Advanced Placement Chemistry s2

Advanced Placement Chemistry Free Response Questions

1) The overall dissociation of oxalic acid, H2C2O4 is represented below. The overall dissociation constant is also indicated. + 2- -6 H2C2O4 <===> 2 H + C2O4 K = 3.78 x 10

a) What volume of 0.400-molar NaOH is required to neutralize completely a 5.00 x 10-3 mole sample of pure oxalic acid? b) Give the equations representing the first and second dissociations of oxalic acid. Calculate the value of the - first dissociation constant, K1, for oxalic acid if the value of the second dissociation constant, K2 is 6.40 x 10 5 2- c) To a 0.015-molar solution of oxalic acid, a strong acid is added until the pH is 0.5. Calculate the [C2O4 ] in the resulting solution. (Assume the change in volume is negligible.) d) Calculate the value of the equilibrium constant, Kb, for the reaction that occurs when solid Na2C2O4 is dissolved in water. 2) 2 A + B ---> C + D The following results were obtained when the reaction represented was studied at 25 °C Initial Initial Initial Rate of Formation Experiment [A] [B] of C (mol L¯1 min¯1) 1 0.25 0.75 4.3 x 10¯4 2 0.75 0.75 1.3 x 10¯3 3 1.50 1.50 5.3 x 10¯3 4 1.75 ?? 8.0 x 10¯3 a) Determine the order of the reaction with respect to A and B. Justify your answer. b) Write the rate law for the reaction. Calculate the value of the rate constant, specifying units. c) Determine the initial rate of change of [A] in Experiment 3. d) Determine the initial value of [B] in Experiment 4. e) Identify which of the reaction mechanisms represented below is consistent with the rate law developed in part (b). Justify your choice. i) A + B ---> C + M Fast ii) B <===> M Fast equilib iii) A + <===> M Fast equilib M + A ---> D Slow M + A ---> C + X Slow M + A ---> C + X Slow A + X ---> D Fast X ---> D Fast

3) Consider the molecules PF3 and PF5. a) Draw the Lewis electron-dot structures for PF3 and PF5 and predict the molecular geometry of each. b) Is the PF3 molecular polar, or is it nonpolar? Explain. c) On the basis of bonding principles, predict whether each of the following compounds exists. In each case, explain your prediction. (i) NF5 (ii) AsF5 4) Explain each of the following observations using principles of atomic structure and/or bonding. a) Potassium has a lower first-ionization energy than lithium. b) The ionic radius of N3- is larger than that of O2-. c) A calcium atom is larger than a zinc atom. d) Boron has a lower first-ionization energy than beryllium. 5) For the gaseous equilibrium represented below, it is observed that greater amounts of PCl3 and Cl2 are produced as the temperature is increased.

PCl5(g) <===> PCl3(g) + Cl2(g)

a) What is the sign of S° for the reaction? Explain. b) What change, if any, will occur in G° for the reaction as the temperature is increased. Explain your reasoning in terms of thermodynamic principles. c) If He gas is added to the original reaction mixture at constant volume and temperature, what will happen to the partial pressure of Cl2? Explain. d) If the volume of the original reaction is decreased at constant temperature to half the original volume, what will happen to the number of moles of Cl2 in the reaction vessel? Explain. 6) An experiment is to be performed to determine the mass percent of sulfate in an unknown soluble sulfate salt. A 0.20 molar solution of BaCl2 is available, as well as a drying oven. a) Briefly devise a list of steps needed to carry out this experiment. b) What experimental data need to be collected to calculate the mass percent of sulfate in the unknown? c) List the calculations necessary to determine the mass percent of sulfate in the unknown. d) Would 0.20-molar MgCl2 be an acceptable substitute for the BaCl2 solution provided for this experiment? Explain. 7) Solve the following problem related to the solubility equilibria of some metal hydroxides in aqueous solution. 6 a) The solubility of Cu(OH)2 is 1.72 x 10¯ gram per 100. milliliters of solution at 25 °C. (i) Write the balanced chemical equation for the dissociation of Cu(OH) 2 (s) in aqueous solution. (ii) Calculate the solubility (in moles per liter) of Cu(OH) 2 at 25 °C. (iii) Calculate the value of the solubility-product constant, Ksp, for Cu(OH) 2 at 25 °C. 17 b) The value of the solubility-product constant, Ksp, for Zn(OH) 2 is 7.7 x 10¯ at 25°C. (i) Calculate the solubility (in moles per liter) of Zn(OH) 2 at 25°C in a solution with a pH of 9.35. (ii) At 25°C, 50.0 milliliters of 0.100-molar Zn(NO3) 2 is mixed with 50.0 milliliters of 0.300-molar NaOH. Calculate the molar concentration of Zn2+(aq) in the resulting solution once equilibrium has been established. Assume that volumes are additive.

