Unit 14: ElectrochemLPChem: Wz. Unit 14: Electrochemistry

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Electrochemistry Oxidation-Reduction

Unit 14: The Chemistry-Electricity Connection

Unit 14: Electrochem

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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-


+ 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-


+ 2 e-

Loss of Electrons is Oxidation

2 e- +

Gain of Electrons is Reduction

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Electrochemistry


2 Na0 2 Na1+

Cl2 0 2 Cl1-


+ 2 e-

Oxidation IsLoss (of electrons)

2 e- +

ReductionIsGain (of electrons)

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Electrochemistry


2 Na0 2 Na1+

Cl2 0 2 Cl1-

Redox Reaction (a complete reaction)


+ 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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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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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


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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.


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).


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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.)


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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:


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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!


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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.


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Zn Zn2+ + 2e-

2e- + Cu2+ Cu

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Voltage

Standard Reduction Potential


Voltage (AKA Electrochem. Potential) is found on a chart like this.

The chart is all REDUCTION half-reactions.

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Voltage



The REDUCTION half-reactions from our battery was:

Cu2+ + 2e- Cu

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Voltage



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



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



The OXIDATION half-reactions from our battery was:

Zn Zn2+ + 2e-

Its opposite would be:Zn2+ + 2e- Zn

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Voltage



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 =


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Zn Zn2+ + 2e-

2e- + Cu2+ Cu 0.34 V

0.76 V+1.10 V

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Voltage


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


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


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.)

Documents

Unit 14: ElectrochemLPChem: Wz. Unit 14: Electrochemistry