First Law of Thermodynamics

Part IIContributions by:

John L. Falconer & Will MedlinDepartment of Chemical and Biological Engineering

University of ColoradoBoulder, 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

D. Need more information

1 kg

Vacuum

Gas

Piston

Block

Stopper

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. A

B. B

C. C

D. All have the same

final temperature

3

2 kgVacuum

Gas

Piston

Block

A

1 kg

Vacuum

Gas

B

Vacuum

Gas

C

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?

A. A

B. B

C. C

D. All have the same

final temperature

4

2 kg

Gas

Piston

Block

Stopper

A1 kg

Gas

B

Gas

C

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  

C. not done  

D. Need more information. 1 kg

Gas

Heat

1 kg

Gas

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?

A. yes 

B. no

C. need more information  

2 kg

Air70°F5 atm

Piston

Weight

Air70°F1 atm

Stopper

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  

 

1 kg

AirHigh P

Piston

Weight

Stopper

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

1 bar

Gas

1 bar

A BIsothermal 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.

1 kg

Air70°F5 atm

Piston

Weight

Air70°F1 atm

Stopper

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 ____________.

A. to the air

B. from the air

C. nowhere. No heat transfer takes place.

D. Need more information.

1 kg

Air70°F1 atm

Piston

Weight

Stopper

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

B. decreases

C. remains the same

Vacuum

Diaphragm

10 MPa300°Cgas

Stoppers

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 _________.

A. increases

B. decreases

C. remains the same

Vacuum

Diaphragm

10 MPa300°Cgas

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 _________.

A. increases

B. decreases

C. remains the same

Vacuum

Diaphragm

10 MPa300°Cgas

10 MPa

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

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

15

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

16

An endothermic reaction takes place in continuous-flow, 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 continuous-flow, 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 1,1,1,2-tetrafluoroethane. 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 ________.

A. increases

B. decreases

C. remains the same

T (°

R)

P (psia)

500

450

4000 5,000 10,000

Isenthalp

Joule-Thomsoninversion curve

A saturated vapor is fed to a turbine:

The exit from the turbine is ______________.

A. saturated vapor

B. superheated vapor

C. a vapor-liquid mixture

P1

sat.vapor

P2

W

High pressure liquid enters a throttle. What is true about the composition at the outlet?

A. It is all liquid

B. It is all vapor

C. It is a mixture of liquid and vapor

5 MPa200°CH2O(l)

0.1 MPaT = ?

Valve

An ideal gas expands through a throttle process:

Is the outlet temperature...

A. > 350 K

B. < 350 K

C. = 350 K

D. Need more information

1 MPa350 KGas

0.1 MPaGas T = ?Throttle

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

C. 400 K

D. Need more information

Gas Vacuum

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.

Gas Vacuum

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