Prelaboratory Questions And Problems

*1. Knowing that Zn(II) hydroxide is amphoteric and that it forms divalent ions in acidic and in basic solutions, a) Replace the alphabet letters in the boxes in Figure 2 with the chemical formulas of the missing species.

b) Write the (balanced) equations that describe each one of the equilibrium lines in this diagram (except the dotted ones, that represent the water equilibria; these two lines do not participate directly in the Zn equilibria).

c) Justify chemically and/or algebraically the type of slope in each Zn equilibrium line.

*2. Calculate and draw the Pourbaix diagram for Cu in the E range of -2 to +2 V vs NHE (normal hydrogen electrode), and pH range 0-20

at 1 M concentration of dissolved species, 25°C, and 1 atm. You may draw a simplified diagram, considering only the following Cu species: (a) dissolved species (Cu+, Cu2+, CuO2-), and (b) solid species [Cu, Cu20(S), Cu(OH)2(S)]. The equations and the associated thermodynamic data necessary to calculate this diagram are given below. The calculations assume that the solutions do not contain dissolved oxygen. For the sake of simplicity, the physical state of the aqueous species is not specified.

Note: If you have not studied the construction of these diagrams, you may obtain one from the literature and insert it here.

Pourbaix Diagram For Cuoh
C:\HSc4\tempZn25.iep pH

ELEMENTS Molality Pressure

Zn 1.000E-03 1.000E+00

Figure 2. A simplified Pourbaix diagram of Zn. (Drawn with the HSC Chemistry 4.0 commercial program).

2Cu(OH)2(S) + 2H+ + 2e~ «=* Cu20(s) + 3H20(I) E° = 0.73 V (8)

2Cu022" + 6H+ + 2e~ ^ Cu20(s) + 3H2Oa) E° = 2.67 V (9)

Note that Cu Jq) does not appear in the diagram because its disproportionation reaction into Cu2aq) and Cu(s) is spontaneous, as can be calculated from equations 4 and 6.

*3. a) Balance the reactions for the oxidation and reduction of water (both written as reductions). Note that in the reduction of water, the species that becomes reduced is the hydrogen and not the oxygen; thus, the reduction reaction is often written as the reduction of protons:

b) Write the two algebraic equations that relate the standard potential of each reaction and pH, as derived from the Nernst equation. Make the assumptions and simplifications that you may deem reasonable. Use T = 25°C, 1 atm, concentration of all dissolved species = 1M. You may use (RT/F) In x — 0.06 log x, RT/F = 0.0257, £°h+/h2 = 0.00 V and

c) With these data, draw the £-pH (Pourbaix) for water.

Additional Related Projects

• Observe transitions among different regions involving the production or disappearance of V or Co species, as the pH and/or E vary in an aqueous solution. (See Powell, 1987).

• Predict the change in standard potentials of some metal ion couples upon complexation with selected ligands. For example, predict and then test the oxidizing or reducing power of the Fe3+^2+ couple with a solution of I~ and a solution of I_/I2. Now add some EDTA to these mixtures. Analyze your observations in the light of the new standard potential of the metal ion couple upon complexation (see Section 2.4 and problem 2 above). See Napoli, 1997 and Ibanez, 1988.

• Calculate and draw the Pourbaix diagram for iron. Compare it to those drawn by Pourbaix, 1974 and Barnum, 1982.

* Answer in the book's webpage at www.springer.com

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