1a. Describe three types of equilibria that are found on a E_h-pH (Pourbaix) diagram. Include how and why each differs from the others in terms of their response to changes in pH and electrode potential. (5 points each)

1. Electrochemical half cell reactions involving only charge transfer (i.e., electrons but not H⁺ or OH^- ions). These are horizontal lines, i.e., the reaction is independent of pH.
2. Electrochemical half cell reactions involving electrons and H⁺ or OH^- ions (sloped lines). These reactions are affected by both pH and E.
3. Chemical reactions involving only H⁺ or OH^- ions, i.e. no potential dependency (vertical lines).

1b. Label the following reactions as to their type (i, ii, or iii above): (1 point each)

1c.Calculate the reversible potential for the above H₂ reaction (H⁺ + e = 1/2 H₂) in a pH = 4 solution

2a. Draw an electrical circuit for polarization of a sample in the cathodic (reduction) direction using a battery, and other circuit elements. Label all circuit elements, show the current flow direction, show the anion and cation flow directions in the electrolyte, show the polarity of one of the battery terminals, i.e., (+) if the battery attracts electrons and (-) if the battery repels electrons, and state which meter outputs are the axes of the polarization curve.

Label I 2 Battery polarity 2 WE 1 Flowdir 2 CE 1 RE 1 Meters 2 Axes 2

2b. What are the two circumstances that determine the optimum positioning of the Luggin capillary tip with respect to the W.E. surface? (5 points each)

1. Tip must not be so close to W. E. that it affects the rate in the region of the tip.
2. The closer the tip is to the W. E., the smaller is the contribution of IR voltage in the measurement of E.

In this regard define parameters in the relation 1>2D and explain how to modify them in order to minimize the IR component of the measurement E voltage while remaining consistent with (i) and (ii).

1. The overall reaction divides into oxidation and reduction (partial) reactions
2. Because of a spacial separation of the items in (i), the two conduction processes that occur are motion of electrons in the metal and the motion of cations and anions (or positive and negative ions) in the electrolyte phase.
3. The valency of two or more of the species of the overall reaction change in the following ways:

The valency is higher in the product species for an oxidation reaction, and the valency is lower in product species of a reduction reaction.

4. Describe a physical situation at the electrode surface that is equivalent to the open circuit condition in an electrical circuit.

4a. Would a piece of copper corrode in a 25 C acid solution (pH0, [Cu²⁺] = 10^-6 M) exposed to air? (2 points)

Support your answer with a calculation of the free energy change. The relevant equilibria are some of the following reactions:

Cu2+ + 2e- = Cu Cu2+ + e- = Cu+ Cu(OH)2 + 2e- = Cu + 2 OH- H+ + e- = 1/2 H2 O2 + 4H+ + 4e- = 2H2O O2 + 2H2O + 4e- = 4OH-

Eo = 0.34 V Eo = 0.16 V Eo = -0.224 V Eo = 0 V Eo = 1.229 V Eo = 0.401 V

4b. Crosshatch the E- pH area on the accompanying diagram where Cu will corrode. (5 points)