Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 1 Tuesday, February 13, 2007 Page 1 of 6

Exam 1 Name: ______________________________ Recitation Section Lenny.: 6:30 7:30 (circle one): Greg.: 6:30 7:30 Student Number: _____________________ Nic.: 6:30 7:30 Please show your work and either circle your answers or put your answers in the boxes provided. You may tear off the last page of this exam, which contains useful data etc. 1) (30 pts total) A) When liquid hydrazine is mixed with oxygen gas, the following reaction occurs and the vessel gets warm: N2H4 (l) + 3 O2 (g) 2 NO2 (g) + 2 H2O (l) What is the sign of ΔH for the above reaction (use chemical reasoning and/or observations) (2pts) ? a) ΔH < 0 b) ΔH > 0 c) ΔH is not obvious (need Appendix of ΔHf

o's) Please give your reason (1pt): What is the sign of ΔS for the above reaction (use chemical reasoning and/or observations) (2pts)? a) ΔS < 0 b) ΔS > 0 c) ΔS is not obvious (need Appendix of So's) Please give your reason (1pt): B) When NO gas is mixed with fluorine gas, the following reaction occurs: 2NO (g) + F2(g) 2NOF (g) What is the sign of ΔS for the above reaction (use chemical reasoning and/or observations) (3pts)? a) ΔS < 0 b) ΔS > 0 c) ΔS is not obvious (need Appendix of ΔHf

o's) Please give your reason (1pt):

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 1 Tuesday, February 13, 2007 Page 2 of 6

C) Consider a substance with the following thermodynamic properties for melting (fus) and boiling (vap): ΔHfus = 5.40 kJ/mol Tfus = 23.0 oC ΔHvap = 54.0 kJ/mol Tvap = 230.0 oC For which of processes (melting or boiling) is ΔS the largest? (circle one, 4 pts) a) ΔSfus > ΔSvap b) ΔSfus < ΔSvap c) ΔSfus = ΔSvap Please give your reason (1pt): D) A balloon initially contains 1.00 liter of gas at room temperature. It it then placed into ice water and both its temperature and volume decrease. What are the signs of q and w for the gas in the ballon? q: positive negative zero (circle one, 2pts) w: positive negative zero (circle one, 3pts) E) The heat involved in a constant volume process is equal to: (circle one, 5pts) a) ΔH b) ΔT c) ΔE d) ΔS e) Cp

F) Consider samples of ideal gasses: Sample 1: 1.00 liter of gas at 25.0oC and a pressure of 1.00 atm

Sample 2: 0.500 liter of gas at 25.0oC and a pressure of 2.00 atm 10.0 kJ of heat is transferred to each of the samples at constant volume (assume no heat exchange with the surroundings). What is true of the final temperatures, Tf, of the two samples.

a) The Tf of sample 1 will be larger than the Tf of sample 2 (circle one, 5pts) b) The Tf of sample 2 will be larger than the Tf of sample 1 c) The Tf of sample 1 will be the same as the Tf of sample 2

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 1 Tuesday, February 13, 2007 Page 3 of 6

2) (25pts) Consider the following reaction occurring in water: 2 A + 3 B 2 C ΔHo = ? 120.0 ml of 1.40 M A (at 25.0oC) is mixed with 200.0 ml of 1.50 M B (at 25.0oC) in an insulated container. The final temperature of the solution is 32.4 oC. What is ΔHo for the above reaction? Both solutions are initially at 25.0oC. (Assume the above reaction goes to completion.)

ΔHo = ______________________kJ

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 1 Tuesday, February 13, 2007 Page 4 of 6

3) (25pts total) A) (20pts) A metal bar weighs 92.4 g and is initially at 450.0 oC. It is placed into 425.0 ml of water, which is initially at 25.0 oC. The final temperature of the water is 42.0 oC. What is the heat capacity, Cp, of the metal bar? (Assume that the bar and water are in thermal contact, but that no heat is lost to the surroundings.) B) (5pts) In part A, suppose that instead of putting the bar into 425.0 ml of water, the bar was placed into a 425 ml of a fluid with a heat capacity of 4.314 J/(g oC) and a density of 0.90 g/ml. Would the final temperature of the system be higher or lower than the 42.0 oC obtained in part A? The final temperature will be higher lower the same as in part A.

(circle one)

Cp(bar) = _________________ J/(g oC)

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 1 Tuesday, February 13, 2007 Page 5 of 6

4) (20pts total) A) (15 pts) Using the table of enthalpies of atomization and bond enthalpies on the last page of the exam, estimate ΔHf

o for the following molecule (C2H3SBr):

B) (5pts) Suppose you wanted to use the following reaction as a fuel reaction (i.e. this reaction should give off heat when it occurs)

H – H H H H H H | | | | | | H – C – C – C – X + Y H – C – C – C – Y + X | | | | | | H – H H H – H H H

For this reaction to give off heat, which must be true? (circle one) a) The C-X bond must be stronger than the C-Y bond b) The C-Y bond must be stronger than the C-X bond c) Can't tell from information given

ΔHfo [C2H3SBr]= _________________ kJ/mol

For official use only

5) ____________/30

6) ____________/25

7) ____________/25

8) ____________/20

Total

___________/100

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 1 Tuesday, February 13, 2007 Page 6 of 6

Chemical Data

Average Bond Enthalpies in kJ/mole Single bonds Multiple bonds

C=C 615 C=N 615 O=O 498 N=O 607 N=N 418 C=S 477 C=O 728 (799 in CO2) S=O (532 in SO2); (469 in SO3) C≡C 812 C≡N 891 N≡N 945 C≡O 1072

Cv(H2O(l)) = Cp(H2O(l)) = 1.00cal/(g K) 1 cal = 4.184 J density (H2O(l)) = 1.00 g/ml 1ml = 1 cm3 R = 0.0821 liter atm /( mole K ) R = 8.314 J/( mole K) We would appreciate you writing the symbolic form of equations before filling in the numbers, i.e. n = (PV)/(RT) = (0.1atm) (1.0liter)/(0.0821 l atm/mol K) (298.15K) instead of n= (0.1atm) (1.0liter)/(0.0821 l atm/mol K) (298.15K) This makes it much easier for us to give partial credit. Thanks

Molar Enthalpy of Atomization

H C N O

H 218.0 436

C 716.7 413 348 N 472.2 391 292 161 O 249.2 463 351 139 S 278.8 339 259 F 79.0 563 441 270 185 Cl 121.7 432 328 200 203 Br 111.9 366 276 I 106.8 299 240