BCIT Fall Chem Exam #2

Transcription

1 BCI Fall 2016 Chem 3310 Exam #2 Name: Attempt all questions in this exam. Read each question carefully and give a complete answer in the space provided. Part marks given for wrong answers with partially correct reasoning/calculations. Constants and equations are given below. A Periodic able and Formula Sheet are attached at the back. otal points = 50 Page 1 of 11

2 Section I: (16 points total, 1 point each) Choose the BES answer to the following questions. 1. A chemical reaction that transfers heat from the system (the reaction) to the surroundings is always: a) endothermic b) exothermic c) spontaneous d) not spontaneous e) an electrochemical reaction 2. For the reaction H 2 O( ) H 2 O(g) at 298 Kand 1.0 atm, ΔH is more positive than ΔE by 2.5 kj/mol. his extra 2.5 kj/mol of energy can be considered to be a) the heat flow required to maintain a constant temperature b) the work done in pushing back the atmosphere c) the difference in the H-O bond energy in H 2 O( ) compared to H 2 O(g) d) the value of ΔH e) none of the above 3. Which of the following reactions is the standard formation reaction of CO(g)? a) C graphite (s) + O(g) CO(g) b) C graphite (s) + ½O 2 (s) CO(g) c) C graphite (s) + ½O 2 (g) CO(g) d) 2C graphite (s) + O 2 (s) 2CO(g) e) 2C graphite (s) + O 2 (g) 2CO(g) 4. he specific heat of gold is 0.13 J g -1 K -1, and that of copper is 0.39 J g -1 K -1. Suppose that a 25 g sample of gold and a 25 g sample of copper are both heated to 75 C and then drop each into identical thermally isolated beakers containing 125 ml of water at 21 C. When the beakers reach thermal equilibrium which of the following will be true? a) both beakers will be the same temperature b) the beaker with the gold will be at a higher temperature c) the beaker with the copper will be at higher temperature d) more information is needed 5. What is H for the following reaction? CBr 4 (g) + CCl 4 (g) CBr 3 Cl(g) + CBrCl 3 (g) Consider the Lewis structures of the molecules. a) H << 0 b) H = 0 (exactly 0) c) H is close to 0 d) H >> 0 e) H will depend on the reaction path taken Page 2 of 11

3 6. Which of the following statements correctly describes the signs of q and w for the following process? H 2 O( ) H 2 O(g) a) q and w are both negative. b) q is positive, w is negative. c) q is negative, w is positive d) q and w are both positive. e) q and w are both zero 7. wo 5.0 L flasks are joined together with a valve in the middle. If 0.25 moles of N 2 (g) are held in the left flask and the right flask is empty, what happens to the entropy of the gas upon opening the valve between the flask? Assume that the temperature is constant at a normal room temperature. a) the entropy decreases b) the entropy remains the same c) the entropy increases d) it depends on the room's temperature e) more information is needed to assess the change 8. Consider the dissociation of hydrogen: H 2 (g) 2H(g) his reaction is a) non-spontaneous at any temperature b) spontaneous only at low temperatures c) spontaneous only at high temperatures d) spontaneous at all temperatures e) more information is needed 9. he entropy of a system approaches zero as: a) it reacts to achieve a noble gas configuration b) as the temperature surpasses the boiling point of its highest boiling component c) as the temperature approaches 0 C d) as the temperature approaches 0 K e) all of the above 10. he degradation of phosgene, COCl 2, into CO and Cl 2 happens to be non-spontaneous at room temperature and standard conditions. Which change could make this a spontaneous reaction? COCl 2 (g) CO(g) + Cl 2 (g) a) add a catalyst b) decrease the temperature c) increase the temperature d) two of a-c could make it spontaneous e) more information is needed Page 3 of 11

