First Law of
Thermodynamics
Part II
Contributions by:
John L. Falconer & Will Medlin
Department of Chemical and Biological Engineering
University of Colorado
Boulder, CO 80309-0424
Supported by the National Science Foundation
An ideal gas is at 1 atm in a piston-cylinder. The piston
is weightless and frictionless. When the mass is
removed from the piston, the gas temperature
__________________.
A. increases
B. decreases
C. does not change
Stopper
Vacuum
1 kg
D. Need more information
Block
Piston
Gas
3
In these piston-cylinder systems, when the red stop is
removed, the ideal gas expands, and the piston moves
until it hits the black stopper. Each system is adiabatic
and starts at 10 atm and 25°C.
Which has the highest final temperature?
A
B
C
A. A
B. B
Block
Vacuum
Vacuum
2 kg
1 kg
Vacuum
C. C
D. All have the same
Piston
final temperature
Gas
Gas
Gas
4
In these piston-cylinders, when the red stops are
removed, the gases are compressed, and the pistons
move until they hit the black stopper. Each system is
adiabatic, and each starts at the same temperature
and pressure. Which has the highest final
temperature?
Block
A
B
2 kg
1 kg
C
A. A
B. B
Piston
C. C
D. All have the same
final temperature
Stopper
Gas
Gas
Gas
A gas is heated and expands in a sealed, frictionless,
piston-cylinder arrangement. In this process, work is
________________.
A. done on the gas
B. done by the gas
1 kg
C. not done
D. Need more information.
1 kg
Gas
Gas
Heat
Air in a piston-cylinder device is initially at 70°F and 5
atm in surroundings that are at 70°F, 1 atm. If the locks
holding the piston in place are removed, the piston
moves to the stopper, and the air pressure changes to
2 atm while temperature remains constant. Frictional
effects are negligible. Is there heat transfer involved in
this process?
Air
Stopper
A. yes
B. no
C. need more information
Weight
2 kg
Piston
Air
70°F
5 atm
70°F
1 atm
Air at high pressure and ambient temperature is
contained in a perfectly insulated piston-cylinder
device. If the locks holding the piston in place are
removed, the piston moves upwards to a stopper. The
temperature of the air _____________.
A. increases
B. remains the same
C. decreases
Stopper
Weight
1 kg
Piston
Air
High P
Two different systems, isothermal (A) & adiabatic (B),
contain ideal gases in piston-cylinders at the same
temperature and pressure. If you compress both
systems to 10 bar, which system has the higher
temperature at the end of the compression?
A. A
B. B
C. Both systems have the
same final temperature
D. Need more information.
Gas
Gas
1 bar
1 bar
A
B
Isothermal
Adiabatic
Air in a piston-cylinder is at 70°F and 5 atm. The piston
is locked in place. The locks are removed, and the
piston moves to a stopper where the air is now at 70°F
and 2 atm. Heat is transferred ____________.
A. to the air inside the
cylinder.
B. from the air inside the
cylinder.
C. nowhere. No heat transfer
takes place.
Air
70°F
1 atm
Stopper
Weight
1 kg
Piston
Air
70°F
5 atm
Air in a piston-cylinder is at 70°F and 1 atm. A weight
is added to the piston, and the air increases to 4 atm
at 70°F. Heat is transferred ____________.
1 kg
A. to the air
Weight
B. from the air
C. nowhere. No heat
transfer takes place.
D. Need more information.
Stopper
Piston
Air
70°F
1 atm
Stoppers prevents a piston from moving within a
piston-cylinder system. When the diaphragm is
removed from this piston-cylinder system, the energy
of the gas _________.
