Version 119 – Exam 4 – mccord – (50970)
1
This printout should have 26 questions.
Multiplechoice questions may continue on
the next column or page – fnd all choices
beFore answering.
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.