# AP Chemistry 2011 Scoring Guidelines

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1 AP Chemistry 2011 Scoring Guidelines The College Board The College Board is a not-for-profit membership association whose mission is to connect students to college success and opportunity. Founded in 1900, the College Board is composed of more than 5,700 schools, colleges, universities and other educational organizations. Each year, the College Board serves seven million students and their parents, 23,000 high schools, and 3,800 colleges through major programs and services in college readiness, college admission, guidance, assessment, financial aid and enrollment. Among its widely recognized programs are the SAT, the PSAT/NMSQT, the Advanced Placement Program (AP ), SpringBoard and ACCUPLACER. The College Board is committed to the principles of excellence and equity, and that commitment is embodied in all of its programs, services, activities and concerns. College Board, ACCUPLACER, Advanced Placement Program, AP, AP Central, SAT, SpringBoard and the acorn logo are registered trademarks of the College Board. Admitted Class Evaluation Service is a trademark owned by the College Board. PSAT/NMSQT is a registered trademark of the College Board and National Merit Scholarship Corporation. All other products and services may be trademarks of their respective owners. Permission to use copyrighted College Board materials may be requested online at: AP Central is the official online home for the AP Program: apcentral.collegeboard.com.

2 Question 1 1. Each of three beakers contains 25.0 ml of a M solution of HCl, NH 3, or NH 4 Cl, as shown above. Each solution is at 25 C. (a) Determine the ph of the solution in beaker 1. Justify your answer. ph = -log[h ] = -log(0.100) = point is earned for the correct ph. (b) In beaker 2, the reaction NH 3 (aq) H 2 O(l) NH 4 (aq) OH (aq) occurs. The value of K b for NH 3 (aq) is at 25 C. (i) Write the K b expression for the reaction of NH 3 (aq) with H 2 O(l). K b = 4 [NH ][OH ] [NH ] 3 1 point is earned for the correct expression. (ii) Calculate the [OH - ] in the solution in beaker 2. Let [OH ( x)( x) ] = x, then K b = (0.100 x) Assume that x << M, then = 2 x x = [OH ] = M 1 point is earned for the correct setup. 1 point is earned for the correct answer. (c) In beaker 3, the reaction NH 4 (aq) H 2 O(l) NH 3 (aq) H 3 O (aq) occurs. (i) Calculate the value of K a for NH 4 (aq) at 25 C. K a = K K w b = = point is earned for the correct answer.

3 Question 1 (continued) (ii) The contents of beaker 2 are poured into beaker 3 and the resulting solution is stirred. Assume that volumes are additive. Calculate the ph of the resulting solution. In the resulting solution, [NH 3 ] = [NH 4 ]; K a = = [NH ][H O ] [NH ] Thus [H 3 O ] = ; ph = -log( ) = point is earned for noting that the solution is a buffer with [NH 3 ] = [NH 4 ]. 1 point is earned for the correct ph. (d) The contents of beaker 1 are poured into the solution made in part (c)(ii). The resulting solution is stirred. Assume that volumes are additive. (i) Is the resulting solution an effective buffer? Justify your answer. The resulting solution is not an effective buffer. Virtually all of the NH 3 in the solution formed in (c)(ii) will react with the H 3 O from solution 1: NH 3 H 3 O NH 4 H 2 O leaving mostly NH 4 in the final solution. Since only one member of the NH 4 /NH 3 conjugate acid-base pair is left, the solution cannot buffer both base and acid. 1 point is earned for the correct response with an acceptable justification. (ii) Calculate the final [NH 4 ] in the resulting solution at 25 C. moles = (volume)(molarity) moles H 3 O in sol. 1 = (0.0250)(0.100) = mol moles NH 3 in sol. 2 = (0.0250)(0.100) = mol moles NH 4 in sol. 3 = (0.0250)(0.100) = mol When the solutions are mixed, the H 3 O and NH 3 react to form NH 4, resulting in a total of mol NH 4. The final volume is the sum ( ) = 75.0 ml. 1 point is earned for the correct calculation of moles of NH 4. 1 point is earned for the correct calculation of the final volume and concentration. The final concentration of NH 4 = ( mol/ L) = M.

5 Question 2 (continued) NaHCO 3 (aq) should be used. The HCO 3 ion will react as a base to neutralize the HNO 3. 1 point is earned for the correct choice with explanation. Then the student pours 25 ml of the 6 M HNO 3 into a beaker and adds a g sample of the alloy. After the sample completely reacts with the acid, some saturated NaCl(aq) is added to the beaker, resulting in the formation of an AgCl precipitate. Additional NaCl(aq) is added until no more precipitate is observed to form. The precipitate is filtered, washed, dried, and weighed to constant mass in a filter crucible. The data are shown in the table below. Mass of sample of copper-silver alloy Mass of dry filter crucible Mass of filter crucible and precipitate (first weighing) Mass of filter crucible and precipitate (second weighing) Mass of filter crucible and precipitate (third weighing) g g g g g (b) Calculate the number of moles of AgCl precipitate collected. mass of AgCl collected = ( ) g = g g 1 ( ) g mol = mol AgCl 1 point is earned for the correct mass of AgCl. 1 point is earned for the correct number of moles of AgCl given with the correct number of significant figures. (c) Calculate the mass percent of silver in the alloy of copper and silver g Ag mol Ag 1 mol Ag g 100% = 62.49% Ag g = g Ag 1 point is earned for the correct setup and the correct calculation of the mass of Ag. 1 point is earned for the correct percent of Ag.

