AP CHEMISTRY 2007 SCORING GUIDELINES (Form B)
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1 AP CHEMISTRY 2007 SCORING GUIDELINES (Form B) Question 1 A sample of solid U O 8 is placed in a rigid L flask. Chlorine gas, Cl 2 (g), is added, and the flask is heated to 862 C. The equation for the reaction that takes place and the equilibrium-constant expression for the reaction are given below. ( puo2cl ) ( p 2 O ) 2 U O 8 (s) + Cl 2 (g) UO 2 Cl 2 (g) + O 2 (g) K p = ( p ) When the system is at equilibrium, the partial pressure of Cl 2 (g) is atm and the partial pressure of UO 2 Cl 2 (g) is atm. (a) Calculate the partial pressure of O 2 (g) at equilibrium at 862 C. Cl2 U O 8 (s) + Cl 2 (g) UO 2 Cl 2 (g) + O 2 (g) I ---? 0 0 C E atm atm? One point is earned for the correct answer atm UO 2 Cl 2 (g) (1 mol O 2) ( mol UO Cl ) = atm O 2 (g) 2 2 (b) Calculate the value of the equilibrium constant, K p, for the system at 862 C. ( puo2cl ) ( p 2 O ) 2 K p = ( p ) Cl 2 = 4 4 ( ) ( ) (1.007) One point is earned for the = correct substitution. One point is earned for the correct answer. (c) Calculate the Gibbs free-energy change, ΔG, for the reaction at 862 C. ΔG = RT ln K p = ( 8.1 J mol 1 K 1 )( (862+27) K)(ln ( )) = 272,000 J mol 1 = 272 kj mol 1 One point is earned for the correct setup. One point is earned for the correct answer with units The College Board. All rights reserved.
2 AP CHEMISTRY 2007 SCORING GUIDELINES (Form B) Question 1 (continued) (d) State whether the entropy change, ΔS, for the reaction at 862 C is positive, negative, or zero. Justify your answer. ΔS is positive because four moles of gaseous products are produced from three moles of gaseous reactants. One point is earned for the correct explanation. (e) State whether the enthalpy change, ΔH, for the reaction at 862 C is positive, negative, or zero. Justify your answer. Both ΔG and ΔS are positive, as determined in parts (c) and (d). Thus, ΔH must be positive because ΔH is the sum of two positive terms in the equation ΔH = ΔG + TΔS. One point is earned for the correct sign. One point is earned for a correct explanation. (f) After a certain period of time, mol of O 2 (g) is added to the mixture in the flask. Does the mass of U O 8 (s) in the flask increase, decrease, or remain the same? Justify your answer. The mass of U O 8 (s) will increase because the reaction is at equilibrium, and the addition of a product creates a stress on the product (right) side of the reaction. The reaction will then proceed from right to left to reestablish equilibrium so that some O 2 (g) is consumed (tending to relieve the stress) as more U O 8 (s) is produced. One point is earned for a correct explanation The College Board. All rights reserved.
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9 AP CHEMISTRY 2007 SCORING COMMENTARY (Form B) Question 1 Sample: 1A Score: 9 This response earned all 9 points: 1 for part (a), 2 for part (b), 2 for part (c), 1 for part (d), 2 for part (e), and 1 for part (f). Sample: 1B Score: 6 The point was not earned in part (a) because the student incorrectly attempts to use the Ideal Gas Law to determine the partial pressure of O 2 gas at equilibrium. Both points were earned in part (b) because the student correctly substitutes and calculates a value for the equilibrium constant using the pressure calculated in part (a). Both points were earned in part (c) because the student correctly substitutes and calculates a value for ΔG using the K p from part (b). The point was earned in part (d). The first point was earned for part (e) because the student correctly identifies the sign of ΔH as positive; however, the second point was not earned because the justification given relates to the heat added to start the reaction rather than a connection to the Gibb s equation. The point was not earned in part (f). Sample: 1C Score: The point was not earned in part (a). Both points were earned in part (b) because the student correctly substitutes and calculates a value for the equilibrium constant using the calculated pressure from part (a). Only 1 point was earned in part (c) because although the student substitutes correctly, using the K p value calculated in part (b), the incorrect R is used in the equation. No points were earned for parts (d), (e), or (f) The College Board. All rights reserved.
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