AP Free Response

(a) Some solid NH₄HS is placed in an evacuated vessel at 25 °C. After equilibrium is attained , the total pressure inside the vessel is found to be 0.659 atmosphere. Some solid NH₄HS remains in the vessel at equilibrium. For this decomposition, write the expression for K_p and calculate its numerical value at 25 °C.

(b) Some extra NH₃ gas is injected into the vessel containing the sample described in part (a). When equilibrium is reestablished at 25 °C, the partial pressure of NH₃ is twice the partial pressure of H₂S. Calculate the numerical value of the partial pressure of NH₃ and the partial pressure of H₂S in the vessel after the NH₃ has been added and equilibrium has been reestablished.

(c) In a different experiment, NH₃ gas and H₂S gas are introduced into an empty 1.00-liter vessel at 25 °C. The initial partial pressure of each gas is 0.500 atmosphere. Calculate the number of moles of solid NH₄HS that is present when equilibrium is established.

For the reaction above, carried out in solution at 30 °C , the following kinetic data were obtained:

Experiment	Initial concentration of Reactants mole liter¯¹		Initial Rate of Reaction mole liter¯¹ hour¯¹
	A_o	B_o
1	0.240	0.480	8.00
2	0.240	0.120	2.00
3	0.360	0.240	9.00
4	0.120	0.120	0.500
5	0.240	0.0600	1.00
6	0.0.140	1.35	?

(b) Calculate the value of the specific rate constant k at 30 °C and specify its units.

(c) Calculate the value of the initial rate of this reaction at 30 °C for the initial concentrations shown in experiment 6.

(d) Assume that the reaction goes to completion. Under the conditions specified for experiment 2, what would be the final molar concentration of C?

3) A 1.2156-gram sample of a mixture of CaCO₃ and Na₂SO₄ was analyzed by dissolving the sample and completely precipitating the Ca²⁺ as CaC₂O₄. The CaC₂O₄ was dissolved in sulfuric acid and the resulting H₂C₂O₄ was titrated with a standard KMnO₄ solution.

(a) On a page of your answer booklet, write the balanced equation for the titration reaction, shown balanced below.

Indicate which substance is the oxidizing agent and which substance is the reducing agent.

(b) The titration of the H₂C₂O₄ obtained required 35.62 milliliters of 0.1092-molar MnO₄¯ solution. Calculate the number of moles of H₂C₂O₄ that reacted with the MnO₄¯.

4) Use appropriate ionic and molecular formulas to show the reactants and the products for the following, each of which occurs in aqueous solution except as indicated. Omit formulas for any ionic or molecular species that do not take part in the reaction. You need not balance. In all cases a reaction occurs.

(d) A solution of ammonium sulfate is added to a saturated solution of barium hydroxide.

(g) A drop of potassium thiocyanate is added to a solution of iron(III) chloride.

5) A solution of CuSO₄ was electrolyzed using platinum electrodes by passing a current through the solution. As a result, there was a decrease in both [Cu²⁺] and the solution pH; one electrode gained in weight and a gas was evolved at the other electrode.

(a) Write the cathode half-reaction that is consistent with the observation above.

(b) Write the anode half-reaction that is consistent with the observations above.

(d) List the experimental measurement that would be needed in order to determine from such an experiment the value of the faraday.

6) Assume that you have two different gases that you know are not cyclic (i.e., not ring) compounds, each with the following elementary analysis: C, 85.7 per cent; H, 14.3 per cent. Each gas has a molecular weight of 56 ± 1.

(b) Draw the structural formulas for the four possible noncyclic isomers with this molecular formula.

(c) In the presence of an appropriate catalyst, both gases add hydrogen. The hydrogenated products are identical, their molecular weight is 58. Which of the structures you drew to answer (b) can definitely be eliminated on the basis of this additional information?

(a) Predict whether a 0.10-molar solution of each of the salts above is acidic, neutral, or basic.

(b) For each of the solutions that is not neutral, write a balanced chemical equation for a reaction occurring with water that supports your prediction.

8) The emission spectrum of hydrogen consists of several series of sharp emission lines in the ultraviolet (Lyman series). in the visible (Balmer series). and in the infrared (Paschen series, Brackett series, etc,) regions of the spectrum.

(a) What feature of the electronic energies of the hydrogen atom explains why the emission spectrum consists of discrete wavelengths rather than a continuum of wavelengths?

(b) Account for the existence of several series of lines in the spectrum. What quantity distinguishes one series of lines from another?

(c) Draw an electronic energy level diagram for the hydrogen atom and indicate on it the transition corresponding to the line of lowest frequency in the Balmer series.

(d) What is the difference between an emission spectrum and an absorption spectrum? Explain why the absorption spectrum of atomic hydrogen at room temperature has only the lines of the Lyman series.

(a) Does the tendency of reactions to proceed to a state of minimum energy favor the formation of the products of this reaction? Explain.

(b) Does the tendency of reactions to proceed to a state of maximum entropy favor the formation of the products of this reaction? Explain.

(d) State whether a decrease in the volume of the system at constant temperature drives this reaction to the right, to the left, or has no effect. Explain.

Advanced Placement Chemistry

1981 Free Response Questions