Exam 4B, Spring 2000

1. Complete and balance the following equations. Give the oxidation numbers for all of the atoms in both products and reactants except for hydrogen and oxygen. Predict the sign of ΔS° for each reaction.

a. H₃PO₄(aq) + N₂H₄(aq) → PH₃(g) + N₂(g)

b. Mn(s) + MnO₄^–(aq) → MnO₂(s) + Mn(OH)₂(s)

2. Consider the reaction between solid magnesium chloride and aqueous carbonic acid.

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. Find ΔG° for the reaction at 25 °C in units of kJ.

e. Find ΔG° for the reaction at 75 °C in units of kJ.

3. Estimate K_a1 for carbonic acid using thermochemical data.

4. Estimate the cell potential at standard condition and 25 °C for the cell

Pt(s) | H₃AsO₃(aq) | H₂AsO₄^–(aq) | H⁺(aq) || H₃AsO₄(aq) | H₃AsO₃(aq) | H⁺(aq) | Pt(s)

5. Consider the cell: Au(s) | Au³⁺(aq) || Fe²⁺(aq) | Fe(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?

Answers