Chemistry Reference
In-Depth Information
14.67
2 NO
2
(g) £ N
2
O
4
(
/
)
14.81
¢
H
9.67 kJ
(
67.70 kJ)
58.03 kJ
Because the
N
2
O
4
is much more stable (the
¢
H
is very neg-
Kinetic
ative), the reaction is apt to be spontaneous.
120
100
80
60
40
20
0
2
2
O
2
(g) £ 8 CO
2
(g)
14.70
C
8
H
18
(
/
)
9
H
2
O(
/
)
Potential
(a)
The reactants are present in the same ratio as the coeffi-
cients in the balanced equation. Both will react com-
pletely. For 1 mol of
Kinetic
C
8
H
18
:
¢
H
¢
H
f
(products)
¢
H
f
(reactants)
Kinetic
8
¢
H
f
(CO
2
)
9
¢
H
f
(H
2
O)
¢
H
f
(C
8
H
18
)
Potential
8(
393.5 kJ)
9(
285.9 kJ)
(
208 kJ)
20
5513 kJ
Heat added
For 0.660 mol of
C
8
H
18
:
0.660 mol (
5513 kJ/mol)
3640 kJ
(b)
The same quantity of heat would have been pro-
duced because the octane would have been the limiting
quantity.
14.84
Because the concentration of water that can be held by the
air decreases with decreasing temperature but the actual con-
centration of water in the air does not change, the relative
humidity increases.
14.71
Note that
¢
H
combustion
values, not
¢
H
f
values, are given.
1
2
O
2
(g) £ 6 CO
2
(g)
3 H
2
O(
/
)
C
6
H
6
(
/
)
¢
H
3273 kJ
2
O
2
(g) £ 2 CO
2
(g)
H
2
O(
/
)
C
2
H
2
(g)
¢
H
1305 kJ
a
1.01 g
1 mL
14.85
(a)
50.0 mL
b
50.5 g
For the given reaction,
(50.5 g) (4.10 J/g
#
°C)
¢
t
¢
H
(
3273 kJ)
3(
1305 kJ)
642 kJ
Heat
mc
¢
t
690 J
For 25.0 g of
C
2
H
2
:
¢
t
3.33°C
a
1 mol C
2
H
2
26.0 g C
2
H
2
642 kJ
1 mol C
2
H
2
t
f
25.0°C
3.33°C
28.3°C
25.0 g C
2
H
2
b
a
b
617 kJ
(b)
NaOH(aq)
HCl(aq) £ NaCl(aq)
H
2
O(
/
)
0.0125 mol H
2
O(
55.2 kJ/mol H
2
O)
0.690 kJ
14.76
It came from potential energy—the energy of ordering the
molecules or ions to their regular arrangement in the lattice.
(c)
The final volume is 50.0 mL; the final concentration of
NaCl is The heat
of reaction in part (b) is which is provided to
the solution. Thus, heat added to the solution is
Because all the factors are the same as in
part (a), the final temperature is the same,
(0.0125 mol)/ (0.0500 L)
0.250 M.
14.77
NH
3
and
AsH
3
each has greater intermolecular forces than
0.690 kJ,
does
PH
3
.
(NH
3
has hydrogen bonding, and
AsH
3
has
greater van der Waals forces.)
0.690 kJ.
28.3°C.
14.79
(a)
No
(b)
Because energy is produced,
¢
H
is negative for the
14.86
CaO has doubly charged ions, as opposed to singly charged
ions in KF. Because CaO has greater forces holding the par-
ticles together, it should have the higher melting point. The
actual values are
reaction.
(c)
Because energy is absorbed,
¢
H
is positive for the
warming process.
(d)
The overall enthalpy change is zero. Because the energy
from the reaction is added to the solution, no energy is
gained by or lost from the system.
2580°C
for CaO and
858°C
for KF.
14.88
(a)
of water is present at the
end of the process that was not present initially. It has
condensed from the steam.
(b)
The water has warmed from
15.60 g
15.00 g
0.60 g
14.80
For hydrogen:
15.3°C
to
39.4°C;
the heat
(14.4 J/g
#
°C) (2.016 g/mol)
29.0 J/mol
#
°C
required for this process is
The molar heat capacities of the other elements are calcu-
lated in the same manner, giving the following results
(15.00 g) (4.184 J/g
#
°C) (24.1°C)
Heat
mc
¢
t
151
3
J
(in J/mol
#
°C):
(c)
The water condensed from steam has cooled from
to
100°C
H
2
29.0
Al
24
Fe
24.7
39.4°C.
The heat liberated is
O
2
29.5
Ag
26
Mg
24
(0.60 g) (4.184 J/g
#
°C) (
60.6°C)
Heat
mc
¢
t
N
2
29.1
Au
25.4
Pb
27
Co
27
Sn
26
15
2
J
Cr
23
Zn
25.4
(d)
The total heat supplied is
151
3
J.
If we subtract the
Cu 24.5
Despite a wide range of
specific
heat capacities, the diatomic
gases have
molar
heat capacities of about and
the
molar
heat capacities of all the metallic elements are
close to The latter generalization is known as
the law of Dulong and Petit.
heat supplied by the cooling water,
15
2
J,
we get the
heat supplied by the condensation process:
29 J/mol
#
°C,
151
3
J
(
15
2
J)
136
1
J
1.36
1
kJ
26 J/mol
#
°C.
The heat is negative because the condensation process
supplies heat to the cold water.
