Chem 17 LE 1 Samplex

University of the Philippines Association of Chemistry Majors and Enthusiasts (UP ACME)

SAMPLE FIRST LONG EXAM (CHEM 17) Second Semester, AY 2015-2016

I.

Modified True or False. Write TRUE if the statement is always correct. If the statement is false, CHANGE the underlined word/s to make the statement correct. _________________1. _________________2. _________________3. _________________4. _________________5. _________________6. _________________7. _________________8. _________________9. _________________10.

Presence of catalysts speeds up the rate and increases the yield of reactions. In determining rate laws using transition state theory, the net change of concentration of intermediates is equal to zero. The green-colored cobalt (III) ion hastens the oxidation of tartrate by hydrogen peroxide. A reaction mechanism is a step-by-step description of a chemical reaction consisting of a series of elementary processes. In the Ellingham Diagram, the slope of the curve obtained by plotting ΔG versus T is equal to the ΔS of the system. A second-order bimolecular reaction can be reduced to pseudo-first order reaction if the concentration of one reactant is very high and remains essentially constant. Transition states are species that do not appear in the overall chemical reaction and overall rate law, but can be isolated under certain conditions. If heat is released by the system and work is done by the system, the process has a positive ΔU. If a reaction is said to be spontaneous at all temperatures, then its enthalpy and entropy are always negative. A spontaneous reaction that is strongly endothermic is enthalpy-driven.

II. Cause and Effect. Predict the effect (INCREASE, DECREASE, NO EFFECT, INDETERMINATE) of the given condition in Column A on the indicated parameter in Column B. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Column A (CONDITION) The cork with thermometer was not completely inserted in the test tube. In the calibration of the calorimeter, the HCl solution was not diluted to mark. The cork stopper was placed a minute after the reaction took place. The second dry ice slab was not placed immediately after ignition of the Mg ribbon. Several minutes were needed before the Mg ribbon could be ignited and burned completely. Cu2+ was added to the solution containing permanganate and oxalate. Water used to prepare the KI solution was found to be contaminated with Cu2+ ions. Excess HCl was added to the thiosulfate solution. The cork with thermometer was not completely inserted in the test tube.

CHEM 17 (2nd Sem, AY ‘15-‘16)

Column B (PARAMETER) Magnitude of measured qcalorimeter ∆Hrxn of water

EFFECT

Experimental ∆Hrxn Amount of graphite formed ∆G of formation of magnesium oxide Rate of reaction Rate of reaction Rate constant Magnitude of measured qcalorimeter

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III. Multiple Choice. Encircle the best answer. 1. For the reaction A products, the following data were obtained: t = 0 s, [A] = 0.715 M; 22 s, 0.605 M; 74 s, 0.345 M; 132 s, 0.055 M. What is the half-life of the reaction? A. 24 s C. 48 s B. 72 s D. 96 s 2. The change in slope in the linear curve of an Ellingham diagram indicates which of the following? A. Heating at Constant Pressure B. Heating at Constant Volume C. Isothermal Phase Change D. Isobaric Phase Change 3. What is the change in internal energy of a system if the system evolves 280 cal of heat and has 1.25 kJ of work done on it? A. +2.42 kJ C. -2.42 kJ B. -0.08 kJ D. +0.08 kJ 4. The activation energy for the isomerization of cyclopropane to propene is 274 kJ/mol. By what factor does the rate of reaction increase as the temperature rises from 500:C to 550:C? A. 11.31 C. 13.33 B. 0.088 D. 0.075 5. Which among the following reactions has a half-life that is independent of the original amount of solution? A. Zero-order reactions B. First-order reactions C. Second-order reactions D. None of the above 6. Which of the following is/are true about the Co(II)-ion system? I. Heating CoCl2(aq) induces the production of Co2+ which in turn causes the color change from pink to blue. II. Diluting the solution with water turns the solution into a purple color. III. CoCl4- is produced after the addition of HCl. A. I only B. II and III

C. III only D. None of the above

7. What is the final temperature of a system after 100.0 g of zinc at 95°C is immersed into 50.0 g of water at 15°C? g (CZn = 6.06 cal⁄mol K ; MWZn = 65.37 ⁄mol) A. 28°C B. 42°C C. 30°C D. 53°C For items #8, #9 and #10: Given the following data for the reaction: NH3(g) + HCl(g) ∆H f : (kJ/mol) ∆G f : (kJ/mol) NH3(g) -46.11 -16.48 HCl(g) -92.31 -95.30 NH4Cl -314.4 -202.9

NH4Cl

8. Determine the ∆H:rxn for the reaction using the reaction using the given values. A. +176 kJ C. -452.8 kJ B. -176 kJ D. +452.8 kJ CHEM 17 (2nd Sem, AY ‘15-‘16)

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9. What is the value of the ∆S:rxn for the reaction A. +6.481 J mol-1 B. -6.481 J mol-1

