chem 17 le 1 samplex

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CHEM 17 (2nd Sem, AY ‘15-‘16) UP ACME | of5

University of the Philippines Assoc iatio n of Chemistry Majors and E nthusiasts (UP ACME)

SAMPLE FIRST LONG EXAM (CHEM 17)

Second Semester, AY 2015-2016

. Modified True or False. Write TRUE if the statement is always correct. If the statement is false, CHANGE theunderlined word/s to make the statement correct.

_________________1. Presence of catalysts speeds up the rate and increases the yield of reactions._________________2. In determining rate laws using transition state theory, the net change of concentration of

intermediates is equal to zero._________________3. The green-colored cobalt (III) ion hastens the oxidation of tartrate by hydrogen peroxide._________________4. A reaction mechanism is a step-by-step description of a chemical reaction consisting of a

series of elementary processes.

_________________5. In the Ellingham Diagram, the slope of the curve obtained by plotting ΔG versus T isequal to the ΔS of the system.

_________________6. A second-order bimolecular reaction can be reduced to pseudo-first order reaction if theconcentration of one reactant is very high and remains essentially constant.

_________________7. Transition states are species that do not appear in the overall chemical reaction and

overall rate law, but can be isolated under certain conditions.

_________________8. If heat is released by the system and work is done by the system, the process has apositive ΔU.

_________________9. If a reaction is said to be spontaneous at all temperatures, then its enthalpy and entropyare always negative.

_________________10. A spontaneous reaction that is strongly endothermic is enthalpy-driven.

I. Cause and Effect. Predict the effect (INCREASE, DECREASE, NO EFFECT, INDETERMINATE) of the given conditionin Column A on the indicated parameter in Column B.

Column A (CONDITION) Column B (PARAMETER) EFFECT

1. The cork with thermometer was notcompletely inserted in the test tube.

Magnitude of measuredqcalorimeter

2.

In the calibration of the calorimeter,the HCl solution was not diluted tomark.

∆Hrxn of water

3. The cork stopper was placed a minuteafter the reaction took place.

Experimental ∆Hrxn

4. The second dry ice slab was not placedimmediately after ignition of the Mgribbon.

Amount of graphite formed

5. Several minutes were needed beforethe Mg ribbon could be ignited and

burned completely.

∆G of formation of magnesiumoxide

6. Cu2+ was added to the solution

containing permanganate and oxalate.

Rate of reaction

7. Water used to prepare the KI solutionwas found to be contaminated withCu2+ ions.

Rate of reaction

8. Excess HCl was added to thethiosulfate solution.

Rate constant

9. The cork with thermometer was notcompletely inserted in the test tube.

Magnitude of measuredqcalorimeter




II. 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 sB. 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 PressureB.

Heating at Constant VolumeC. Isothermal Phase ChangeD. 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 workdone on it?

A. +2.42 kJ C. -2.42 kJB. -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 therate of reaction increase as the temperature rises from 500C to 550C?

A.

11.31 C. 13.33B. 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 reactionsB. First-order reactionsC. Second-order reactionsD. 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 C. III onlyB. II and III 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?

(CZn = 6.06 calmolK⁄ ; MWZn = 65.37

gmol⁄ )

A. 28°C

B. 42°CC. 30°CD. 53°C

For items 8, 9 and 10: Given the following data for the reaction: NH3(g) + HCl(g) NH4Cl

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

8. Determine the ∆Hrxn for the reaction using the reaction using the given values.A. +176 kJ C. -452.8 kJB. -176 kJ D. +452.8 kJ




9. What is the value of the ∆Srxn for the reactionA. +6.481 J mol-1 C. -284.77 J mol-1 B. -6.481 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 25C 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?

a(s) H2(l) 12

H2(g) aH(aq) 184 J

A. 6.0 kJB. -6.0 kJ

C. 12 kJD. -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. I onlyB. I and II

C. I and IIID. None of the above.

15. In a bomb calorimeter, the measurement is done on what type of condition?A. Isobaric

B. IsothermalC. ReversibleD. Isochoric

V. Problem Solving.

1. For Questions 1-3, Please refer to the following Ellingham Diagram. The ΔG* values are for the XIDATI (e.g2Ag + O2 --> 2Ag2O) reactions of the compounds whose formula is listed on top of the plots.

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




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

2. For the reaction A products, the following data give [A] as a function of time: (General Chemistry 10 th Ed.Petrucci, Chapter 14 Problem 25)

Time (s) [A] (M)

0 0.600

100 0.497

200 0.413300 0.344400 0.285

600 0.198

1000 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 C2H6(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 scientisthad no access to reference materials during the experiment, he had to experimentally determine the enthalpiesof the 2 gases. 20.00 mL of C2H6(g) initially under STP was initially placed under a bomb calorimeter with a

heat capacity of 472.0J

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, itis the data needed to determine the enthalpy of combustion of CH4(g).

C(g) 12

2(g) C2(g) ∆H° = -283.0 kJ




CH4(g) C2(g) 2C(g) 2H2(g) ∆H° = 247.0 kJ

H2(g) 12

2(g) H2(l) ∆H° = -285.8 kJ

A. Determine the ∆Hrxn of the combustion of C2H6(g). Express in 4 significant digits.

B. Determine the ∆Hrxn of the combustion of CH4(g).

C. Determine the % composition of C2H6(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: 2(g) F2(g) 2F2(g) (fast)

Step 2: 2F2(g) 2F(g) F(g) (slow)

Step 3: F(g) 2(g) 2F(g) (fast)

A. What is the overall reaction?B. What are the intermediates in the mechanism?

C. What is the molecularity of each of the elementary reactions?D. What is the rate determining step?E. What is the rate law predicted by the mechanism?

5. For the hypothetical reaction aA b cC dD eE , the following data were obtained:

[A], M [B], M [C], M Rate, M*s

-1

Temperature, °C

0.10 0.20 0.15 5.20 x10-3

280.25 0.20 0.15 3.25 x10-2

0.10 0.10 0.10 2.60 X10-3

0.10 0.10 0.20 3.12 X10-3

300.20 0.15 0.20 1.87 X10-2

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 le 1 samplex

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