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Version 119 – Exam 4 – mccord – (50970)
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001
10.0 points
Reactions with positive values oF Δ
S
◦
r
always
become spontaneous at low temperatures.
1.
True
2.
±alse
correct
Explanation:
Δ
G
= Δ
H

T
Δ
S
is used to predict spon
taneity. (Δ
G
is negative For a spontaneous
reaction.)
T
is always positive. When Δ
S
is
positive and
T
is small, then in order For Δ
G
to be negative, Δ
H
must be negative, zero
or have a very small positive value. But this
is not always the case, so not ALL reactions
with a positive Δ
S
are spontaneous at low
temperatures.
002
10.0 points
The sublimation oF solid carbon dioxide is a
spontaneous process. Predict the sign (+,

,
or 0) oF Δ
G
◦
r
, Δ
H
◦
r
, and Δ
S
◦
r
, respectively.
1.
0, +, +
2.

,

,

3.

, +, +
correct
4.

, 0, +
5.

, +,

Explanation:
Δ
G
is negative For a spontaneous reaction.
Sublimation requires energy to Facilitate the
solid becoming a gas, so the process is en
dothermic (Δ
H
is positive). ±inally, the en
tropy oF a gas is more than that oF a solid, so
disorder increases (Δ
S
is positive).
003
10.0 points
A sample oF 1
.
55 grams oF compound Y is
burned completely in a bomb calorimeter
which contains 2500 g oF water. The temper
ature rises From 24
.
358
◦
C to 24
.
762
◦
C. What
is Δ
U
rxn
For the combustion oF compound Y?
The hardware component oF the calorimeter
has a heat capacity oF 3
.
18 kJ/
◦
C. The spe
cifc heat oF water is 4.184 J/g
·
◦
C, and the
MW oF Y is 128 g/mol.
1. 256.5
2. 529.6
3. 477.3
4. 189.3
5. 419.8
6. 225.9
7. 350.1
8. 366.7
9. 455.1
10. 552.3
Correct answer:

455
.
1 kJ
/
mol oF Y.
Explanation:
SH = 4.184 J/g
·
◦
C
HC = 3
.
18 kJ/
◦
C
m
Y
= 1
.
55 g
MW
Y
= 128 g/mol
m
water
= 2500 g
Δ
T
= 24
.
762
◦
C

24
.
358
◦
C = 0
.
404
◦
C
Heat = (SH)(m
water
)(Δ
T
) + (HC)(Δ
T
)
,
Then divide the heat by the number oF moles
which is mass/MW.
004
10.0 points
All systems undergoing spontaneous reaction
at constant
T
and
P
1.
have Δ
H >
0.
2.
have Δ
S >
0.
3.
give o² heat.
4.
have Δ
H <
0.
5.
are slow.
6.
have Δ
S <
0.
7.
are Fast.
8.
have Δ
G >
0.
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View Full DocumentVersion 119 – Exam 4 – mccord – (50970)
2
9.
have Δ
G <
0.
correct
10.
absorb heat.
Explanation:
Δ
G
is always negative (Δ
G <
0) for spon
taneous reactions.
005
10.0 points
The enthalpy of fusion of methanol (CH
3
OH)
is 3.16 kJ/mol.
How much heat would be
absorbed or released upon freezing 25.6 grams
of methanol?
1.
0.253 kJ released
2.
0.253 kJ absorbed
3.
3.95 kJ released
4.
3.95 kJ absorbed
5.
2.52 kJ absorbed
6.
2.52 kJ released
correct
Explanation:
MW CH
3
OH = 32.042g
ΔH(fusion) =

ΔH(freezing)
q = 25
.
6 g
×
1mol
32
.
042g
×

3
.
16 kJ
mol
=

2
.
525 kJ
006
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 Fall '07
 Fakhreddine/Lyon
 Chemistry, Reaction

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