past_exam_ans (1)

8/11/2019 past_exam_ans (1)

1/8

CHE 302 (Fall 97) __________________

LAST NAME, FIRST

Quiz #1

(Closed notes and closed books, 35 minutes)

1. A. Temperature and pressure are sometimes extensive properties of a system.B. An adiabatic wall prevents energy transfer as heat but not as work.

1. A and B are true 2. Only A is true 3. Only B is true 4. A and B are false

2. A. An isolated system has a boundary which allows only a particular energy interactionwith its environment.

B. A steady-state open system is an example of a system that is at equilibrium.


3. One gram mole of a gas at a temperature of 25oC anda pressure of 1 bar is compressed to 10

bar by a process in which PVg= constant, where g= CP/CV. Determine:

a. (5 pts) The work required. Ans: 5797 J

b. (5 pts) The change in internal energy of the gas. Ans: 5797 JGas is ideal with CP= 38 J/mol

.oK. Gas constant R = 8.314 J/mol.oK.

4. (6 pts) A 4 m3 well-insulated storage tank containing 2 m3of liquid is to be pressurizedadiabatically with air from a large, high pressure reservoir through a valve at the top of the tank

to permit rapid ejection of the liquid. The air in the reservoir is maintained at 100 bar and 300oK.

The gas space above the liquid contains initially air at 1 bar and 280oK. When the pressure in the

tank reaches 5 bar and the liquid transfer valve is still closed, what is the air temperature?Air is

an ideal gas with CP= 38 J/mol.oK. Gas constant R = 8.314 J/mol.oK

Ans: 357.5 K


2/8

CHE 302 (Fall 97) __________________

LAST NAME, FIRST

Quiz #2


1. A. At constant pressure, enthalpy change for a closed system equals the heat effect for areversible process doing PV work.

B. Cpfor an ideal gas is independent of temperature.1. A and B are true 2. Only A is true 3. Only B is true 4. A and B are false

2.A rigid tank 3 m3 in total volume initially contains 1200 kg of saturated liquid water; the

remainder of the volume is filled with saturated vapor steam. The initial temperature is 200 oC; at

that time 800 kg of liquid water at 60oC are added to the tank, along with enough heat to keep

the temperature at 200oC. Steam still presents at the end of the process.

2a) A. The initial specific properties of the saturated water and steam can be determined from

the steam table.

B. The initial and final specific properties of the saturated water and steam are the same.1. A and B are true 2. Only A is true 3. Only B is true 4. A and B are false

2b) (4 pts) The initial specific volumes of saturate water and steam are: V l= 1.157x10-3m3/kg,

Vv= 127.4x10-3m3/kg. Determine the inital vapor mass in kg.

3. a) (6 pts) Calculate the minimum amount of work required to cool 1 mol of oxygen gas at

298oK and 1 atm to 90.13oK in a reversible isobaric process in which heat is transferred to a heat

reservoir at 298oK.

b) (6 pts) Calculate the minimum amount of work required to convert 1 mol of oxygen gas

at 298oK and 1 atm to the liquid state at 90.13 oK (the atmospheric boiling point) in a reversible

isobaric process in which heat is transferred to a heat reservoir at 298oK.

Data for 3a and 3b:

For oxygen gas: Cp= 6.96 cal/mol.oK, heat of vaporization at 90.13oK = 1628.8 cal/mol.


3/8

CHE 302 (Fall 97) __________________

LAST NAME, FIRST

Quiz #3


1. A. The absolute value of internal energy can be determined experimentally.B. In an adiabatic closed system, the entropy change for some process may be less than

zero.1. A and B are true 2. Only A is true 3. Only B is true 4. A and B are false

2. A. The area under the T-S diagram path is the reversible heat exchange.B. The entropy change for an irreversible process between states A and B is different from

the entropy change for a reversible process between states A and B.


3.(8 pts) A block of polymer weighing 150 g is to be compressed from a pressure of 1 atm to a

pressure of 1501 atm. Assume that the volume of the polymer does not change much during the

compression, determine the heat in J required (added or removed) to maintain the polymer at aconstant temperature of 300oK. Data: 1 atm = 1.013x105Pa.

For polymer: density = 1.2 g/m3, v=P

T

V

V

1

= 8.5x10-7K-1.

4. (8 pts) A steady stream of air at 300oF and 5 atm is available. How much work can be

obtained from each lbmol of air if it flow through a reversible device and leaves at 77 oF and1

atm? The device may exchange heat with the surroundings at 77 oF. Assume ideal gas behavior

and constant heat capacity (Cp= 7.0 Btu/lbmol.oR, gas constant R = 1.987 Btu/lbmol.oR)


4/8

CHE 302 (Fall 97) __________________

LAST NAME, FIRST

Quiz #4

(Closed notes and closed books, 35 minutes)Note: Your answers must be correct to 3 significant figures and have the appropriate units.

