Exam 4B, Spring 2003

1. Complete and balance the following reactions.

a. magnesium (II) hydroxide plus phosphoric acid

b. Cu(OH)2(s) + H2SO4(aq)

c. Co3+(aq) + NH3(aq)

d. HNO3(aq) + SO2(g) → NO(g) + H2SO4(aq)

e. Co(OH)2(s) + ClO4–(aq) → Co(OH)3(s) + ClO3–(aq)

2. Look at the periodic trend for the enthalpies of formation and absolute entropies of the group 2 chlorides and hydroxides. Notice that the values for strontium hydroxide are missing. Make an estimate of the enthalpy of formation and the absolute entropy of strontium hydroxide, accurate to 10%.

3. Consider the reaction between solid calcium hydroxide and aqueous hydrochloric acid at 25 °C.

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. Consider the zinc air cell: Zn(s)|Zn(OH)42–(aq), pH=11 || O2(g), pH=11 |Pt(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 of decrease as the pH increases? Explain.

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