Exam 4A, Spring 2002

1. Complete and balance the following equations.

a. manganese (II) hydroxide plus phosphoric acid

b. SrCl2(s) + Na2SO4(aq)

c. S2O82–(aq) + H3PO3(aq) → SO42–(aq) + H3PO4(aq)

d. Ni(OH)3(s) + As(s) → Ni(OH)2(s) + AsO2–(aq)

2. Does Zn(OH)2 have a higher solubility in an unbuffered solution with pH =10.00 or a buffered solution with pH = 10.00? Explain your answer using balanced chemical reactions, showing how you interpret the reactions to arrive at your conclusion. Ksp for Zn(OH)2 = 1.2×10–17.

a. Write the balanced reaction.

b. Find ΔH° for the reaction in units of kJ.

c. Find ΔS° for the reaction in units of J/K.

d. Without doing any calculations, would you expect the reaction to be spontaneous or nonspontaneous? Why?

4. Ammonium ion, NH4+, is a weak acid.

a. Write the reaction in water.

b. The Ka for ammonium ion was measured at several temperatures and the following plot was made:

Find the standard enthalpy change (in units of kJ/mol) and standard entropy change (in units of J/mol·K).

c. For a 0.1 M solution, does the pH increase or decrease as the temperature is raised? Explain.

5. Consider the cell: Zn(s) | Zn2+(aq) || O2(g) | H+(aq) | C(s)

a. What is the balanced oxidation half-reaction?

b. What is the balanced reduction half-reaction?

c. What is the balanced net reaction?

d. What is the standard potential for the cell?

e. Is the reaction spontaneous or nonspontaneous? Why?

f. Does the cell potential increase or decrease as the pH increases? Explain.

Answers