Exam 1. Name: Recitation Section Lenny.: 6:30 7:30 (circle one): Greg.: 6:30 7:30 Student Number: Nic.: 6:30 7:30

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1 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: N 2 H 4 (l) + 3 O 2 (g) 2 NO 2 (g) + 2 H 2 O (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 ΔH o f 's) 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 S o 's) B) When NO gas is mixed with fluorine gas, the following reaction occurs: 2NO (g) + F 2(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 ΔH o f 's) Exam 1 Tuesday, February 13, 2007 Page 1 of 6

2 C) Consider a substance with the following thermodynamic properties for melting (fus) and boiling (vap): ΔH fus = 5.40 kj/mol T fus = 23.0 o C ΔH vap = 54.0 kj/mol T vap = o C For which of processes (melting or boiling) is ΔS the largest? (circle one, 4 pts) a) ΔS fus > ΔS vap b) ΔS fus < ΔS vap c) ΔS fus = ΔS vap 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) C p F) Consider samples of ideal gasses: Sample 1: 1.00 liter of gas at 25.0 o C and a pressure of 1.00 atm Sample 2: liter of gas at 25.0 o C 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, T f, of the two samples. a) The T f of sample 1 will be larger than the T f of sample 2 (circle one, 5pts) b) The T f of sample 2 will be larger than the T f of sample 1 c) The T f of sample 1 will be the same as the T f of sample 2 Exam 1 Tuesday, February 13, 2007 Page 2 of 6

3 2) (25pts) Consider the following reaction occurring in water: 2 A + 3 B 2 C ΔH o =? ml of 1.40 M A (at 25.0 o C) is mixed with ml of 1.50 M B (at 25.0 o C) in an insulated container. The final temperature of the solution is 32.4 o C. What is ΔH o for the above reaction? Both solutions are initially at 25.0 o C. (Assume the above reaction goes to completion.) ΔH o = kj Exam 1 Tuesday, February 13, 2007 Page 3 of 6

4 3) (25pts total) A) (20pts) A metal bar weighs 92.4 g and is initially at o C. It is placed into ml of water, which is initially at 25.0 o C. The final temperature of the water is 42.0 o C. What is the heat capacity, C p, of the metal bar? (Assume that the bar and water are in thermal contact, but that no heat is lost to the surroundings.) C p (bar) = J/(g o C) B) (5pts) In part A, suppose that instead of putting the bar into ml of water, the bar was placed into a 425 ml of a fluid with a heat capacity of J/(g o C) and a density of 0.90 g/ml. Would the final temperature of the system be higher or lower than the 42.0 o C obtained in part A? The final temperature will be higher lower the same as in part A. (circle one) Exam 1 Tuesday, February 13, 2007 Page 4 of 6

5 4) (20pts total) A) (15 pts) Using the table of enthalpies of atomization and bond enthalpies on the last page of the exam, estimate ΔH f o for the following molecule (C 2 H 3 SBr): ΔH f o [C 2 H 3 SBr]= kj/mol 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 For official use only 5) /30 6) /25 7) /25 8) /20 Total /100 Exam 1 Tuesday, February 13, 2007 Page 5 of 6

6 Chemical Data Average Bond Enthalpies in kj/mole Single bonds Molar Enthalpy of H C N O Atomization H C N O S F Cl Br I 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 CO 2 ) S=O (532 in SO 2 ); (469 in SO 3 ) C C 812 C N 891 N N 945 C O 1072 C v (H 2 O (l) ) = C p (H 2 O (l) ) = 1.00cal/(g K) 1 cal = J density (H 2 O (l) ) = 1.00 g/ml 1ml = 1 cm 3 R = liter atm /( mole K ) R = 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)/( l atm/mol K) (298.15K) instead of n= (0.1atm) (1.0liter)/( l atm/mol K) (298.15K) This makes it much easier for us to give partial credit. Thanks Exam 1 Tuesday, February 13, 2007 Page 6 of 6

Chapter 5 Thermochemistry. 許富銀 ( Hsu Fu-Yin)

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