1. First Law of Thermodynamics Part 2
Contributions by:
John L. Falconer & Will Medlin Department of Chemical and Biological Engineering University of Colorado Boulder, CO
Supported by the National Science Foundation

2. An ideal gas is at 1 atm in a piston-cylinder
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 __________________.
1 kg
Vacuum
Gas
Piston
Block
Stopper
increases
decreases
does not change
Need more information
ANSWER: C. does not change. For an ideal gas, its internal energy is only a function of its temperature. No work is done, so there is no energy change and therefore no temperature change.

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
2 kg
Vacuum
Gas
Piston
Block
1 kg
Vacuum
Gas
Vacuum
Gas A B C
All have the same final temperature
ANSWER: C. C, the cylinder without the weight. The system without the weight will have the highest final temperature because the gas does the least work on the piston and therefore has the greatest final internal energy (less energy is used for work). For an ideal gas, the greater the internal energy, the greater the temperature.

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?
2 kg
Gas
Piston
Block
Stopper A
1 kg B C A B C
All have the same final temperature
ANSWER: A. A, the cylinder with the largest weight. The system with the greatest weight has the highest final temperature because it does the most work done on the gas. That work is converted to internal energy and the higher the internal energy for an ideal gas, the higher the temperature.

5. A gas is heated and expands in a sealed, frictionless, piston-cylinder arrangement. In this process, work is ________________.
done on the gas  
done by the gas  
not done  
Need more information. 
1 kg
Gas
Heat
ANSWER: B. done by the gas.

6. 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?
2 kg
Air
70°F
5 atm
Piston
Weight
1 atm
Stopper
yes  no
need more information  
ANSWER: A. yes. For the temperature to remain constant while the pressure and volume change, there must be heat transfer.

7. 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 _____________.
1 kg
Air
High P
Piston
Weight
Stopper
increases  
remains the same  
decreases  
ANSWER: C. decreases. The air did work in raising the weight and thus energy was removed from the air.

8. 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?
Gas
1 bar
Gas
1 bar A B
Both systems have the same final temperature
Need more information.
ANSWER: B. B, the adiabatic system. By doing work on the gas (compression) energy is added into the system and the temperature increases. The isothermal system removes energy in the form of heat to remain isothermal, but the adiabatic system does not and therefore has the higher final temperature. A B
Isothermal
Adiabatic

9. Air in a piston-cylinder is at 70°F and 5 atm
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 ____________.
1 kg
Air
70°F
5 atm
Piston
Weight
1 atm
Stopper
to the air inside the cylinder.
from the air inside the cylinder.
nowhere. No heat transfer takes place.
ANSWER: A. to the air inside the cylinder. The air did work raising the weight and thus energy was removed from the air inside the cylinder. For the temperature to remain the same, heat was transferred from the outside air to the air inside the cylinder.

10. Air in a piston-cylinder is at 70°F and 1 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
Air
70°F
1 atm
Piston
Weight
Stopper
to the air
from the air
nowhere. No heat transfer takes place.
Need more information.
ANSWER: B. from the air. The weight does work on the air in the cylinder and thus increases its internal energy which is an increase in temperature. For the temperature to remain the same, heat must be transferred out of the air inside the cylinder.

11. Stoppers prevents a piston from moving within a piston-cylinder system
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 _________.
Vacuum
Diaphragm
10 MPa
300°C
gas
Stoppers
increases
decreases
remains the same
ANSWER: C. Energy of the gas is not transferred elsewhere.

12. A piston pushing on a gas holds it at a pressure of 10 MPa
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 _________.
Vacuum
Diaphragm
10 MPa
300°C
gas
increases
decreases
remains the same
ANSWER: A. when the diaphragm is moved the piston pushes down the gas and does work on the gas and this increases the energy and therefore increases the temperature of the gas above 300 C

13. A piston pushing on a gas holds it at a pressure of 10 MPa
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 _________.
Vacuum
Diaphragm
10 MPa
300°C
gas
increases
decreases
remains the same
ANSWER: A. when the diaphragm is moved the piston pushes down the gas and does work on the gas and this increases the energy and therefore increases the temperature of the gas above 300 C

14. What must be changed about the First Law for a system with a chemical reaction?
Add equilibrium term
Add heat of reaction term
Don’t change anything
ANSWER: C. Don’t change anything.

15. The effluent temperature is higher when __________
An endothermic reaction takes place in an adiabatic reactor. The reaction (A  B) goes to completion.
The effluent temperature is higher when __________
pure A is fed to the rector
a mixture of A and inert is fed to the reactor
in neither case
ANSWER: B. a mixture of A and inert is fed to the reactor.