4 11. Which of the following is true for the freezing of water at -10 C and 1.0 atm? a) S universe < 0 b) S universe = 0 c) S universe > 0 d) depends on the pathway taken e) more information is needed 12. Under what minimum conditions can G system be used to tell if a chemical reaction is spontaneous? a) constant pressure b) constant temperature c) constant volume d) constant temperature and pressure e) constant temperature and volume 13. S surroundings is for exothermic reactions and for endothermic reactions. a) favorable, favorable b) favorable, unfavorable c) unfavorable, favorable d) unfavorable, unfavorable e) cannot tell 14. For a spontaneous exothermic process, which of the following must be true? a) G > 0 b) S > 0 c) S < 0 d) two of these (a-c) must be true e) none of these (a-c) must be true 15. For which of the following reactions is S > 0? a) NH 4 NO 3 (s) NH 4 +(aq) + NO 3 (aq) b) Cl 2 (g) Cl 2 ( ) c) 2NO 2 (g) N 2 O 4 (g) d) H 2 O(g) H 2 O(s) e) more than one has S > In which case will a reaction at constant pressure not be spontaneous at any temperatures? a) ΔH is positive, ΔS is positive. b) ΔH is positive, ΔS is negative. c) ΔH is negative, ΔS is positive. d) ΔH is negative, ΔS is negative. e) none of these Page 4 of 11

5 Section II: Calculations and reasoning must be shown (34 points total) 17. Hydrogen peroxide, H 2 O 2, is a colorless liquid whose solutions are used as bleach and an antiseptic. H 2 O 2 can be prepared in a process whose overall chemical reaction is H 2 (g) + O 2 (g) H 2 O 2 ( ). ΔH (kj/mol) H 2 O 2 ( ) H 2 O(g) + ½O 2 (g) -98 2H 2 (g) + O 2 (g) 2H 2 O( ) -572 H 2 O(g) H 2 O( ) -44 Calculate H for the reaction: H 2 (g) + O 2 (g) H 2 O 2 ( ) (4 points) 18. A reaction that releases 200. kj of energy occurs in a 1.00 kg Fe container. he heat capacity of Fe is J g -1 C -1. Assume that all the heat from the reaction goes into heating up the Fe container. he melting point of Fe is 1535 C. he initial temperature of the container is 25 C. What is the final temperature of the Fe container? (3 points) 19. Use the values of the bond energies to calculate the ΔH for the following reaction? (3 points) 2C 2 H 4 (g) CH 3 CHCHCH 3 (g) Page 5 of 11

6 20. When ignited, solid ammonium dichromate decomposes in a fiery display. his is the reaction for a volcano demonstration. he decomposition produces nitrogen gas, water vapor, and chromium(iii) oxide. he temperature is constant at 25 C. he reaction is: (NH 4 ) 2 Cr 2 O 7 (s) N 2 (g) + 4H 2 O(g) + Cr 2 O 3 (s) Substance ΔH f (kj/mol) S (J mol -1 K -1 ) Cr 2 O 3 (s) H 2 O(g) N 2 (g)? 192 (NH 4 ) 2 Cr 2 O 7 (s) a) Determine ΔS for the reaction (in J mol -1 K -1 ). (2 points) b) Determine ΔH for the reaction (in kj/mol). (2 points) c) Determine ΔG for the reaction (in kj/mol). (2 points) d) Is this reaction spontaneous at 25 C? Explain your reasoning (1 point) Page 6 of 11

7 21. ΔH f of H 2 O(g) at 25 C is kj/mol. What is ΔH f of H 2 O(g) at 100 C? (8 points) C p (J mol -1 K -1 ) H 2 (g) O 2 (g) H 2 O(g) Page 7 of 11

8 22. Rubidium, Rb, has a heat of vaporization of 69.0 kj/mol at its boiling point (686 C). For the vaporization of rubidium at 686 C and 1.00 atm, find w, q, ΔE, ΔH, ΔS and ΔG? (9 points) Page 8 of 11

9 Formula Sheet: Gas constant, R = J mol -1 K -1 = cal mol -1 K -1 = L atm mol -1 K -1 Faraday's constant, F = C mol -1 (K) = ( C) PV = nr q = m c q = n C V 2 w = P ext dv w = nr ln V 2 w = nr ln P 1 V 1 V 1 P 2 E = q + w E = nc v H=q p H = nc p H = E + (PV) H = Σ(Energy of bonds broken) Σ(Energy of bonds formed) H o =Σn prod H f o (products) - Σn react H f o (reactants) C v = 1.5R E = 1.5nR E = 1.5 (PV) C p C v = R ln p 2 p 1 = H R S universe = S system + S surroundings S o =Σn prod S o (products) - Σn react S o (reactants) q S = rev 2 rev = = 1 dq 1 2 ncd S = nc ln 2 1 S = nr ln V 2 V 1 S = nr ln P 1 P 2 G o = H o - S o S vap = H vap bp G = G o + R ln Q G o = -R ln K G o =Σn prod G o f (products) - Σn react G o f (reactants) G o = - nfe Page 9 of 11

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