CHM 1046
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
where Q is the reaction quotient

DG = -nFE
where n = moles of electrons transferred, F = 96,500 J/V mole e-, E = EMF in volts

-nFE = -nFE0 + RT lnQ

solving for E yields:

This is the Nernst Equation. It allows us to do the following for redox reactions:

  1. If we know the value of E0 and we measure the EMF of the cell (under non-standard conditions) we can determine the value of Q (and therefore the concentrations of reactants and products)
  2. If we know the value of E0 and the concentrations of reactants and products, then we can determine the resulting EMF of the cell

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