General Chemistry II Exam 1 (175 points) (Answer Key)
Name_______________________________________________
Multiple Choice (10 points each)
Identify the letter of the choice that best completes the statement or answers the question.
__E__ 1. For a certain process at 27°C, ∆G = +210.6 kJ and ∆H = -168.2 kJ. What is the entropy change for this process at this temperature? Express your answer in the form, ∆S = __________ J/K.
a. +628.3 J/K d. 1.26 x 103 J/K b. -141.3 J/K e. -1.26 x 103 J/K c. +141.3 J/K Answer: 3 3 27 273 300 ( 168.2 ) ( 210.6 ) 1.26 300 1.26 10 1.26 10 o G H T S T C K H G kJ kJ kJ S T K K kJ J J K kJ K ∆ = ∆ − ∆ = + = ∆ − ∆ − − + ∆ = = = − − − × =
_A___ 2. Which response includes all of the following substances that have ∆Hof = 0, and no other substances? I. HCl(g)
II. Na(s) III. HCl(aq) IV. F2(g)
a. II and IV d. I, II, III, and IV
b. II e. I, II, and IV
c. I and II
Answer: All elements in their natural standard state have a ∆Ho f = 0. __A__ 3. From the following data at 25°C,
2 NOCl(g) → 2 NO(g) +Cl2g) ∆H0 = +75.56 kJ 2 NO(g) + O2(g) → 2 NO2g) ∆H0 = –113.05 kJ
2 NO2(g) → N2O4(g) ∆H
0 = –58.03 kJ Calculate ∆H0 at 25°C for the reaction below.
N2O4(g) + Cl2(g) → 2 NOCl(g) + O2(g)
a. +95.52 kJ d. –85.52 kJ
b. +299 kJ e. –246.65 kJ
c. –186.8 kJ
Answer: Reverse all reactions and change the sign of the ∆H0 for each. 2 NO(g) +Cl2 (g) → 2 NOCl(g) ∆H0 = –75.56 kJ 2 NO2 (g) → 2 NO(g) + O2(g) ∆H0 = +113.05 kJ N2O4(g) → 2 NO2(g) ∆H0 = +58.03 kJ N2O4(g) + Cl2(g) → 2 NOCl(g) + O2(g) ∆H0 = +95.52 kJ
__A__ 4. Assuming the gases are ideal, calculate the amount of work done, in joules, for the conversion of 1.00 mole of Ni to Ni(CO)4 at 75°C in the reaction below. The value of R is 8.314 J/mol•K.
Ni(s) + 4CO(g) → Ni(CO)4(g)
a. 8.68 × 103 J d. 1.80 × 103 J b. -1.80 × 103 J e. - 494 J c. -8.68 × 103 J
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4 3Answer: Ni(s) + 4CO(g) Ni(CO) (g) 4 mol gas 1 mol gas 1 mol gas 4 mol gas 3 mol gas 8.314 75 273 348 8.314 3 mol gas 348 8.68 10 o n J R mol K T C K w nRT J K mol K J → ∆ = − = − = = + = = − ∆ = − − = + ×
_B___ 5. Which of the following techniques cannot be used to calculate ∆Hrxn? a. Using of Heats of Formation of reactants and products
b. Using melting points of reactants and products c. Hess's Law
d. Using bond energies of reactants and products e. Calorimetry
__C__ 6. Consider the reaction below at 25°C for which ∆S0 = 16.1 J/K. CH4(g) + N2(g) + 163.8 kJ → HCN(g) + NH3(g) +∆H
Which one of the following statements describes the reaction?
a. Insufficient information to estimate temperature range of spontaneity b. Spontaneous at relatively low temperatures only
c. Spontaneous at relatively high temperatures only d. Spontaneous at all temperatures
e. Nonspontaneous at all temperatures
Answer: Under conditions of positive enthalpy and positive entropy, the temperature must be relatively high to achieve a negative free energy.
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answer ∆ = ∆ − ∆G H T S
__A__ 7. At 25°C ∆H = 128.9 kJ and ∆G = 33.5 kJ for a reaction. Above what minimum temperature will this reaction become spontaneous? a. 403 K d. 332 K b. 530 K e. 298 K c. 1150 K
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: . 1) . 128.9 33.5 0.320 298Answer This is a two part answer Determine S G H T S kJ H G mol kJ S T K mol K − ∆ ∆ = ∆ − ∆ − ∆ − ∆ ∆ = = = 2) 0. 128.9 403 0.320 Determine T by letting G G H T S H kJ T kJ K S mol K ∆ = ∆ = ∆ − ∆ ∆ = = = ∆
____ 8. Which one of the following thermodynamic quantities is not a state function?
a. work d. internal energy
b. free energy e. enthalpy
c. entropy
__B__ 9. Which response includes all the following processes that are accompanied by an increase in entropy, and only those processes?
