Unit 4 Exam-solutions

Version 181 Unit 4 Exam vandenbout (51335) 1

This print-out should have 20 questions.Multiple-choice questions may continue onthe next column or page find all choicesbefore answering.

vandenbout/labrake - ch301

EXAM49:30 am class only - 51335

7-9pm Dec 5, 2012

Thermodynamic Data at 25C

Hf G

f S

Substance kJ/mol kJ/mol J/mol K

Al (s) 28.33Al2O3 (s) -1675.7 -1582.4 50.92H2O (`) -286 -237 70H2O (g) -242 -229 189CO (g) -111 -137 198CO2 (g) -394 -395 214CH4 (g) -75 -51 186C3H8 (g) -104 -24 270C6H12O2 (`) -555 C12H22O11 (s) -2222 -1545 360O2 (g) 205.1Fe2O3 (s) -824.2 -742.2 87.4Fe3O4 (s) -1118.4 1015.4 146.4

Single Bond Energies (kJ/mol of bonds)H C N O S F Cl

H 436C 413 346N 391 305 163O 463 358 201 146S 347 272 226F 565 485 283 190 284 155Cl 432 339 192 218 255 253 242

Multiple Bond Energies (kJ/mol of bonds)C=C 602 C=N 615 C=O 799CC 835 CN 887 CO 1072N=N 418 O=O 498 NN 945

001 4.0 points

If the change in internal energy for a givenprocess is zero, which of the following must betrue?

1. q = -w correct

2. w = 0

3. E = H

4. q = 0

Explanation:

E = q + w = 0q = -w

002 4.0 points

Carbon monoxide reacts with oxygen to formcarbon dioxide by the following reaction:

2CO(g) + O2(g) 2CO2(g)

H for this reaction is 135.28 kcal.If 811.7 kcal is released, how many moles of

CO must have reacted?

1. 12.0 mol correct

2. 16.3 mol

3. 6.00 mol

4. 270.5 mol

5. 1.35 mol

6. 40.5 mol

7. 5.41 mol

Explanation:

H = 135.28 kcal q = - 811.7 kcal( 1 mol rxn135.28 kcal

)(2 mol CO1 mol rxn

)

(811.7kcal) = 12 mol CO

so 12 mol CO were reacted.

003 4.0 points

At constant temperature and pressure, how isSuniv related to Gsys?


1. Gsys = Suniv

2. Gsys = T Suniv correct

3. Gsys = Suniv

4. Gsys = T Suniv

5. Gsys = SunivT

6. Gsys =SunivT

Explanation:

004 4.0 points

A 22.9 g sample of metal X requires136 calories of energy to heat it from 10.0Cto 81.0C. Calculate the specific heat of metalX.1. 0.26152. 0.213. 0.25484. 0.20385. 0.35626. 0.39027. 0.30818. 0.21879. 0.3510. 0.2733

Correct answer: 0.35 J/g C.

Explanation:

005 4.0 points

Chlorine monofluoride (ClF) will react withcarbon monoxide (CO) to give carbonyl chlo-rofluoride (COClF):

ClF + CO COClF

Use bond energies (provided elsewhere) toestimate the change in enthalpy (H) forthis reaction.

1. -298 kJ/mol correct

2. -571 kJ/mol

3. -444 kJ/mol

4. -376 kJ/mol

5. -193 kJ/mol

Explanation:

ClF + CO ClC=O|F

Break: ClF and CO253 1072

Make: C=O and CCl and CF799 339 485

breaking : +1325 (reactants)making : -1623 (products)

H = 298 kJ/mol

006 4.0 points

What is the change in entropy (Svap)for the vaporization of ethanol (Hvap =

38.6 kJ mol1) at its standard boiling tem-perature (78.4 C)?