1. A. In a closed system at constant Tand P, a spontaneous process will increase Gibbs freeenergy, G, until a maximum value for Gis reached at equilibrium.

B. Chemical potential is an intensive property.


2. A. The size and thermodynamic state of a pure single phase system would be fixed by two ofits intensive state variables.

B. The chemical potential of a pure substance is equal to its specific Gibbs free energy, G.


3. A. P=S

V

U

B. T=V

S

U


4. A. Entropy change of matter from (T1, V1) to (T2, V2) can be determined from

dS=T

CV dT+

T

PdV

B. Entropy change of matter from (T1, P1) to (T2, P2) can be determined from

dS =T

CP dT+

T

PdV


5. (8 pts) 20 g of water at 0oC is placed in a large cold space (a reservoir) at - 15 oC until it

completely freezes (still at 0oC). The heat of fusion of water at 0oC is -335 J/g.

a. The entropy change of the water is __________

b. The entropy change of the surrounding is __________

If the final state is ice at -15oC and CP,ice= 2.1 J/g.oK.

c. The entropy change of the water is __________

d. The entropy change of the surrounding is __________

6.(4 pts) Determine volume of pure water at 25oC required to form 200 cm3of 72.2 mole %

solution of water in methanol.

Data: Water (1) V1(at x1 = 0.722) = 17.840 cc/mol, V1= 18.0686 cc/mol

Methanol (2) V2(at x1 = 0.722) = 38.486 cc/mol, V2= 40.7221 cc/mol

V1= ____________


5/8

CHE 302 (Fall 97) __________________

LAST NAME, FIRST

Quiz #5

(Closed notes and closed books, 35 minutes)Note: Your answers must be correct to 3 significant figures and have the appropriate units.

I. Crystalline sodium sulfate, in the presence of water vapor, may form a decahydrate.

Na2SO4(s) + 10H2O(g) = Na2SO4.10H2O(s) (1)

Data: (Note: R = 1.987 cal/mol.oK)

Species Hf(25oC), kcal/mol Gf(25

oC), kcal/mol

Na2SO4(s) - 330.50 - 302.33

H2O(g) - 57.798 - 54.635

Na2SO4.10H2O(s) - 1033.85 - 870.52

a. The standard heat of reaction at 25oC for reaction (1) is __________

b. The standard Gibbs free energy change at 25oC for reaction (1) is __________

c. If Grxn(25oC) = - 25.7 kcal/mol, Ka(at 25

oC) is __________

d. If Hrxn= -200 kcal/mol = constant, Ka= 1.21012at 300oK, Kaat 280

oK is __________

Note:T

Ka

)(ln=

2RT

Hrxn

e. If Ka(at 25oC) = 2.5x1012, the partial pressure of water at equilibrium is __________

II. A heat engine operating between reservoirs at 1100o

K and 300o

K is found to deliver 1200J/hr of work with 2100 J/hr of high-temperature heat absorbed.

a. The heat dumped to the low-temperature sink in J/hr is __________

b. The maximum work that could be obtained from the engine is __________

c. The entropy production in J/hr.oK is __________

III. Nitrogen gas flowing at the rate of 1200 kg/h enters a compressor at 1 bar and 300 oK and

leaves at 12 bar and 450

o

K. Cooling water enters the compressor at 300

o

K and leaves at 350

o

K.Cooling water flow rate is 1500 kg/hr. Nitrogen (MW = 28) is an ideal gas with Cp= 1.05

kJ/kg.oK. Heat capacity of water is 4.184 kJ/kg.oK. Gas constant R = 8.314 J/mol.oK.

a. The entropy change of nitrogen is __________

b. The entropy change of water is __________


6/8

CHE 302 (Fall 97) _______________________

LAST NAME, FIRST (8 pts)

Final Exam(Closed notes and closed books)

(Note: Your numerial answers must be correct to 3 signifcant fgures. There are 26questions with 4 points each howe!er "ou can onl" obtain a ma#imum o$ 1%% points. Thatmeans "ou can miss 3 questions an& still get 1%%' score..