I. boiling water l Æ g (+∆S)
II. freezing water l Æ
s
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∆S)III. N2(g) + 3H2(g) → 2NH3(g) 4 mol gas Æ 2 mol gas (
–
∆S) IV. Br2(l) → Br2(g) (+∆S)a. I and II d. II, III, and IV
b. I and IV e. III and IV
c. another one or another combination
_B___ 10. Estimate the enthalpy change for the reaction below from the average bond energies given. There are two Cl-Cl and two C-H bonds in CH2Cl2. Remember that energy is absorbed when bonds are broken and released when they are formed. CH4(g) + 2Cl2(g) → CH2Cl2(g) + 2HCl(g)
Average Bond Energies
C-H 413 kJ/mol Cl-Cl 242 kJ/mol H-Cl 432 kJ/mol C-Cl 339 kJ/mol a. +578 kJ/mol d. -578 kJ/mol b. -232 kJ/mol e. +541 kJ/mol c. +232 kJ/mol
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: 4 4(413 ) 1652 2 2(242 ) 484 2136 2 2(413 ) 826 2 2(339 ) 678 2 2(432 ) 864 2368 . 2136 23 Answer reactants C H bonds kJ kJ Cl Cl bonds kJ kJ kJ products C H bonds kJ kJ C Cl bonds kJ kJ H Cl bonds kJ kJ kJ B E reactants products kJ − = = + − = = = − = = + − = = + − = = = = − = −∑
∑
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68kJ)
= −232kJ_D___ 11. How much heat is given off in the formation of 35.0 grams of Fe2O3(s) at 25°C and 1.00 atm pressure by the following reaction? The molar mass of iron(III) oxide is 159.7 g/mol.
4Fe(s) + 3O2(g) → 2Fe2O3(s) ∆H (kJ/mol) -824.2 a. 243. 9 kJ d. 180.7 kJ b. 90.4 kJ e. 360.1 kJ c. 151 kJ Answer:
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2 3 2 3 2 3 2 3 ( ) ( ) 2( 824.2 ) 0 0 1648 35.0 1 1 1648 180.6 159.7 2 1 o rxn H n products n reactants kJ kJ g Fe O mol Fe O mol rxn kJ kJ g Fe O mol Fe O mol rxn ∆ = − = − − + = − = ∑
∑
_A___ 12. Which one of the following statements is not correct?
a. When ∆H for a reaction is negative, the reaction is never spontaneous. b. When ∆G for a reaction is negative, the reaction is spontaneous. c. When ∆G for a reaction is positive, the reaction is nonspontaneous. d. When ∆G for a reaction is zero, the system is at equilibrium.
Answer: Most reactions exothermic, and as a result, the change in entropy is positive. A negative change in enthalpy and positive change in entropy will result in a negative free energy and a spontaneous reaction.
_E___ 13. Which statement concerning sign conventions for ∆E = q + w is false? a. For heat absorbed by the system, q is positive.
b. For an expansion, w is negative.
c. When energy is released by the reacting system, ∆E is negative. d. For work done by the system, w is negative.
e. If ∆E is positive, energy can be written as a product in the equation for the reaction.
Answer: If the change in internal energy of a system is positive, then the system gained energy and or had work done on it by the surroundings. In terms of a chemical reaction, this means the reactants added heat or had work done on it (such as gases going to liquids) during the reaction process.
Problem You must show your work for the following questions and it must logically lead to your answer to receive full credit for a question. The calculations and answers should be shown with the correct units and significant figures. 14. (45 total points) A 1.000 g sample of ethanol, C2H5OH, was burned in a bomb calorimeter whose heat capacity had been
determined to be 2.71 kJ/oC. The temperature of 3.000 x 103 grams of water rose from 24.284 oC to 26.225 oC. The molar mass of ethanol is 46.07 g/mol.
a) (20 points) Determine the ∆H for the reaction in joules per gram of ethanol. b) (10 points) Determine the ∆H for the reaction in kilojoules per mole of ethanol. c) (10 points) What is the ∆E for the reaction at constant pressure?
d) (5 points) Is work done by or on the system?
C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) + 3 H2O (l) Answer:
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3 3 3 4 4 2.71 10 ) ; ; 26.255 24.284 1.941 4.184 2.71 10 3.000 10 1.941 1.941 2.96 10 2.96 10 ( , ) o o o o o o o J a q mC T calorimeter constant T C C C J J q g C C J g C C q H JH remember the rxn is exothermic
g EtOH b × = ∆ = ∆ = − = × = × + = × = ∆ × ∆ = −
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4 3 3 3 3 2.96 10 46.07 1 ) 1.36 10 10 ) 2 3 1 8.341 1 (298 ) 2.48 10 2.48 1.36 10 o o J g EtOH kJ kJg EtOH mol EtOH J mol
c E q w
n mol gas mol gas mol gas
w nRT J mol gas K J kJ mol K E q w kJ mol × − = − × ∆ = + ∆ = − = − = − ∆ = − − = × = ∆ = + = − × +
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3 2.48 1.36 10 ) . kJ kJ mold Work is done on the system by the surroundings
