081 Version Quiz 6 laude (50995)
This print-out should have 8 questions.
Multiple-choice questions may continue on
the next column or page nd all choices
before answering.
Constants
R = 8.314 J K1 mol1
= k NA
k = 1.381 1023 J K1
1
2. I only
3. II only
4. III only
5. I, II
NA = 6.626 1023 mol1
6. I, II, III
Equations
U = 0. 5 k T
S = k ln W
q
S =
T
G = H T S
U = q + w
= qv
= H + P V
H = qp
q = mcT
= H
w = P V
= ngas RT
7. II, III
Suniverse 0
Suniverse = Ssys + Ssurr
Gsys = T Suniverse
001 5.0 points
When calculating positional entropy at absolute zero using the Boltzmann formula
(S = k ln W ), which of the following statements is/are true?
I) An experimentally determined value of
W is less than the ideal value.
II) If S = R ln 2 for 1 mol of some compound, then a single molecule of the same
compound has a positional entropy of
S = NA R ln 2, where NA is Avogadros
number.
III) The positional entropy of a group of
molecules with one orientation will equal
zero.
1. I, III correct
Explanation:
Statement I is true because ideal values assume that IMF arent present and therefore
dont inuence the likelihood of a molecule
adopting a particular orientation. Statement II is false because for a single molecule
R
S = k ln 2 =
ln 2. Statement III is true
NA
because 1 raised to any power is 1 and the
natural log of 1 is 0.
002
5.0 points
A man loads boxes onto a delivery truck,
doing 200 kJ of work and releasing 800 kJ of
heat. What is his change in internal energy?
1. 1000 kJ correct
2. 1000 kJ
3. 600 kJ
4. 600 kJ
Explanation:
U = q + w = 800 kJ + 200 kJ =
1000 kJ
003 5.0 points
Note: E and U are both symbols for internal
energy. They are the exact same thing.
Which of the following statements concerning internal energy is/are true?
I) If the expansion work is small, H and
U are close in value.
II) U for a system is equal to q at constant
volume.
Version 081 Quiz 6 laude (50995)
III) Assuming no heat is exchanged, when
pressure-volume work is done on the system, U is positive.
1. III only
2
standard pressure at 101 C. Even without knowing the exact values of Svap and
Hvap , we can predict that Suniverse is (positive/negative) and relatively (large/small) in
magnitude.
2. II, III
1. negative; large
3. I, II, III correct
2. negative; small correct
4. I, III
3. positive; small
5. I only
4. positive; large
6. II
7. I, II only
Explanation:
Statement I follows from the identity H =
U + pV , because pV is the expansion
work. Statement II follows from U = q + w,
because w = 0 for processes that occur at
constant volume. Statement III also follows
from U = q + w, because w is the pressurevolume work. (Note: for reversible processes,
expansion work and pressure-volume work are
identical.)
Explanation:
The process would be non-spontaneous
(water doesnt condense at 1 atm at 101 C),
so Suniverse would be negative. We are near
the phase change temperature of water, so it
would also be small.
006 5.0 points
Note: E and U are both symbols for internal
energy. They are the exact same thing.
How much internal energy is associated
with just the vibrational motion of 10
molecules of CO2 ?
1. 15kT
004
5.0 points
2. 45kT
If you have 1 mol of HF molecules, each of
which has two orientations, what is the total theoretical positional entropy at absolute
zero?
1. 7.65 1022 J K1
2. 461 J K1
3. 9.57 1024 J K1
4. 5.76 J K1 correct
Explanation:
S = k ln(2NA ) = R ln 2 = 5.76 J K1
005
5.0 points
Consider the condensation of water at
3. 20kT correct
4. 40kT
5. 30kT
Explanation:
The energy of any given mode of movement for any given atom or molecule is equal
to 1/2kT . Since carbon dioxide has 9 total modes, 3 of which are translational and
2 of which are rotational (because it is linear), it has 4 vibrational modes. Since there
are 10 molecules and 4 modes, each of which
are worth 1/2kT , we have a total of 20kT in
internal energy.
007
5.0 points
Version 081 Quiz 6 laude (50995)
Rank the following systems by entropy, from
least to greatest:
a) 1 mol of pure ice
b) 1 mol of water with 1 mol of salt dissolved
in it.
c) 1 mol of pure water
1. c < b < a
2. b < a < c
3. c < a < b
4. a < c < b correct
5. b < c < a
6. a < b < c
Explanation:
Entropy increases as systems go through
endothermic phase transitions and when there
is more matter or more dispersed matter
present.
008
5.0 points
A system releases 900 J of heat to the surroundings, which are at a constant 27 C.
What is S of the surroundings?
1. 33.3 J K1
2. 33.3 J K1
3. 3 J K1
4. 3 J K1 correct
Explanation:
Heat released by the system will increase
the entropy of the surroundings by an amount
equal to q/T , in this case
900 J
= 3 J K1 .
300 K
3