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13
Questions on Concepts
Q2.1)
Electrical current is passed through a resistor immersed in a liquid in an adiabatic
container.
The temperature of the liquid is raised by 1ºC. The system consists solely of
the liquid.
Does heat or work flow across the boundary between the system and
surroundings?
Justify your answer.
Although work is done on the resistor, this work is done in the surroundings. Heat flows
across the boundary between the surroundings and the system because of the temperature
difference between them.
Q2.2)
Explain how a mass of water in the surroundings can be used to determine
q
for a
process.
Calculate
q
if the temperature of 1.00 kg of water in the surroundings increases
by 1.25ºC.
Assume that the surroundings are at a constant pressure.
If heat flows across the boundary between the system and the surroundings, it will lead to
a temperature change in the surroundings given by
P
q
T
C
∆=
.
For the case of interest,
1
1
3
1000 g 4.19 J g K
1.25 K
5.24 10 J
surroundings
P
qq
m
C
T
=−
∆ =−
×
×
×
.
Q2.3)
Explain the relationship between the terms
exact differential
and
state function
.
In order for a function
f
(
x,y
) to be a state function, it must be possible to write the total
differential
df
in the form
y
x
ff
df
dx
dy
xy
⎛⎞
∂∂
=+
⎜⎟
⎝⎠
. If the form
df
as written exists, it is
an exact differential.
Q2.4)
Why is it incorrect to speak of the heat or work associated with a system?
Heat and work are transients that exist only in the transition between equilibrium states.
Therefore, a state at equilibrium is not associated with values of heat or work.
Q2.5)
Two ideal gas systems undergo reversible expansion starting from the same
P
and
V
. At the end of the expansion, the two systems have the same volume.
The pressure in
the system that has undergone adiabatic expansion is lower that in the system that has
undergone isothermal expansion.
Explain this result without using equations.
In the system undergoing adiabatic expansion, all the work done must come through the
lowering of
∆
U
, and therefore of the temperature.
By contrast, some of the work done in
the isothermal expansion can come at the expense of the heat that has flowed across the
boundary between the system and surroundings.
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Q2.6)
A cup of water at 278 K (the system) is placed in a microwave oven and the the
oven is turned on for one minute, during which it begins to boil. Which of
q
,
w
, and
∆
U
are positive, negative or zero?
The heat
q
is positive because heat flows across the systemsurrounding boundary into
the system. The work
w
is negative because the vaporizing water does work on the
surroundings.
∆
U
is positive because the temperature increases and some of the liquid is
vaporized.
Q2.7)
What is wrong with the following statement?:
Because the well insulated house
stored a lot of heat, the temperature didn't fall much when the furnace failed
. Rewrite the
sentence to convey the same information in a correct way.
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This note was uploaded on 01/27/2010 for the course CHEM 3321 taught by Professor Goux during the Fall '09 term at University of Texas at Dallas, Richardson.
 Fall '09
 Goux
 Physical chemistry, pH

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