8) C6H5OH(s) + 7 O2 (g) ---> 6 CO2 (g) + 3H2O(l) When a 2.000-gram sample of pure phenol, C6H5OH (s), is completely burned according to the equation above, 64.98 kilojoules of heat is released. Use the information in the table below to answer the questions that follow. a) Calculate the molar heat of combustion of phenol in kilojoules per mole at 25°C. b) Calculate the standard heat of formation, ∆ H°f, of phenol in kilojoules per mole at 25°C. c) Calculate the value of the standard free-energy change, ∆ G° for the combustion of phenol at 25°C. d) If the volume of the combustion container is 10.0 liters, calculate the final pressure in the container when the temperature is changed to 110°C. (Assume no oxygen remains unreacted and that all products are gaseous.) Standard Heat of Formation, ∆ H°f, Absolute Entropy, S°, Substance at 25°C (kJ/mol) at 25°C (J/mol-K) C(graphite) 0.00 5.69 CO2(g) -395.5 213.6 H2(g) 0.00 130.6 H2O(l) -285.85 69.91 O2(g) 0.00 205.0 C6H5OH(s) ? 144.0 9) An approximately 0.1-molar solution of NaOH is to be standardized by titration. Assume that the following materials are available. Clean, dry 50 mL buret Analytical balance 250 mL Erlenmeyer flask Phenolphthalein indicator solution Potassium hydrogen phthalate, KHP, a pure Wash bottle filled with distilled water solid monoprotic acid (to be used as the primary standard) (a) Briefly describe the steps you would take, using materials listed above, to standardize the NaOH solution. (b) Describe (i.e., set up) the calculations necessary to determine the concentration of the NaOH solution. (c) After the NaOH solution has been standardized, it is used to titrate a weak monoprotic acid, HX. The equivalence point is reached when 25.0 mL of NaOH solution has been added. In the space provided at the right, sketch the titration curve, showing the pH changes that occur as the volume of NaOH solution added increases from 0 to 35.0 mL. Clearly label the equivalence point on the curve. (d) Describe how the value of the acid-dissociation constant, Ka, for the weak acid HX could be determined from the titration curve in part (c). (e) The graph above shows the results obtained by titrating a different weak acid, H2Y, with the standardized NaOH solution. Identify the negative ion that is present in the highest concentration at the point in the titration represented by the letter A on the curve.

6) Answer the following questions regarding the kinetics of chemical reactions. (a) The diagram below at right shows the energy pathway for the reaction O3 + NO --> NO2 + O2.

Clearly label the following directly on the diagram. (i) The activation energy (Ea) for the forward reaction (ii)The enthalpy change (∆ H) for the reaction (b) The reaction 2 N2O5 --> 4 NO2 + O2 is first order with respect to N2O5.

(i) Using the on the previous page, complete the graph that represents the change in [N2O5] over time as the reaction proceeds. (ii) Describe how the graph in (i) could be used to find the reaction rate at a given time, t. (iii) Considering the rate law and the graph in (i), describe how the value of the rate constant, k, could be determined. (iv) If more N2O5 were added to the reaction mixture at constant temperature, what would be the effect on the rate constant, k? Explain.

