1. 18.4 Standard Electrode Potentials
By:
Khedup Lodoe

2. Half Cell Potential
Standard cell potential (SCP) is the potential energy difference between two electrodes
SCP is dependant on each half reaction occurring in the half cells
Each electrode has its own potential
This is the standard electrode potential
The overall SCP is the difference between the two standard electrode potentials
Electrons flow from the electrode with more negative charge to the electrode with more positive charge
This negative charge is comparable to a greater potential energy
And a positive charge implies a lower potential energy

3. Standard Hydrogen Electrode(SHE)
We cannot measure electrode potential in a half cell directly
It is only possible for us to measure the overall potential when two half cells are combined
But we use the SHE as a marker to see where other half cells are
This half cell contains an inert platinum electrode in 1M HCl with hydrogen gas at 1 atm.
The formula for this reaction is:
2H+ (aq)+ 2e- → H2(g) Eocathode = 0.00 V
Now the electrode potential can be measured from knowing that half of the cell is at 0 V

4. Measuring Half Cell Potential
Eocell is the standard electrode potential which is the difference in voltage between the cathode (initial state) and the anode (final state)
Eocell= Eofinal - Eoinitial (Neither of these equations is on the ref. Table, so its important to remember them)
Eocell= Eocathode - Eoanode
Both of these equations are used to determine standard electrode potential
In a cell with a SHE a +Eocell means that the other half cell is is at a negative potential

5. Measuring Half Cell Potential (cont.)
The more negative electrode potential is at the anode, the greater the potential energy of an electron at that electrode is because electrons are repelled by negative voltage
Reactions with these potentials are spontaneous
Eocell is positive
With a greater positive electrode potential than the SHE, then that electrode has a higher voltage (lower potential energy)
These reactions are nonspontaneous because the anode has a higher voltage than the SHE
Eocell is negative

6. Full diagram on page 873. This is a summarized version I found online
Full diagram on page 873. This is a summarized version I found online. Not the full one in the book. Mr. Stevenson is handing out a copy of the full version.

7. Question
A) The anode is the Cr(s) and the cathode is the Fe(s). The electrons flow from Cr to Fe.
B) Cr(s) + Fe3+(aq) → Cr3+(aq) + Fe(s) Eocell = 0.69V
C) The Cathode is positive, and the anode is negative.

8. Predicting Spontaneous Direction of Redox Reactions
If the overall cell potential is positive it is spontaneous
This would mean a more negative potential in the anode than the cathode
You can use Eocell= Eocathode - Eoanode
Another method of finding Eocell is to compare where they are on the table of reduction half potentials
Any reduction half reaction listed is spontaneous if it is paired with the reverse of a half reaction that is below it on the table
Half reactions near the top tend to undergo reduction and half reactions listed near the bottom tend to undergo oxidation
Remember NIO PIR
NIO: more Negative (the standard potential) Is Oxidation
PIR: more Positive (the standard potential) Is Reduction

9. Will a metal dissolve in acid?
Metals listed below the reduction of hydrogen ions will dissolve in acid
Whereas metals listed above the reduction of hydrogen ions will not dissolve in acid
There is one exception:
Nitric acid can dissolve metals through this reaction
NO3-(aq) + 4 H+(aq) + 3 e- → NO(g) + 2 H2O(L) Eo= 0.96V

10. Question
A) This reaction does not occur spontaneously in the forward direction.
B) This reaction does occur spontaneously in the forward direction.
C) This reaction does occur spontaneously in the forward direction.

11. College Board Stuff

12. College Board Stuff (Cont.)

16. Copy and pastes
Eocathode
Eoanode
Eocell
Eofinal
Eoinitial H+
2e-

17. Links