6 Question 3 Hydrogen gas burns in air according to the equation below. 2 H 2 (g) O 2 (g) 2 H 2 O(l) (a) Calculate the standard enthalpy change, (The molar enthalpy of formation, DH o 298, for the reaction represented by the equation above. DH o f, for H 2 O(l) is kj mol 1 at 298 K.) D o = [2 ( 285.8)] [2(0) 1(0)] = kj mol 1 1 point is earned for the correct answer. H 298 (b) Calculate the amount of heat, in kj, that is released when 10.0 g of H 2 (g) is burned in air. 1 mol H kj q = 10 g H g H 1 mol H = kj point is earned for the correct setup. 1 point is earned for the correct answer. (c) Given that the molar enthalpy of vaporization, H o 298 DH o vap, for H 2 O(l) is 44.0 kj mol 1 at 298 K, what is the standard enthalpy change, D, for the reaction 2 H 2 (g) O 2 (g) 2 H 2 O(g)? 2 H 2 (g) O 2 (g) 2 H 2 O(l) kj 2 H 2 O(l) 2 H 2 O(g) 2(44.0) kj 2 H 2 (g) O 2 (g) 2 H 2 O(g) kj 1 point is earned for the correct answer. A fuel cell is an electrochemical cell that converts the chemical energy stored in a fuel into electrical energy. A cell that uses H 2 as the fuel can be constructed based on the following half-reactions. Half-reaction E o (298 K) 2 H 2 O(l) O 2 (g) 4 e 4 OH (aq) 0.40 V 2 H 2 O(l) 2 e H 2 (g) 2 OH (aq) 0.83 V

7 Question 3 (continued) (d) Write the equation for the overall cell reaction. 2 H 2 O(l) O 2 (g) 4 e 4 OH (aq) 2 H 2 (g) 4 OH (aq) 4 H 2 O(l) 4 e 2 H 2 (g) O 2 (g) 2 H 2 O(l) 1 point is earned for the correct equation. (e) Calculate the standard potential for the cell at 298 K. E = 0.40 V ( 0.83 V) = 1.23 V 1 point is earned for the correct answer. (f) Assume that 0.93 mol of H 2 (g) is consumed as the cell operates for 600. seconds. (i) Calculate the number of moles of electrons that pass through the cell mol H 2 2 mol e = 1.9 mol e 1 point is earned for the correct answer. 1 mol H 2 (ii) Calculate the average current, in amperes, that passes through the cell. 1.9 mol e 96,500 C 1 mol e = C q 5 I = = C = t 2 amps 600. s 1 point is earned for calculation of the charge in coulombs. 1 point is earned for calculation of the current in amperes. (g) Some fuel cells use butane gas, C 4 H 10, rather than hydrogen gas. The overall reaction that occurs in a butane fuel cell is 2 C 4 H 10 (g) 13 O 2 (g) 8 CO 2 (g) 10 H 2 O(l). What is one environmental advantage of using fuel cells that are based on hydrogen rather than on hydrocarbons such as butane? Hydrogen fuel cells produce only water as a product, unlike fuel cells that use hydrocarbons, which release carbon dioxide. Carbon dioxide contributes to global warming via the enhanced atmospheric greenhouse effect. 1 point is earned for an acceptable environmental advantage.

8 Question 4 For each of the following three reactions, write a balanced equation for the reaction in part (i) and answer the question about the reaction in part (ii). In part (i), coefficients should be in terms of lowest whole numbers. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solutions as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You may use the empty space at the bottom of the next page for scratch work, but only equations that are written in the answer boxes provided will be scored. (a) Solid magnesium hydroxide is added to a solution of hydrobromic acid. 1 point is earned for the correct reactants. (i) Mg(OH) 2 2 H Mg 2 2 H 2 O 2 points are earned for the correct products. 1 point is earned for correctly balancing the equation for both mass and charge. (ii) What volume, in ml, of 2.00 M hydrobromic acid is required to react completely with 0.10 mol of solid magnesium hydroxide? mol H or HBr = 0.10 mol Mg(OH) mol H 1.00 L 2.00 mol H 2 mol H 1 mol Mg(OH) 1,000 ml = 100 ml 1.00 L 2 = 0.20 mol H 1 point is earned for the correct volume. (b) Excess hydrochloric acid is added to a solution of cobalt(ii) nitrate to produce a coordination complex. (i) Co 2 4 Cl [CoCl 4 ] 2 Note: any number of coordinated Cl from 1 to 6 is acceptable. ions 2 points are earned for the correct reactants. 1 point is earned for the correct product. 1 point is earned for correctly balancing the equation for both mass and charge. (ii) Which species in the reaction acts as a Lewis base? Cl functions as a Lewis base. 1 point is earned for the correct identification of the Lewis base.