1. 110 J mol1 K1

2. 492 J mol1 K1

3. 492 J mol1 K1

4. 110 J mol1 K1 correct

Explanation:

Svap =HvapT

=38, 600 J mol1

351.4 K= 110 J mol1 K1

007 4.0 points

Given that fH

Fe2O3(s)= 826 kJ mol1

what would you predict for the spontaneityof the following reaction

2Fe2O3(s) 4Fe(s) + 3O2(g)

1. there is no way to predict the spontaneityof this reaction

2. it would be spontaneous only at hightemperatures correct


3. it would never be spontaneous

4. it would be spontaneous at all tempera-tures

5. it would be spontaneous only at low tem-peratures

Explanation:

The reaction as written is the reverse oftwice the formation of Fe2O3. As such thisreaction is extremely endothermic (H >>0). However, the entropy change for thisreaction should be positive as it generatesthree moles of gas. The enthalpy changescause an increase in the free energy, but theentropy change will cause the free energy todecrease. At high temperatures the entropywill dominate and the free energy change willbe negative.

008 4.0 points

Which of the following is always true fora spontaneous process at constant temper-ature?

1. S =q

T

2. S > 0

3. Ssystem + Ssurroundings > 0 correct

4. S 0).

009 4.0 points

Rank the following systems by entropy, fromleast to greatest:a) 1 mol of pure iceb) 1 mol of water with 1 mol of salt dissolved

in it.

c) 1 mol of pure water

1. c < b < a

2. a < b < c

3. c < a < b

4. b < c < a

5. b < a < c

6. a < c < b correct

Explanation:

Entropy increases as systems go throughendothermic phase transitions and when thereis more matter or more dispersed matterpresent.

010 4.0 points

Consider the reaction

Ni(s) + 4CO(g) Ni(CO)4(g)

Assuming the gases are ideal, calculate thework done on the system at a constant pres-sure of 1 atm at 75C for the conversion of1.00 mole of Ni to Ni(CO)4.

1. 2.89 103 J

2. 5.79 103 J

3. 1.80 103 J

4. 1.87 103 J

5. 1.16 104 J

6. 8.68 103 J correct

Explanation:

T = 75C+ 273 = 348 K nNi = 1 mol

ni = 1 mol Ni 4 mol CO

1 mol Ni= 4 mol

nf = 1 mol Ni 1 mol Ni(CO)4

1 mol Ni= 1 mol


n = (1 4) mol = 3 mol

w = P V = nRT

= (3 mol) 8.31451 J

mol K 348 K

= 8680.35 J

011 4.0 points

Calculate the standard enthalpy of combus-tion (cH) of t-butyl acetate, C6H12O2(`).

1. -125 kJ/mol

2. -4080 kJ/mol

3. -4635 kJ/mol

4. -3525 kJ/mol correct

5. -2970 kJ/mol

6. -4371 kJ/mol

7. -3261 kJ/mol

Explanation:

The balanced equation is:C6H12O2(`) + 8O2(g) 6CO2(g) + 6H2O(`)cH = 6(394) + 6(286) (555)cH = 2364 +1716 + 555cH = 3525

012 4.0 points

Calculate the entropy change for the systemwhen 40 g of ethanol are heated from 25C to40C given that the heat capacity for ethanolis 2.44 J g1 K1.

1. 4.8 J K1 correct

2. -45.9 J K1

3. 0.12 J K1

4. 45.9 J K1

5. -2.3 J K1

Explanation:

The entropy change is S = ClnTfTi

S =

(40 * 2.44) ln(313.15/298.15) = 4.8 J K1

013 4.0 points

Consider the reaction

2 SO2(g) + O2(g) 2 SO3(g)

Which statement is true for this reaction?

1. S < 0 correct

2. Sm = 0 for O2(g)

3. S > 0

4. S = 0

Explanation:

For the reaction 3 mol gas 2 mol gas. Thefewer the moles of gas, the lower the disorder,so S is negative.

014 4.0 points

Calculate the standard reaction enthalpy forthe reaction

NO2(g) NO(g) + O(g)

Given:O2(g) 2O(g) H

= +498.4 kJ/molNO(g) + O3(g) NO2(g) + O2(g)

H = 200 kJ/mol3O2(g) 2O3(g) H

= +285.4 kJ/mol.

