Final Exam, Spring 1999

1. Complete and balance the following reactions.

a. HCl(aq) + NaOH(aq)

b. NiCl₂(aq) + NaOH(aq)

c. (CH₃)₃N(aq) + H₂O(l)

d. Zn²⁺(aq) + H₂O(l)

e. Zn²⁺(aq) + NH₃(aq)

f. Ca(OH)₂(s) + H₂SO₄(aq)

g. HCl(aq) + HCO₃^–(aq)

h. Co(NO₃)₂(aq) + H₂S(aq)

i. Cr₂O₇^2–(aq) + NO(g) → Cr³⁺(aq) + HNO₃(aq)

j. H₂O₂(aq) + Fe(s) → Fe(OH)₂(s)

2. Methane, CH₄, is the primary component of natural gas and is a clean burning, high energy density fuel:

CH₄(g) + O₂(g) → CO₂(g) + H₂O(g)

a. Find ΔH° for the combustion of methane.

b. Find ΔS° for the combustion of methane.

c. Find ΔG° for the combustion of methane at 25 °C.

d. Estimate ΔG° for the combustion of methane at 400 °C

e. Estimate the maximum amount of work possible for the combustion of 1.00 mol of methane at 400 °C.

3. The following rate data was determined for the decomposition of nitrogen dioxide into nitrogen and oxygen.

T (°C)k (L/mol·s)

3200.514

3300.789

3401.006

3501.710

a. Write the balanced reaction.

b. Make an Arrhenius plot of the data.

c. From your plotted data, estimate the activation energy for the reaction.

d. What is the order of the reaction?

4. Find the value of K_a for the hydronium ion.

5. A 0.0010 M solution of (CH₃)₂NH is prepared at 25 °C.

a. What is the pH of the solution?

b. 50.0 mL of the dimethylamine solution is mixed with 1.0 mL of 0.025 M HCl. What is the pH of the mixed solution at 25 °C?

6. This question is concerned with the chemistry of carbon dioxide. Consider the reaction of carbon dioxide with water at 25 °C:

CO₂(g) + H₂O(l) → ← H₂CO₃(aq)

a. Find ΔH°.

b. Find ΔS°.

c. Find ΔG° at 25 °C.

d. Find the thermodynamic equilibrium constant for the reaction at 25 °C.

e. The atmospheric pressure of carbon dioxide is 0.00036 atm. Find the concentration of carbonic acid under these conditions.

f. Find the pH of an aqueous solution in equilibrium with atmospheric carbon dioxide.

7. Consider CoS.

a. What is the molar solubility in pure water at 25 °C?

b. What is the molar solubility in 0.010 M Co(NO₃)₂ solution?

8. The commercial alkaline dry cell is Zn(s) | Zn(OH)₂(s) || MnO₂(s) | Mn₂O₃(s) | C(s) and the electrolyte for both half-cells is an aqueous paste of potassium hydroxide.

a. Write the oxidation half-reaction under the pH conditions of the cell.

b. Write the reduction half-reaction under the pH conditions of the cell.

c. Write the net reaction under the pH conditions of the cell.

d. E° for the cell is 1.5 V. Find the cell potential when the reactants are 25% consumed.

e. A cell is built using 10.0 g of each reactant. Assume that the cell dies because the limiting reagent is consumed. If you are using this battery in your portable CD player that draws 1.0 mA, how many hours can you listen to music at the beach?

Answers