ap07_chemistry_form_b_sgs_final_complete - AP Chemistry...

AP
®
Chemistry
2007 Scoring Guidelines
Form B
The College Board: Connecting Students to College Success
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 association is composed of more than 5,000 schools, colleges, universities, and other
educational organizations. Each year, the College Board serves seven million students and their parents, 23,000 high schools, and
3,500 colleges through major programs and services in college admissions, guidance, assessment, financial aid, enrollment, and
teaching and learning. Among its best-known programs are the SAT
®
, the PSAT/NMSQT
®
, and the Advanced Placement
Program
®
(AP
®
). 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.
© 2007 The College Board. All rights reserved. College Board, Advanced Placement Program, AP, AP Central, SAT, and the
acorn logo are registered trademarks of the College Board. PSAT/NMSQT is a registered trademark of the College Board and
National Merit Scholarship Corporation.
Permission to use copyrighted College Board materials may be requested online at:
www.collegeboard.com/inquiry/cbpermit.html.
Visit the College Board on the Web: www.collegeboard.com.
AP Central is the official online home for the AP Program: apcentral.collegeboard.com.

AP
®
CHEMISTRY
2007 SCORING GUIDELINES (Form B)
© 2007 The College Board. All rights reserved.
Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).
Question 1
A sample of solid U
3
O
8
is placed in a rigid 1.500 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.
U
3
O
8
( s ) + 3 Cl
2
( g ) R 3 UO
2
Cl
2
( g ) + O
2
( g ) 2 2 2
2
3
UO Cl O
3
Cl
( ) ( )
( )
p
p p
K
p
=
When the system is at equilibrium, the partial pressure of Cl
2
( g ) is 1.007 atm and the partial pressure of
UO
2
Cl
2
( g ) is 9.734 × 10
− 4
atm.
(a) Calculate the partial pressure of O
2
( g ) at equilibrium at 862 ° C.
U
3
O
8
( s ) + 3 Cl
2
( g ) R
3 UO
2
Cl
2
( g ) + O
2
( g )
I --- ? 0 0
C
E 1.007 atm 9.734 × 10
− 4
atm ?
9.734 × 10
− 4
atm UO
2
Cl
2
( g ) ×
2
2 2
(1mol O )
(3 mol UO Cl )
= 3.245 × 10
− 4
atm O
2
( g )
One point is earned for
the correct answer.
(b) Calculate the value of the equilibrium constant, K
p
, for the system at 862 ° C.
K
p
=
2 2 2
2
3
UO Cl O
3
Cl
( ) ( )
( )
p p
p
=
4 3 4
3
(9.734 10 ) (3.245 10 )
(1.007)
− −
× ×
= 2.931 × 10
− 13
One point is earned for the
correct substitution.
One point is earned for the
correct answer.
(c) Calculate the Gibbs free-energy change, Δ G ° , for the reaction at 862 ° C.
Δ G ° = − RT ln K
p
= ( − 8.31 J mol
− 1
K
− 1
)( (862+273) K)(ln (2.931 × 10
− 13
))
= 272,000 J mol
− 1
= 272 kJ mol
− 1
One point is earned for
the correct setup.