c
=
q
m
×Δ
T
=
41.0 J
12.3 g
×
24.7
o
C
−
17.3
o
C
⎛
⎝
⎜
⎞
⎠
⎟
=
41.0 J
12.3 g
×
7.4
o
C
=
0.45
J
g
o
C
(b) The molar mass of iron is 55.85 g/mol.
Use that to calculate the molar heat capacity:
0.45
J
g
o
C
×
55.845 g
1 mol
=
25
J
mol
o
C
41
. The thermal energy enters the surroundings as heat, so the temperature in the surroundings,
T
surroundings
, rises:
T
f,surroundings
> T
i,surroundings
Δ
T
surroundings
= T
f,surroundings
– T
i,surroundings
= positive
Here, the surroundings gets energy from the system, so the internal energy of the system, E
system
,
lowers:
E
f,system
< E
i,system
Δ
E
system
= E
f,system
– E
i,system
= negative
42. (a)
The thermal energy from the surroundings is used to cause the physical phase transition, so:
T
f,alloy
= T
i,alloy
Δ
T
alloy
= T
f,alloy
– T
i,alloy
= zero (neither positive nor negative)
(b) Here, the system gets energy from the surroundings, so the internal energy of the system, E
alloy
,
rises.
E
f,alloy
> E
i,alloy
Δ
E
alloy
= E
f,alloy
– E
i,alloy
= positive
46. Define the system as the H
2
O.
m
ice
= (4 cubes)
×
60.1 g H
2
O
cube
= 240. g = m
water
= m
vapor
To change solid to liquid in the system, use q = m
×
Δ
H
fus
.
q