9 Question 4 (continued) (c) A copper wire is dipped into a solution of silver(i) nitrate. (i) Cu 2 Ag Cu 2 2 Ag 1 point is earned for the correct reactants. 2 points are earned for the correct products. 1 point is earned for correctly balancing the equation for both mass and charge. (ii) Describe what is observed as the reaction proceeds. Silver metal will appear on the surface of the copper wire. OR The solution will turn blue. OR The copper wire will lose mass. 1 point is earned for any one of the observations.

10 Question 5 Hydrazine is an inorganic compound with the formula N 2 H 4. (a) In the box below, complete the Lewis electron-dot diagram for the N 2 H 4 molecule by drawing in all the electron pairs. The correct Lewis diagram has single bonds between each pair of atoms and a lone pair of electrons on each N atom (a total of 14 e ). 1 point is earned for the correct Lewis diagram. (b) On the basis of the diagram you completed in part (a), do all six atoms in the N 2 H 4 molecule lie in the same plane? Explain. No, they do not. The molecular geometry surrounding both nitrogen atoms is trigonal pyramidal. Therefore the molecule as a whole cannot have all the atoms in the same plane. 1 point is earned for a correct answer with a valid explanation. (c) The normal boiling point of N 2 H 4 is 114 C, whereas the normal boiling point of C 2 H 6 is 89 C. Explain, in terms of the intermolecular forces present in each liquid, why the boiling point of N 2 H 4 is so much higher than that of C 2 H 6. N 2 H 4 is a polar molecule with London dispersion forces, dipole-dipole forces, and hydrogen bonding between molecules, whereas C 2 H 6 is nonpolar and only has London dispersion forces between molecules. It takes more energy to overcome the stronger IMFs in hydrazine, resulting in a higher boiling point. 1 point is earned for correct reference to the two different types of IMFs. 1 point is earned for a valid explanation based on the relative strengths of the IMFs.

11 Question 5 (continued) (d) Write a balanced chemical equation for the reaction between N 2 H 4 and H 2 O that explains why a solution of hydrazine in water has a ph greater than 7. N 2 H 4 H 2 O N 2 H 5 OH 1 point is earned for a valid equation. N 2 H 4 reacts in air according to the equation below. N 2 H 4 (l) O 2 (g) N 2 (g) 2 H 2 O(g) H = 534 kj mol 1 (e) Is the reaction an oxidation-reduction, acid-base, or decomposition reaction? Justify your answer. The reaction is an oxidation-reduction reaction. The oxidation state of N changes from 2 to 0 while that of O changes from 0 to 2. 1 point is earned for the correct choice with a valid justification. (f) Predict the sign of the entropy change, S, for the reaction. Justify your prediction. The entropy change for the reaction is expected to be positive. There are three moles of gas produced from one mole of liquid and one mole of gas. The net increase of two moles of gas results in a greater entropy of products compared to the entropy of reactants. 1 point is earned for the correct prediction with a valid justification. (g) Indicate whether the statement written in the box below is true or false. Justify your answer. The large negative ΔH for the combustion of hydrazine results from the large release of energy that occurs when the strong bonds of the reactants are broken. The statement is false on two counts. First, energy is released not when bonds are broken, but rather when they are formed. Second, the bonds in the reactants are relatively weak compared to the bonds in the products. 1 point is earned for correctly identifying the statement as false along with a valid justification.

13 Question 6 (continued) In a second experiment, which is performed at a much higher temperature, a sample of ethanol gas and a copper catalyst are placed in a rigid, empty 1.0 L flask. The temperature of the flask is held constant, and the initial concentration of the ethanol gas is M. The ethanol begins to decompose according to the chemical reaction represented below. CH 3 CH 2 OH(g) Cu æææ CH 3 CHO(g) H 2 (g) The concentration of ethanol gas over time is used to create the three graphs below. (c) Given that the reaction order is zero, one, or two, use the information in the graphs to respond to the following. (i) Determine the order of the reaction with respect to ethanol. Justify your answer. The order of the reaction is zero. The plot on the left is a straight line, indicating that the rate of decrease in [ethanol] is constant as [ethanol] changes. Therefore the rate of reaction does not depend on [ethanol]. 1 point is earned for the correct choice with a valid justification. (ii) Write the rate law for the reaction. rate = k 1 point is earned for the correct rate law. (iii) Determine the rate constant for the reaction, including units. D[ethanol] ( ) mol/l rate = k = - = - Dt 2000 s = M s 1 1 point is earned for the correct setup. 1 point is earned for the correct units.

14 Question 6 (continued) (d) The pressure in the flask at the beginning of the experiment is 0.40 atm. If the ethanol completely decomposes, what is the final pressure in the flask? The final pressure is 0.80 atm (twice the original pressure because the products represent twice as many moles of gas as the reactant). 1 point is earned for the correct final pressure.

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