Expert: Dr. Henry Boyter, Jr. - 11/29/2008

Question
Can you explain what reduction potentials are? Specifically, how do they relate
whether a reaction will take place?

Thank you.

Answer
Let's take a look at some.  They are representatives of how atoms and molecules lose or gain electrons in reactions.  A reduction potential can not stand alone.  There has to be two of them to have a reaction.  I'm using the list at

http://www.jesuitnola.org/upload/clark/refs/red_pot.htm

Take the two extremes

F2 + 2e- = 2F-         Eo= +2.87
Li+ + e- = Li   Eo= -3.05

Now to make a reaction, you add the reduction potentials in a balanced fashion so the electrons cancel out.  So we have two possibles.

F2 + 2e- = 2F-         Eo= +2.87
2Li = 2e- + 2Li+       Eo= +3.05 (reversed reaction -change the sign)
F2 + 2Li = 2F- + 2Li+  Eo=+5.92 (add them up)


2F- = F2 + 2e-           Eo= -2.87 (sign reversed)
2Li+ + 2e- = 2Li         Eo= -3.05 (don't multiply the Eo by 2)
2F- + 2Li+ = F2 + 2Li     Eo=-5.92

Now from your earlier studies of atoms and ions, you should know that Li likes being Li+ and F likes being F-, so you would think the first version more likely and it is the likely one.  Notice the Eo is positive.  So any reaction where the total Eo is positive is more likely to happen.

See these websites for more examples


http://www.chem.vt.edu/chem-ed/echem/redox-std-potentials.html

http://www.science.uwaterloo.ca/~cchieh/cact/c123/balance.html