(c) Data for the chemical reaction 2A --> B + C were collected by measuring the concentration of A at 10-minute intervals for 80 minutes. The following graphs were generated from analysis of data. Use the information in the graphs above to answer the following. (i) Write the rate-law expression for the reaction. Justify your answer. (ii) Describe how to determine the value of the rate constant for the reaction. 7) C(s) + H2O(g) <===> CO(g) + H2 (g) ∆H° = +131 kJ A rigid container holds a mixture of graphite pellets (C(s)), H2O(g), CO(g), and H2 (g) at equilibrium. State whether the number of moles of CO(g) in the container will increase, decrease, or remain the same after each of the following disturbances is applied to the original mixture. For each case, assume that all other variables remain constant except for the given disturbance. Explain each answer with a short statement. (a) Additional H2 (g) is added to the equilibrium mixture at constant volume. (b) The temperature of the equilibrium mixture is increased at constant volume. (c) The volume of the container is decreased at constant temperature. (d) The graphite pellets are pulverized.

8) Answer the following questions regarding the electrochemical cell shown above. (a) Write the balanced net-ionic equation for the spontaneous reaction that occurs as the cell operates, and determine the cell voltage. (b) In which direction do anions flow in the salt bridge as the cell operates? Justify your answer. (c) If 10.0 mL of 3.0-molar AgNO3 solution is added to the half-cell on the right, what will happen to the cell voltage? Explain. (d) If 1.0 grams of solid NaCl is added to each half-cell, what will happen to the cell voltage? Explain. (e) If 20.0 mL of distilled water is added to both half-cells, the cell voltage decreases. Explain. 9) Answer each of the following using appropriate chemical principles. (a) Why does it take longer to cook an egg in boiling water at high altitude than it does at sea level? (b) When NH3 gas is bubbled into an aqueous solution of CuCl2, a precipitate forms initially. On further bubbling, the precipitate disappears. Explain these two observations. (c) Dimethyl ether, H3C-O-CH3, is not very soluble in water. Draw a structural isomer of dimethyl ether that is much more soluble in water and explain the basis of its increased water solubility. (d) Identify a chemical species that is (i) capable of oxidizing Cl¯(aq) under standard conditions (ii) capable of reducing Cl2(aq) under standard conditions. In each case, justify your choice. Copyright © 1998 by College Entrance Examination Board and Educational Testing Service. All rights reserved.

7) (a) (i) Cu(OH)2(s) <===> Cu2+(aq) + 2 OH¯ (aq) (one point) Correct stoichiometry and charges (but not phases) necessary No credit earned if water as a reactant or product (ii) 1.72 x 10¯ 6 g / 97.57 g/mol = 1.763 x 10¯ 8 mol Cu(OH)2 (one point) 1.763 x 10¯ 8 mol Cu (OH)2 / 0.100 L = 1.76 x 10¯ 7 moles per liter (one point) One point earned for conversion of mass to moles (need not to be computed explicity) One point earned for calculation of moles per liter (iii) [ Cu2+] = 1.76 x 10¯ 7 M [OH¯ ] = 2 x (1.76 x 10¯ 7 M ) = 3.52 x 10¯ 7 (one point) Ksp = [Cu2+] [OH¯ ]2 = (1.76 x 10¯ 7) (3.52 x 10¯ 7)2 = 2.18 x 10¯ 20 (one point) One point earned for correct [Cu2+] and [OH¯ ] One point for correct substitution into Ksp expression and answer Response need not include explicit statement of [OH¯ ] if Ksp expression is written with correct values of [Cu2+] and [OH¯ ] (b) (i) pH =9.35 ----> pOH = 4.65 ----> [OH¯ ] = 2.24 x 10¯ 5 M (one point) [Zn2+] = Ksp / [OH¯ ]2 = 7.7 x 10¯ 17 / (2.24 x 10¯ 5)2 = 1.5 x 10¯ 7 M (one point) One point earned for correct determination of [OH¯ ] One point for correct answer (assume [Zn2+] equals solubility in moles per liter) No points earned if [OH¯ ] is assumed equal to twice [Zn2+] (b) (ii) Zn2+ + 2 OH¯ ----> Zn(OH)2 initial amount 0.0050 mol 0.0150 mol 0 mol reacting final amount 0 mol 0.0050 mol 0.0050 mol

