn Zn2+ + 2e- 2e- + Cu2+ Cu
KCl
Cl- K+
Anode Bridge Cathode
At the Anode is Oxidation
At the Cathode is Reduction
Galvanic Cells:
Unit 14: Electrochemistry
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Zn Zn2+ + 2e- 2e- + Cu2+ Cu
KCl
Cl- K+
Anode Bridge Cathode
At the Anode is Oxidation
At the Cathode is Reduction
The cathode (-) attracts cations.
The anode (+) attracts anions.
Galvanic Cells:
If you put something in the middle of the wire, the electrons go through it on their way from anode to cathode.
Electricity!
Unit 14: Electrochemistry
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Zn Zn2+ + 2e- 2e- + Cu2+ Cu
KCl
Cl- K+
Separating half-reactions makes galvanic reactions into batteries.
Now let’s calculate its voltage!
Voltage
Voltage is the “push” behind electric current.
It depends on the two half-reactions used:
Because every element has different electron affinity, the voltage is different for each element.
Unit 14: Electrochemistry
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Zn Zn2+ + 2e-
2e- + Cu2+ Cu
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Voltage
Standard Reduction Potential
Unit 14: Electrochemistry
Voltage (AKA Electrochem. Potential) is found on a chart like this.
The chart is all REDUCTION half-reactions.
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Voltage
Standard Reduction Potential
Unit 14: Electrochemistry
The REDUCTION half-reactions from our battery was:
Cu2+ + 2e- Cu
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Voltage
Standard Reduction Potential
Unit 14: Electrochemistry
The chart is in order of potential/voltage. Eo (V)
Elements at the top are most desperate to reduce (gain electrons).
Reactions with the most positive voltages are the most spontaneous reductions.
Reactions with the least positive (most negative) voltages are least spontaneous reductions.
Hydrogen is zero.
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Voltage
Standard Reduction Potential
Unit 14: Electrochemistry
If charts are always of reduction potential, how do we find the voltage for our oxidation half-reaction?
If oxidation is the opposite of reduction, then the opposite of the original chart should do the trick!
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Voltage
Standard Reduction Potential
Unit 14: Electrochemistry
The OXIDATION half-reactions from our battery was:
Zn Zn2+ + 2e-
Its opposite would be:Zn2+ + 2e- Zn
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Voltage
Standard Reduction Potential
Unit 14: Electrochemistry
The reduction potential of:
Zn2+ + 2e- Zn= - 0.76 V
Its opposite Zn Zn2+ + 2e-
would be worth the opposite voltage:= 0.76 V
Voltage
The two half-reactions used:Reduction:
Oxidation:Added together:Complete Reaction =
Unit 14: Electrochemistry
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Zn Zn2+ + 2e-
2e- + Cu2+ Cu 0.34 V
0.76 V+1.10 V
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Voltage
Unit 14: Electrochemistry
The further apart the two half reactions are on the chart, the greater the voltage of the overall reaction. Voltage is a potential– like
gravitational potential energy.
Larger separation between the half-reactions means the electrons have further to “fall.” More potential!
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Voltage
Unit 14: Electrochemistry
The anode (oxidation) half reaction will always need to be “flipped” because the chart is for reductions.
That means changing the sign on its voltage.
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Voltage
Unit 14: Electrochemistry
If the anode (oxidation) half-reaction were above the cathode half-reaction on the chart, what would happen??
Things don’t fall “up.” No reaction would take
place. Unless… …external current were
applied. (Electrolysis.)
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