1. Consider the following gas phase reaction (Note: R = 8.314 J/mol .oK)

C3H8+ 3H2O = 3CO + 7H2 (1)

Data

Species Hf(25oC), kcal/mol Gf(25

oC), kcal/mol Inital moles

C3H8 - 24.82 - 5.614 2

H2O - 57.798 - 54.635 3

CO - 26.416 - 32.808 0

H2 0 0 0

a. The standard heat of reaction at 25o

C for reaction (1) is __________b. The standard Gibbs free energy change at 25oC for reaction (1) is __________

c. If Grxn(975oK) = - 195.74 kJ/mol, Ka(at 975

oK) is __________

d. If Hrxn= 530 kJ/mol = constant, Ka= 82.1 at 750oK, Kaat 900

oK is __________

Note:T

Ka

)(ln=

2RT

Hrxn

e. When 0.6 mole of C3H8has reacted, the mole fraction of CO is __________

f. When 0.6 mole of C3H8has reacted, the mole fraction of C3H8is __________

2. A heat engine operating between reservoirs at 1000o

K and 300o

K is found to deliver 1100 J/hrof work with 2000 J/hr of high-temperature heat absorbed.

a. The heat dumped to the low-temperature sink in J/hr is __________

b. The maximum work that could be obtained from the engine is __________

c. The entropy production in J/hr.oK is __________

3. Nitrogen gas flowing at the rate of 800 kg/h enters a compressor at 1 bar and 300 oK and

leaves at 12 bar and 450oK. Cooling water enters the compressor at 300oK and leaves at 340oK.

Cooling water flow rate is 1,000 kg/hr. Nitrogen (MW = 28) is an ideal gas with Cp= 1.05

kJ/kg.oK. Heat capacity of water is 4.184 kJ/kg.oK. Gas constant R = 8.314 J/mol.oK.

a. The entropy change of nitrogen is __________

b. The entropy change of water is __________

c. The work provided by the compressor is __________

d. If the entire process is reversible, the temperature of the exit water is __________


7/8

4.A block of polymer weighing 100 g is to be compressed from a pressure of 1 atm to a pressure

of 1501 atm. The volume of the polymer does not change much during the compression. The

polymer is maintained at a constant temperature of 300oK. Data: 1 atm = 1.013x105Pa.

For polymer: density = 1.1 g/cm3, v=P

T

V

V

1

= 6.510-7K-1.

The entropy change of polymer is __________

5. The following closed-loop steam cycle has been proposed to generate work from burning fuel.

Q

Wnet

Q

Boiler

Pump

Condenser

TurbineW

The temperature of the burning fuel is 1100oC, and cooling water is available at 15oC. The steam

leaving the boiler is at 20 bar and 700oC, and the condenser produces a saturated liquid at 0.2

bar. The steam lines are well insulated, the turbine and pump operate reversibly and

adiabatically, and some of the mechanical work generated by the turbine is used to drive the

pump. The steam leaving the turbine is superheated vapor and the steam leaving the condenser is

saturated liquid.

Your numerial answers must be correct to 4 signifcant fgures $or problem ).

Data:

T(oC) P(bar) H(kJ/kg) S(kJ/kg.oK) V(m3/kg)

700 20 3917.4 7.9487

60.1 20 254.3 0.8320

67 0.2 2623.2 7.9487

60.1 0.2 251.4 0.8320 0.00102

a. The work supplied by the turbine per kg of steam generated in the boiler is __________

b. Heat discarded in the condenser per kg of steam generated in the boiler is __________

c. The work used by the pump per kg of steam generated in the boiler is __________

d. Heat absorbed in the boiler per kg of steam generated is __________

e. The efficiency of a Carnot cycle operated at this cycle temperature range is __________

f. A. The fluid leaving the pump is saturated liquid.

B. There is no change in entropy across the turbine.



8/8

6. Consider a pure material with N moles

A. (U/N)at constant S and V = 0

B. (U/N)at constant S and V = (U/N)at constant S and V


7.Consider 500 cm3of 72.2 mole % solution of water in methanol.

Data: Water (1) V1(at x1 = 0.722) = 17.840 cc/mol, V1= 18.0686 cc/mol

Methanol (2) V2(at x1 = 0.722) = 38.486 cc/mol, V2= 40.7221 cc/mol

Moles of water in the solution is __________

8.A strong capillary tube filled with mercury and closed initially at 20oC and 1 atm is heated to

25oC at constant volume. For mercury v=P

T

V

V

1

= 16x10-5K-1 and = T

P

V

V

1

=

3.5x10-6

atm-1

. The resulting pressure is__________

9.One of the processes in our plant uses ammonia as a raw material. It is stored as a saturated

liquid at 77oF and is vaporized in a steam-heated heat exchanger and then expanded through a

turbine before being fed into the process at essentially atmospheric pressure. The exhaust from

the turbine should be saturated vapor at 15 psia and the turbine is assumed to be both reversible

and adiabatic.

a. The temperature of the ammonia vapor prior to entering the turbine is

A) 210oF B) 250oF C) 290oF D) 330oF

b. Work available from the turbine in Btu per lb of ammonia is

A) 140 B) 180 C) 220 D) 260

c. Heat added in the heat exchanger in Btu per lb of ammonia is

A) 490 B) 530 C) 570 D) 610

Figure 1. Temperature-entropy diagram for ammonia

past_exam_ans (1)

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