General Chemistry II
Dr. Michael Blaber
Effect of Concentration on Cell EMF
The EMF of a redox reaction in a voltaic cell is determined not only by the type of redox reaction, but also the concentrations of the reactants and products (i.e. the reducing agent and oxidizing agent)
Hint: Next time you find a dead battery, don't say "hey, this battery is dead". Instead, say "hey, the redox reaction in this voltaic cell has attained equilibrium, and now the electromotive force is zero". Do this when you visit your folks at home (don't do this in the dorm).
The Nernst Equation
Walther Hermann Nernst (1864 - 1941) was a German chemist who came up with an equation that related the EMF of a redox reaction on the concentration of reactants and products
DG = DG0 + RT lnQ
DG = -nFE
-nFE = -nFE0 + RT lnQ
solving for E yields:
This is the Nernst Equation. It allows us to do the following for redox reactions:
A redox reaction for the oxidation of zinc by copper ion is set up with an initial concentration of 5.0M copper ion and 0.050M zinc ion. What is the cell EMF at 298K?
Zn(s) + Cu2+(aq)® Zn2+(aq) + Cu(s)
In this case the value of n (the number of electrons transferred from Zn to Cu2+ in the redox reaction) is 2. The Standard EMF, E0, is 1.10 volt. Therefore, at 298K the Nernst equation gives:
Note: remember to ignore the "concentration" of solids in the equilibrium expression
E = 1.10 - 0.0296*log(0.05/5)
E = 1.10 + 0.0592 = 1.16V
This would seem to follow Le Chatelier's principle; we have loaded up the reaction with a high relative concentration of reactant (Cu2+) and the EMF is higher than the EMF under standard conditions (i.e. 1M or 1atm concentration for all reactants and products)
Equilibrium Constants for Redox Reactions
What happens when the redox reaction achieves equilibrium concentrations of reactants and products?
or, rearranging to solve for K:
What does this equation tell us?
A redox reaction where the equilibrium lies far to the right, will have a large value for K, and a large value for the standard EMF
2000 Dr. Michael Blaber