16. 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 __________.
increases
decreases
remains the same
ANSWER: C. remains the same.

17. An endothermic reaction takes place in continuous- flow, adiabatic reactor. The enthalpy of the effluent is __________ the inlet enthalpy.
higher than
lower than
the same as
ANSWER: C. the same as the inlet enthalpy. Because the reactor is adiabatic, no energy is removed or added.

18. An endothermic reaction takes place in continuous- flow, isothermal reactor. The enthalpy of the effluent is ___________ the inlet enthalpy.
higher than
lower than
the same as
ANSWER: A. higher than the inlet enthalpy. Because the reactor is isothermal, energy must be added to the system for the temperature to remain constant.

19. A can of office duster (used for cleaning cameras and keyboards) contains a liquid-vapor mixture of 1,1,1,2-tetrafluoroethane. If the container is used continuously by depressing the nozzle, the flow rate of air ____________ with time.
decreases
remains constant
increases
remains constant and then decreases
ANSWER: A. Decreases with time. Evaporative cooling lowers the temperature, the can feels cold, and the vapor pressure decreases.

20. The temperature ________.
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 ________.
T (°R)
P (psia)
500
450
400
5,000
10,000
Isenthalp
Joule-Thomson
inversion curve
increases
decreases
remains the same
ANSWER: B. Decreases. For a valve, the energy balance states that DeltaH = 0, therefore from the starting point, follow the isenthalp to the final pressure. As illustrated by the graph, the starting point of 5,000 psia is to the left of the inversion curve so we expect the temperature to drop.

21. A saturated vapor is fed to a turbine:
The exit from the turbine is ______________. P1 P2
sat.
vapor W
saturated vapor
superheated vapor
a vapor-liquid mixture
ANSWER: C. a vapor-liquid mixture. The saturated vapor loses energy as it does work in a turbine, so some of the vapor must condense.

22. High pressure liquid enters a throttle
High pressure liquid enters a throttle. What is true about the composition at the outlet?
5 MPa
200°C
H2O(l)
0.1 MPa
T = ?
Valve
It is all liquid
It is all vapor
It is a mixture of liquid and vapor
ANSWER: C. It is a mixture of liquid and vapor. Some of the liquid evaporates at the lower outlet pressure and the evaporative cooling lowers the temperature because the throttle system is adiabatic. (Delta H is zero for a throttle).

23. An ideal gas expands through a throttle process:
Is the outlet temperature...
1 MPa
350 K
Gas
0.1 MPa
Gas
T = ?
Throttle
> 350 K
< 350 K
= 350 K
Need more information
ANSWER: C.=350 K. Enthalpy of an ideal gas does not depend on pressure, so there is no change in enthalpy for a throttle and the temperature remains the same.

24. Liquid water at 175°C and 20 bar is fed to a throttle:
The outlet pressure is 1 bar. What is the most likely outlet temperature? Assume one of the following is correct.
20 bar
175°C
Liquid
1 bar
T = ?
Throttle
175°C
150°C
100°C
50°C
ANSWER: C.

25. Steam at 150°C and 2 bar is expanded to 1 bar in turbine
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.
lower
higher
same
ANSWER: B. higher. The entropy of the stream for the set expansion (outlet pressure) will be higher for the irreversible case, indicating that a larger fraction of the fluid is in the higher-entropy vapor phase.

26. The temperature at the exit of a reversible turbine will be ________ than the entrance.
higher
lower
the same as
ANSWER: B. lower than the entrance. The work done on the turbine removes energy from the gas and results in a lower exit temperature.

27. A membrane divides a rigid, well-insulated 2 m3 tank into equal parts
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?
Gas
Vacuum
200 K
300 K
400 K
Need more information
ANSWER: B 300K. No work is done, so no energy is removed from the gas. For an ideal gas, the internal energy is only a function of temperature.
Membrane

28. What would cause the temperature of an ideal gas in a closed system (similar to the membrane system below) to change?
Work is done by the gas.
Interactions between molecules change.
Gas occupies more volume when the membrane ruptures.
None of the above.
Gas
Vacuum
ANSWER: A. Work is done on the system. If work is done by the gas, energy is removed from the gas and the temperature changes. For an ideal gas, the internal energy is only a function of temperature.
Membrane

29. 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.
greater than
less than
equal to
ANSWER: C. equal to the enthalpy of the feed. The throttle system is adiabatic, and deltaH = 0 for a throttle. The temperature will change to offset the effect of the reaction exotherm.