A. increases
Stoppers
B. decreases
C. remains the same
10 MPa
300°C
gas
Diaphragm
Vacuum
A piston pushing on a gas holds it at a pressure of 10
MPa. When the diaphragm is removed from this
piston-cylinder system, the energy of the gas
_________.
10 MPa
A. increases
10 MPa
300°C
gas
B. decreases
C. remains the same
Diaphragm
Vacuum
A piston pushing on a gas holds it at a pressure of 10
MPa. When the diaphragm is removed from this
piston-cylinder system, the temperature of the gas
_________.
10 MPa
A. increases
10 MPa
300°C
gas
B. decreases
C. remains the same
Diaphragm
Vacuum
What must be changed about the First Law for a
system with a chemical reaction?
A. Add equilibrium term
B. Add heat of reaction term
C. Don’t change anything
1
5
An endothermic reaction takes place in an adiabatic
reactor. The reaction (A  B) goes to completion.
The effluent temperature is higher when __________
A. pure A is fed to the rector
B. a mixture of A and inert is fed to the reactor
C. in neither case
1
6
An endothermic reaction takes place in an adiabatic
reactor and the conversion is 100%. If the inlet flow
rate is decreased by a factor of two, and the
conversion remains at 100%, then the exit temperature
__________.
A. increases
B. decreases
C. remains the same
An endothermic reaction takes place in continuousflow, adiabatic reactor. The enthalpy of the effluent is
__________ the inlet enthalpy.
A. higher than
B. lower than
C. the same as
An endothermic reaction takes place in continuousflow, isothermal reactor. The enthalpy of the effluent
is ___________ the inlet enthalpy.
A. higher than
B. lower than
C. the same as
A can of office duster (used for cleaning cameras and
keyboards) contains a liquid-vapor mixture of 1,1,1,2tetrafluoroethane. If the container is used continuously
by depressing the nozzle, the flow rate of air
____________ with time.
A. decreases
B. remains constant
C. increases
D. remains constant and then decreases
Methane undergoes a rapid expansion through an
insulated valve. The starting pressure is 5,000 psia
and the starting temperature is 475°R.
The temperature ________.
500
A. increases
C. remains the
same
T (°R)
B. decreases
Isenthalp
450
Joule-Thomson
inversion curve
400
0
5,000
P (psia)
10,000
A saturated vapor is fed to a turbine:
P1
P2
sat.
vapor
W
The exit from the turbine is ______________.
A. saturated vapor
B. superheated vapor
C. a vapor-liquid mixture
High pressure liquid enters a throttle. What is true
about the composition at the outlet?
5 MPa
200°C
H2O(l)
Valve
A. It is all liquid
B. It is all vapor
C. It is a mixture of liquid and vapor
0.1 MPa
T=?
An ideal gas expands through a throttle process:
1 MPa
350 K
Gas
Throttle
Is the outlet temperature...
A. > 350 K
B. < 350 K
C. = 350 K
D. Need more information
0.1 MPa
Gas
T=?
Steam at 150°C and 2 bar is expanded to 1 bar in
turbine. The turbine has an efficiency of 40%. The
quality of the steam will be ________ compared to a
turbine with 100% efficiency.
A. lower
B. higher
C. same
The temperature at the exit of a reversible turbine will
be ________ than the entrance.
A. higher
B. lower
C. the same as
A membrane divides a rigid, well-insulated 2 m3 tank
into equal parts. The left side contains an ideal gas
(Cp = 30 J/mol K) at 300 K and 10 bar and the right
side is a vacuum. If the membrane ruptures, what will
be the final temperature of the gas?
A. 200 K
B. 300 K
Vacuum
Gas
C. 400 K
D. Need more information
Membrane
What would cause the temperature of an ideal gas in a
closed system (similar to the membrane system
below) to change?
A. Work is done by the
gas.
B. Interactions between
molecules change.
C. Gas occupies more
volume when the
membrane ruptures.
D. None of the above.
Vacuum
Gas
Membrane
A liquid mixture at 20 bar is fed to an adiabatic throttle
that is also a catalyst. The exit from the throttle is a
gas at 1 bar and has 50% of the feed converted to
reaction products. If the reaction is exothermic, the
enthalpy of the exit stream is __________ the enthalpy
of the feed.
A. greater than
B. less than
C. equal to