CHEM

CHEM*105
Practice TEST #1

This practice exam is representative of the material that may appear on TEST #1. The topics covered on this test are thermochemistry and thermodynamics (chapter 6, sections 8.3, 8.4, 9.6, 9.7 and chapter 17 and corresponding problem assignments). Please note, the presence or absence of material on this practice exam has no bearing on what may appear on TEST #1.

1.[3 points each]
For each question, click on the letter of the one correct answer. There is no penalty for incorrect answers.

1. The reaction CO(g) + ½O₂(g) -----> CO₂(g) is extremely important in efforts to reduce poisonous emissions from automobile exhausts. At constant T and P, which one of the following is correct for this reaction?
A. DH = DU
B. DH is less than DU
C. DH is greater than DU
D. DH is independent of the physical states of the reactants and product
E. w = 0

2. For a certain reaction at constant pressure, DU = –125 kJ and 22 kJ of expansion work is done by the system. What is DH for this process?
A. +147 kJ
B. +103 kJ
C. –147 kJ
D. –103 kJ
E. –125 kJ

3. Calculate the work needed to make room for products in the combustion of 1 mole of CH₄(g) to carbon dioxide and water vapor at STP (1 L.atm = 101 J).
A. –11.3 kJ
B. –4.52 kJ
C. no work is needed
D. –6.79 kJ
E. –2.26 kJ

4. Calculate the heat required to raise the temperature of 50.0 g of water from 25.0^oC to 39.0^oC. The specific heat capacity of water is 4.184 J.(^oC)^-1.g^-1.
A. 5.23 kJ
B. 2.93 kJ
C. 4.18 kJ
D. 0.163 kJ
E. 8.16 kJ

5. At 0^oC the enthalpy of melting of water is 6.0 kJ.mol^-1. The enthalpy change when 36 g of water freezes is
A. –6.0 kJ
B. +6.0 kJ
C. –12 kJ
D. –216 kJ
E. +12 kJ

6. When sulfur dioxide is formed by burning monoclinic sulfur, the reaction enthalpy is –297.16 kJ, and when rhombic sulfur is burned to form SO₂(g), the reaction enthalpy is –296.83 kJ. Calculate the reaction enthalpy for
S(monoclinic) -----> S(rhombic)
A. –0.33 kJ
B. +593.99 kJ
C. –593.99 kJ
D. +0.33 kJ
E. 0

7. Which of the following has a standard enthalpy of formation equal to zero?
A. S(g)
B. Na(l)
C. I₂(g)
D. C(g)
E. Hg(l)

8. The standard enthalpies of combustion of C₂H₂(g), C(s), and H₂(g) are –1300, –394, and –286 kJ.mol^-1, respectively. What is the standard enthalpy of formation of C₂H₂(g)?
A. –620 kJ.mol^-1
B. +226 kJ.mol^-1
C. –226 kJ.mol^-1
D. –520 kJ.mol^-1
E. +620 kJ.mol^-1

9. Consider the following reaction:
N₂H₄(l) + 2H₂O₂(l) -----> N₂(g) + 4H₂O(g) DH_r^o = -530 kJ.mol^-1 at 298 K
Which of the following is correct regarding this reaction?
A. The reaction is spontaneous at all temperatures.
B. The reaction is spontaneous only at very high temperature.
C. The reaction is spontaneous only at very low temperatures.
D. The reaction is not spontaneous at any temperature.
E. DS_r^o must be given to determine if the reaction is spontaneous.

10. Given the data below, estimate the normal boiling point of bromine.

Thermodynamic data at 298 K

Substance	DH_f^o, kJ.mol^-1	S^o, J.K^-1.mol^-1	DG_f^o, kj.mol^-1
Br₂(l)	0	152.23	0
Br₂(g)	+30.91	245.46	+3.11

A. 58^oC
B. 304^oC
C. 100^oC
D. 31^oC
E. 331^oC

11. Which of the following reactions has the largest positive molar entropy change?
A. N₂(g) + 3H₂(g) -----> 2NH₃(g)
B. CH₄(g) + 2O₂(g) -----> CO₂(g) + 2H₂O(g)
C. KClO₄(s) + 4C(s) -----> KCl(s) + 4CO(g)
D. PCl₅(g) -----> PCl₃(g) + Cl₂(g)
E. H₂O(s) -----> H₂O(g)

12. Using the data below, calculate the equilibrium constant at 25^oC for the reaction
NH₃(g) + HCl(g) <-----> NH₄Cl(s)

Thermodynamic data at 298 K

Substance	DH_f^o, kJ.mol^-1	S^o, J.K^-1.mol^-1	DG_f^o, kj.mol^-1
NH₃(g)	-46.11	192.45	-16.45
HCl(g)	-92.31	186.91	-95.30
NH₄Cl(s)	-314.43	94.6	-202.87