1. +306 kJ/mol correct

2. +555 kJ/mol

3. +192 kJ/mol

4. +355 kJ/mol

5. +93.5 kJ/mol

6. +413 kJ/mol

7. +592 kJ/mol

Explanation:


O2(g) 2O(g)

H = +498.4 kJ/mol

NO(g) + O3(g) NO2(g) + O2(g)

H = 200 kJ/mol

The standard formation of ozone is

3

2O2(g) O3(g)

H = +142.7 kJ/mol

We calculate the Hrxn using Hess Law:To combine the reactions and get the de-

sired reaction, reverse the second and thirdequations and add half of the first one:

NO2(g) + O2(g) NO(g) + O3(g)

H = +200 kJ/mol

O3(g)3

2O2(g)

H = 142.7 kJ/mol1

2O2(g) O(g)

H =1

2(498.4 kJ/mol)

NO2(g) NO(g) + O(g)

Hrxn = 306.5 kJ/mol

015 4.0 points

What is Suniv for a reaction conducted at100C and for which S is 170 J mol1 K1

and H is 43.2 kJ mol1.

1. 262 J mol1 K1

2. 285.8 J mol1 K1

3. 602 J mol1 K1

4. 54.2 J mol1 K1 correct

Explanation:

Suniv = Ssys +Ssurr

= 170 J mol1 K1

43200 J mol1

373 K= 54.2 J mol1 K1

016 4.0 points

Which of the following reactions as writtencorresponds to a standard formation reaction?

1. SO2(g) +1

2O2(g) SO3(g)

2. 2H (g) + O2(g) H2O2 (`)

3. Na(s) +1

2Cl2(g) NaCl(s) correct

4. 2 H2(g) + O2(g) 2 H2O(`)

5. CO2(s) CO2(g)

Explanation:

All of the reactants in a formation reactionmust be ELEMENTS in their thermochemicalstandard state. AND, to match table values,only 1 MOLE of product can be formed be-cause standard table values are in kJ/mol.Only the NaCl fits this definition. The H2Oreaction unfortunately is forming 2 moles ofwater instead of one.

017 4.0 points

The enthalpy of fusion of methanol (CH3OH)is 3.16 kJ/mol. How much heat would beabsorbed or released upon freezing 25.6 gramsof methanol?

1. 0.253 kJ absorbed

2. 3.95 kJ absorbed

3. 2.52 kJ absorbed

4. 0.253 kJ released

5. 3.95 kJ released

6. 2.52 kJ released correct

Explanation:

MW CH3OH = 32.042g

H(fusion) = H(freezing)


q = 25.6 g1mol

32.042g3.16 kJ

mol= 2.525 kJ

018 4.0 points

A sample of 2 grams of compound Y is burnedcompletely in a bomb calorimeter which con-tains 2500 g of water. The temperature risesfrom 25.529C to 25.991C.What is Urxn forthe combustion of compound Y? The hard-ware component of the calorimeter has a heatcapacity of 3.78 kJ/C. The specific heat ofwater is 4.184 J/g C, and the MW of Y is129 g/mol.1. -311.82. -572.33. -503.54. -235.95. -424.36. -213.57. -165.28. -552.39. -269.510. -335.2

Correct answer: 424.3 kJ/mol of Y.

Explanation:

SH = 4.184 J/g C HC = 3.78 kJ/CmY = 2 g MWY = 129 g/molmwater = 2500 gT = 25.991C 25.529C = 0.462C

Heat = (SH)(mwater)(T ) + (HC)(T ) ,

Then divide the heat by the number of moleswhich is mass/MW.

019 4.0 points

Sodium is a metal that reacts violently withwater. Chlorine is a toxic, choking gas. So itsa really good thing that when you put salt onyour fries (chips) that it doesnt suddenly turninto sodium metal and chlorine gas. Why isthis the case?

1. The standard enthalpy of formation ofsodium chloride is negative.2. The standard Gibbs free energy of for-

mation of sodium chloride is negative. COR-RECT

3. The standard entropy change for theformation of sodium chloride is negative.4. Salt dissolves in the water in your food.5. Salt has a really high melting point.

Explanation:

The standard Gibbs free energy of forma-tion of sodium chloride is negative so the pro-cess is spontaneous. Thus unforming sodiumchloride is nonspontaneous, so sodium chlo-ride is stable.


020 24.0 points

This question is merely a placeholder forthe points in the hand-graded portion of theexam.

Explanation:

This question is merely a placeholder forthe points in the hand-graded portion of theexam.

Unit 4 Exam-solutions

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