C. -284.77 J mol-1 D. +284.77 J mol-1

10. What is the minimum temperature (in :C) in which the reaction will not occur spontaneously? A. Starting at 345 :C C. Starting at 618 :C B. Starting at 25:C D. Starting at 298 :C 11. Which of the following is FALSE about a first-order reaction? A. ln [A]t = -kt + ln [A]0 B. The plot of ln [A]0 against time is linear with a negative slope. C. Doubling the concentration will double the reaction rate. D. The individual half-lives of all first-order reactions are always constant. E. 12. Which of the following is TRUE in the calorimetry experiment? A. A negative ∆T indicates an exothermic reaction. B. The reaction between a strong base and a strong acid is more exothermic than the reaction between a strong acid and a weak base. C. In the adiabatic system studied, the Styrofoam ball is part of the surroundings. D. In an adiabatic system, qrxn = qcal. 13. What is the qrxn for the reaction below if 0.75 g of Na(s) (µ = 23.0 g/mol) reacts with excess H2O? 1 a(s) H2 (l) H a H(aq) 184 J 2 2 (g) A. 6.0 kJ B. -6.0 kJ C. 12 kJ D. -3.0 kJ 14. Which of the following is TRUE about the Ellingham diagram? I. It is a plot of ∆G against T. II. Its slope is equal to -∆S. III. Its slope is equal to ∆S. A. B. C. D.

I only I and II I and III None of the above.

15. In a bomb calorimeter, the measurement is done on what type of condition? A. Isobaric B. Isothermal C. Reversible D. Isochoric IV. Problem Solving. 1. For Questions 1-3, Please refer to the following Ellingham Diagram. The ΔG* values are for the XIDATI 2Ag + O2 --> 2Ag2O) reactions of the compounds whose formula is listed on top of the plots.

(e.g.

A. Complete the statement: A change in the slope of a line in the Diagram indicates a _________. B. Given the following information from Ti2 oxidation: ΔG*500C= -800 KJ/mol, ΔG*750C= -750 KJ/mol, ΔG*1000C= -700 KJ/mol, and ΔG*1250C= -650 KJ/mol calculate ΔS* of the reaction. C. Determine ΔH* for the formation of Ca CHEM 17 (2nd Sem, AY ‘15-‘16)

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D. Determine the oxidation process/es that can drive the decomposition of Al2O3 at (a) 1000:C, (b) 2000:C, and (c) 3000:C.

2. For the reaction A

products, the following data give [A] as a function of time: (General Chemistry 10th Ed.,

Petrucci, Chapter 14 Problem 25)

Time (s) 0 100 200 300 400 600 1000

[A] (M) 0.600 0.497 0.413 0.344 0.285 0.198 0.094

A. Show that the reaction is first-order? B. What is the value of the rate constant, k? C. What is [A] at t = 750 s? 3. Newly explored fuel composed of a mixture of CH4 (g) and C2 H6 (g) only was analyzed using thermodynamics. Upon complete combustion, initially at STP, a 1.00 L sample of gas releases 43.6 kJ. However, since the scientist had no access to reference materials during the experiment, he had to experimentally determine the enthalpies of the 2 gases. 20.00 mL of C2 H6 (g) initially under STP was initially placed under a bomb calorimeter with a J heat capacity of 472.0 ⁄ C . After combustion, it was determined that the temperature rose to 27.89°C. After conducting this experiment, you, being a good friend, gave the following data to the scientist. Coincidentally, it is the data needed to determine the enthalpy of combustion of CH4 (g). C

(g)

1 2

2 (g)

CHEM 17 (2nd Sem, AY ‘15-‘16)

C

2 (g)

∆H° = -283.0 kJ

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CH4 (g) H2 (g)

C 1 2

2C

2 (g)

H2

2 (g)

2H2 (g)

(g)

∆H° = 247.0 kJ ∆H° = -285.8 kJ

(l)

A. Determine the ∆H rxn of the combustion of C2 H6 (g). Express in 4 significant digits. B. Determine the ∆H rxn of the combustion of CH4 (g) . C. Determine the % composition of C2 H6 (g) and CH4 (g) in the fuel, by volume. 4. The following mechanism has been proposed for the gas-phase reaction of nitrogen dioxide and fluorine: Step 1: Step 2: Step 3: A. B. C. D. E.

2 (g)

F2 (g)

2 F2(g)

F(g)

2 F2 (g) 2 F(g)

(fast) (slow) (fast)

F(g) 2 F(g)

2 (g)

What is the overall reaction? What are the intermediates in the mechanism? What is the molecularity of each of the elementary reactions? What is the rate determining step? What is the rate law predicted by the mechanism?

5. For the hypothetical reaction aA [A], M 0.10 0.25 0.10 0.10 0.20

b

[B], M 0.20 0.20 0.10 0.10 0.15

cC

dD

eE, the following data were obtained: [C], M 0.15 0.15 0.10 0.20 0.20

Rate, M*s - 1 5.20 x10-3 3.25 x10-2 2.60 X10-3 3.12 X10-3 1.87 X10-2

Temperature, °C 28 30

A. Write the rate law expression for the reaction. B. Determine the order of the reaction with respect to A, B, and C. What is the overall order of reaction? C. Solve for the rate of reaction when 0.12 M A, 0.15 M B and 0.10 M C is made to react at 30°C.

CHEM 17 (2nd Sem, AY ‘15-‘16)

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