Quiz 4 Section C DL#_______ Name(last)__________________________________ (first) _______________________ |___|___|___|
first 3 letters of last name
Student ID:________________________________ Signature________________________________________ Grade:
I certify by my signature that I will abide by the code of academic conduct of the University of California.
ANSWERS WITHOUT AN EXPLANATION WILL RECEIVE NO CREDIT.
1. At T=0, solid X takes a simple cubic structure. At T=100 K, it sublimates.
Shown below to the right is a model of a simple cubic structure. Each side has the same length as the equilibrium bond length,
r0. The interaction well-depth between two X molecules, ε, is 120 × 10-21 J. On the left is a table of useful PE values as a
function of center-to-center separation, r, in units of r0:
r (r0) PE
1 -ε 12
√2 -0.11ε 12
√3 -0.036ε
4
2 -0.015ε
a i) Find the exact Ebond for these 8 molecules of solid X. Report it in units of Joules.
-
Es = 12 ( 2) + 12 (0 .
11E)
+ 12 (- 0 . 0357 E
a ii) Given there are 6 nearest neighbors per molecule, what is the molar heat of sublimation of X? Approximate it by counting
the contribution from nearest neighbors only. Report it in units of Joules.
Es = x6
E bond = ∑ P E pair-wise (at T = 0), N A =6.02×1023 , ①
all pairs
−number of molecules×number of nearest neighbors per molecule
E bond≈ ×ε
2
first 3 letters of last name
Student ID:________________________________ Signature________________________________________ Grade:
I certify by my signature that I will abide by the code of academic conduct of the University of California.
ANSWERS WITHOUT AN EXPLANATION WILL RECEIVE NO CREDIT.
1. At T=0, solid X takes a simple cubic structure. At T=100 K, it sublimates.
Shown below to the right is a model of a simple cubic structure. Each side has the same length as the equilibrium bond length,
r0. The interaction well-depth between two X molecules, ε, is 120 × 10-21 J. On the left is a table of useful PE values as a
function of center-to-center separation, r, in units of r0:
r (r0) PE
1 -ε 12
√2 -0.11ε 12
√3 -0.036ε
4
2 -0.015ε
a i) Find the exact Ebond for these 8 molecules of solid X. Report it in units of Joules.
-
Es = 12 ( 2) + 12 (0 .
11E)
+ 12 (- 0 . 0357 E
a ii) Given there are 6 nearest neighbors per molecule, what is the molar heat of sublimation of X? Approximate it by counting
the contribution from nearest neighbors only. Report it in units of Joules.
Es = x6
E bond = ∑ P E pair-wise (at T = 0), N A =6.02×1023 , ①
all pairs
−number of molecules×number of nearest neighbors per molecule
E bond≈ ×ε
2