A. 6.00 x 10³⁶
B. 3.88 x 10¹⁹
C. 1.42 x 10⁵⁵
D. 9.39 x 10¹⁵
E. 3.68 x 10¹

13. The standard enthalpy of combustion of propanol, CH₃CH₂CH₂OH(l), is -2005.8 kJ.mol^-1 at 25^oC. Calculate the standard enthalpy of formation of propanol given the standard enthalpy of formation of CO₂(g), -393.51 kJ.mol^-1, and of H₂O(l), -285.83 kJ.mol^-1.
A. +318.1 kJ.mol^-1B. -2005.8 kJ.mol^-1C. -156.0 kJ.mol^-1D. +2005.8 kJ.mol^-1E. -318.1 kJ.mol^-1

14. Calculate the free energy change at 298 K for the reaction
CO₂(g) + 2NH₃(g) -----> CO(NH₂)₂(s) + H₂O(l) DG^o = -7.2 kJ
when all gases are present at 76.0 Torr pressure.
A. -7.2 kJ
B. +9.9 kJ
C. +24.3 kJ
D. -24.3 kJ
E. +17.1 kJ

15. Which of the following ionic compounds has the largest lattice enthalpy?
A. BeO
B. MgO
C. BeCl₂D. MgCl₂E. CaO

16. Calculate the vapour pressure of ethanol at 298 K given
CH₃CH₂OH(l) <-----> CH₃CH₂OH(g) DG^o = 6.2 kJ
A. 1.0 atm
B. 0.0031 atm
C. 0.0020 atm
D. 0.082 atm
E. 0.65 atm

17. Processes can be characterized by the sign of DH and DS, as in the following table.

Process	DH	DS
1	-	+
2	+	-
3	-	-
4	+	+

Which processes are definitely spontaneous, and which are possibly spontaneous at constant T and P?
A. definitely 1, possibly 2
B. definitely 2 and 3, possibly 4
C. definitely 1, possibly 3 and 4
D. definitely 3, possibly 1 and 2
E. definitely 4, possibly 1

18. The standard free energy of formation of H₂O(l) is -237.13 kJ.mol^-1 and of H₂O(g) is -228.57 kj.mol^-1. What is DG when 1 mole of water is converted reversibly, at 100^oC and 1 atm, to steam at 100^oC and 1 atm?
A. +8.56 kJ
B. -4.10 kJ
C. 0
D. +4.10 kJ
E. -8.56 kJ

19. Estimate the molar enthalpy of vapourization of carbon tetrachloride if the molar entropy of vapourization is 85.7 J.K^-1.mol^-1 and the boiling point is 77^oC.
A. 30.0 kJ.mol^-1B. 6.6 kJ.mol^-1C. 1.1 kJ.mol^-1D. 4.1 kJ.mol^-1E. 16.8 kJ.mol^-1

20. Which one of the following compounds is most thermodynamically stable with respect to decomposition into its elements at 25^oC?
A. HCN(g), DG_f^o = +124.7 kJ.mol^-1B. HCl(g), DG_f^o = -95.30 kJ.mol^-1C. CCl₄(l), DG_f^o = -65.21 kJ.mol^-1D. N₂H₄(l), DG_f^o = +149.34 kJ.mol^-1E. I₂(g), DG_f^o = +19.33 kJ.mol^-1

2. [20 points]

(a) Calculate the average H-S bond enthalpy in H₂S(g) given the data below.

Compound	DH_f^o, kJ.mol^-1
H₂S(g)	-20.1
H(g)	217.9
S(g)	222.8

(b) Calculate the standard molar enthalpy of formation of NH₃(g) given the data below.

Species	DH_f^o, kJ.mol^-1	bond enthalpy, kJ.mol^-1
N(g)	472.6
H(g)	217.9
N-H		390

(c) What mass of propane is required to heat 240 L (about 50 gal) of water from 15^oC to 60^oC for a bath? The standard enthalpy of combustion of propane is 2200 kJ.mol^-1 and the specific heat capacity of water is 4.18 J.(^oC)^-1.g^-1.

3. [20 points]

(a) Copper(II) sulfate exists as white CuSO₄(s) and also as blue CuSO₄·5H₂O(s). An industrial firm considering the anhydrous material as a potential dehumidifying agent hired a student to answer the following question. Will CuSO₄(s) spontaneously pick up water from the atmosphere at 10^oC when the vapour pressure of water in the atmosphere is 1.33 kPa? Use the following equation as a model.

CuSO₄(s) + 5H₂O(g) -----> CuSO₄·5H₂O(s)

Some of the following data at 298 K may be useful.

Compound	DH_f^o, kJ.mol^-1	DG_f^o, kJ.mol^-1	S_m^o, J.mol^-1.K^-1
H₂O(g)	-241.82	-228.57	188.83
CuSO₄(s)	-771.4	-661.8	109.0
CuSO₄·5H₂O(s)	-2279.7	-1879.7	300.4

(b) If stretching a rubber band is exothermic and if the stretched state is more ordered than the unstretched state, answer the following questions.
(i) Is stretching spontaneous?
(ii) What is the sign of DH, DS and DG for stretching?
(iii) What is the sign of DH, DS and DG for a stretched rubber band going to its normal shorter shape?

(c) Without using thermodynamic data, predict how you would expect the spontaneity of the following reaction to change with temperature.

2NO₂(g) -----> 2NO(g) + O₂(g)