OR [OH¯ ] = 0.0050 mol / 0.100 L = 0.0050 M (one point) One point earned if precipitation reaction is clearly indicated and moles or concentration of OH¯ is calculated correctly Zn(OH)2(s) ---> Zn2+ + OH¯ x (0.050 + 2x) Ksp = 7.7 x 10¯ 17 = [Zn2+] [OH¯ ]2 = (x)(0.050 + 2x)2 = (x)(0.050)2 ----> [Zn2+] = 3.1 x 10¯ 14 M (one point) OR Zn(OH)2(s) ---> Zn2+ + 2 OH¯ (0.050-x (0.150-2x) Ksp = 7.7 x 10¯ 17 = [Zn2+] [OH¯ ]2 = (0.050 - x)(0.150 - 2x)2 (one point) Solve for x, then subtract x from 0.050 M to obtain [Zn2+] (one point)

8) (a) Assume a 100 gram sample ( not necessary for credit ): 65.60g C x (1 mol C / 12.01 g C) = 5.462 mol C 9.44g H x (1 mol H / 1.0079 g H) = 9.366 mol H mass O = [100 - (65.60 + 9.44)] = 24.96 g O 24.96 g O x (1 mol O / 15.9994 g O) = 1.560 mol O C5.462H9.366O1.560 ---> C3.5H6.0O1.0 ---> C7H12O2 One point earned for determining moles of C and moles of H One point earned for determining moles of O One point earned for correct empirical formula (b) m = DT / Kf = 15.2 °C /40.0 K kg mol¯ 1 = 0.380 mol / kg 0.01608 kg x (0.380 mo / 1 kg) = 0.00611 mol molar mass = 1.570 g/ 0.00611 mol = 257 g / mol One point earned for determination of molarity One point earned for conversion of molarity to molar mass OR, moles solute = (DT x kg solvent) / Kf = 0.00611 mol (one point) molar mass = 1.570 g / 0.00611 mol = 257 g / mol (one point) OR, molar mass = (mass x Kf) / (DT x kg solvent) = 257 g / mol (two points) empirical mass of C7H12O2 = 7(12) + 12(1) + 2(16) = 128 g/mol 128 g/mol = 1/2 molar mass ---> molecular formula = 2x ( empirical formula) -----> molecular formula = C14H24O4 (one point) One point earned if molecular formula is wrong but is consistent with empirical formula and molar mass No penalty for simply ignoring the van't Hoff factor Only one point earned for part (b) if response indicates that DT= (15.2 + 273) = 288 K and molar mass = 13.6 g / mol (c) n = (pV) / (RT) = [(1 atm) (0.577 L)] / [(0.0821 L atm mol°1 K°1) (573 K)] = 0.0123 mol (one point) molar mass = mass of sample / moles in sample = 1.570 g / 0.0123 mol = 128 g/mol (one point) Only one point can be earned for part (c) if wrong value for R is used and/or T is not converted from C to K (d) The compound must form a dimer in solution, because the molar mass in solution is twice that it is in the gas phase, OR, the compound must dissociate in the gas phase ( A (g) --> 2B (g)) because the molar mass in the gas phase is half that it is in solution. One point earned for a reference to either or both the ideas of dimerization and dissociation,br> -No point earned for a " non - ideal behavior " argument

9) (a) 4 essential steps (2 points) 1) weigh KHP 2) fill buret with NaOH solution 3) add indicator (phenolphthalein) 4) titrate to endpoint (color change) Two points earned for all 4 steps; one point earned for 2 or 3 steps Titration of acid into base accepted if described correctly (b) moles KHP = Mass KHP / molar mass KHP (one point) moles KHP = moles OH¯ at equivalence and (moles OH¯ / liters NaOH) = [ OH¯ ] (one point) Acceptable if some parts of part (b) appear in (a) (c) Curve should have 3 important features: (2 points) 1) Curve begins above pH 1, but below pH 7 2) Equivalence point at 25 mL 3) Equivalence point above pH 7 Both points earned for all 3 features One point earned for any 2 of the 3 features (d) At the halfway point in the titration, pH = pKa. (one point) (e) A point A in the titration, the anion in highest concentration is Y2¯ . Also acepted: Y¯ 2, Y¯ ¯ , Y=, and specific anions such as SO42¯ , SO32¯ HY¯ , Y¯ , and "Y ion" not accepted