J. RuslingSpring, 2008
CHEM 336 - ELECTROANALYTICAL CHEMISTRY
Exam 1, 21 February 2008
(20%)
1. Electrochemical reductions in electrolytic cells involve transfer of electrons across the electrode-solution interface to a molecular or ionic species that accepts those electrons. For a one electron reduction of a neutral organic molecule, illustrate by a diagram with electrode potential on the Y-axis, what happens to the simple molecular orbitals of the electron acceptor molecule when the electron is transferred. Use a simple representation of the highest occupied molecular orbital (HOMO), and the lowest unoccupied MO (LUMO).
(20%)
2. Consider what happens during a potential step applied to a 3-electrode electrochemical cell with a time constant (CdRs) of 2 ms having a gold working electrode and a Ag/AgCl reference electrode in an aqueous solution of 0.1 M NaNO3.The initial potential at t<0 of -0.2 V vs. Ag/AgCl is slightly negative of the potential of zero charge (pzc) of the Au electrode. A potential pulse of +1.0 V is applied at t=0, after which the Au electrode has a rather large positive charge (qm). Assume that no reduction or oxidation processes occur in this experiment.
(a)Draw schematic diagrams of the Au electrode/solution interface representing the structure of the electric double layer on the solution side at t<0 and at t=20 ms, that is, before and well after the potential pulse. Show ions on the solution side.
(b)How does the current (I) vary depend on time at t>0. Draw an appropriate I vs. t graph and indicate its mathematical form.
(10%)
3. Beginning with the Nernst equation, show why a (Ag/AgCl , saturated KCl) electrode
maintains a constant half-cell potential and thus can be used as a reference electrode in an electrochemical cell. Explain your answer.
(10%)
4. Consider the following electrochemical half reactions:
Cu2+ + 2 e Cu(s)Eo = 0.340 V
Hg2+ + 2 e Hg(l)Eo = 0.851 V
If you place a pure, clean copper wire into a solution of mercuric nitrate, what would you expect to occur ( or what would you see) on the surface of the wire? (Note: elemental copper forms an amalgam with mercury.) Write the chemical equation for your proposed reaction, and estimate its standard reduction potential.
(20%)
5. The Butler-Volmer equation is the classic kinetic relationship between current (i) in and electrochemical cell and the applied potential (E) for the reaction:
kf
O + ne R
kb
(a) What is the relationship of kf and kb to the terms in the above equation? Write the Butler-Volmer equation using kf and kb.
(b) In terms of the electrochemical free energy of the reaction, explain why kf and kb depend on the potential applied to the electrochemical cell.
(20%)
6. Diffusion and mass transport:
(a) (10%) What is the driving force for diffusion? Explain how this driving force develops during electrolysis of O to R at a working electrode in a quiet solution.
kf
O + ne R
kb
(b) (10%) Draw the shapes of the two different linear scan voltammetry I vs E curves that would be obtained in a quiet and in a stirred solution. Use a qualitative description of mass transport combined with Butler-Volmer theory to describe the shape differences in the two I vs E curves