AP Chemistry Unit 4 Test Review

Topics Covered: Thermochemistry, Spontaneity, Entropy, Free Energy, & Kinetics

Section I: Multiple Choice (~50%) Questions 1-5: The set of lettered choices is a list of symbols used in thermodynamics and thermochemistry. Select the one lettered choice that best fits each statement. A choice maybe used more than once or not at all.

a. Ea b. ΔS c. Lattice Energy d. ΔG e. ΔH

1. Its value is negative for any exothermic reaction. 2. Its value is negative for any spontaneous reaction. 3. Its value changes when a catalyst is added to the system 4. Its value represents the change in randomness as the reaction proceeds. 5. Its value is higher in ionic bonds when compared to covalent.

6. For which of the following is ΔH most nearly equal to ΔE? a. CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (l) b. N2 (g) + 3H2 (g)  2NH3 (g) c. H2 (g) + I2 (g)  2HI d. H2 (g) + ½ O2 (g)  H2O (g)

7. Which applies to any reaction that proceeds spontaneously to form products from initial standard conditions? I. Keq > 1 (the equilbrium constant here represents that products are favored) II. ΔH < 0 III. ΔG < 0 a. I only b. II only c. I and II only d. II and III only e. I, II, and III

8. Is a gas in its standard state at 298 K

a. Lithium b. Fluorine c. Bromine d. Uranium e. Nickel

9. A cube of ice is added to some hot water in a rigid, Energy Entrop. insulated container, which is then sealed. There is no (A) Remains constant Remains constant heat exchange with the surroundings. What has (B) Remains constant Decreases happened to the total energy and the total Entrop. when (C) Remains constant Increases the system reaches equilibrium? (D) Decreases Increases (E) Increases Decreases 12. When solid ammonium chloride, NH4Cl(s) is ΔH ΔS added to water at 25 °C, it dissolves and the A Postive Positive temperature of the solution decreases. Which of the B) Positive Negative following is true for the values of ΔH and ΔS for the C) Positive Equal to zero dissolving process? D Negative Positive E) Negative Negative 13. N2(g) + 3 H2(g) ---> 2 NH3(g) The reaction indicated above is thermodynamically spontaneous at 298 K, but becomes nonspontaneous at higher temperatures. Which of the following is true at 298 K? (A) ΔG, ΔH, and ΔS are all positive. (B) ΔG, ΔH, and ΔS are all negative. (C) ΔG and ΔH are negative, but ΔS is positive. (D) ΔG and ΔS are negative, but ΔH is positive. (E) ΔG and ΔH are positive, but ΔS is negative.

14. 2 A(g) + B(g) <===> 2 C(g) When the concentration of substance B in the reaction above is doubled, all other factors being held constant, it is found that the rate of the reaction remains unchanged. The most probable explanation for this observation is that

(A) the order of the reaction with respect to substance B is 1 (B) substance B is not involved in any of the steps in the mechanism of the reaction (C) substance B is not involved in the rate-determining step of the mechanism, but is involved in subsequent steps (D) substance B is probably a catalyst, and as such, its effect on the rate of the reaction does not depend on its concentration (E) the reactant with the smallest coefficient in the balanced equation Genlly has little or no effecton the rate of the reaction

15. rate = k[X] For the reaction whose rate law is given above, a plot of which of the following is a straight line? (A) [X] versus time (B) log [X] versus time (C) 1/[X] versus time (D) [X] versus 1/time (E) log [X] versus 1/time

Initial Initial Rate of Initial [O ] 16. The initial-rate data in the table above were Experiment [NO] 2 Formation of NO (mol L¯1 2 obtained for the reaction represented below. (mol L¯1 (mol L¯1 s¯1) What is the experimental rate la for the reaction? 1 0.10 0.10 2.5 x 10¯4 (A) rate = k[NO] [O2] (B) rate = k[NO] [O2]2 2 0.20 0.10 5.0 x 10¯4 (C) rate = k[NO]2 [O2] 3 0.20 0.40 8.0 x 10¯3 (D) rate = k[NO]2 [O2]2 (E) rate = k[NO] / [O2]

17. Which of the following best helps to explain why the value ΔHo for the dissolving of NaCl in water is positive? (A) NaCl (s) is insoluble in water (B) NaCl (s) dissolves in water to form NaCl (aq) particles (C) Na+1 ions have very strong ion-ion interaction with Cl-1 ions in the crystal lattice. (D) Na+1 ions have very strong ion-dipole interactions with water molecules in the solution.

18. Which of the following equations represents a reaction for which the standard entropy change is negative? (A) 2NO2 (g)  N2O4 (g) (B) 2H2O2 (aq)  2H2O (l) + O2 (g) (C) 2KClO3 (s)  KCl (s) + 3O2 (g) (D) 2HgO (s)  2Hg (l) + O2 (g)

19. A 10. g cube of copper at a temperature, T1 is placed in an insulated cup containing 10. g of liquid mercury at T2. If T2 > T1, which of the following is true of the system when it has attained thermal equilibrium? (the specific heat of copper is 0.385 J/g oC and the specific heat of mercury is 0.14 J/g oC) (A) The temperature of the mercury changed more than the temperature of the copper. (B) The temperature of the copper changed more than the temperature of the mercury. (C) The temperature of the mercury and the copper changed by the same amount. (D) The relative temperature changes of the copper and the mercury cannot be determined without knowing T1 and T2.

N2O4 (g)  2NO2 (g) ΔH > 0 20. Which of the following statements is true about bond energies in the above reaction? (A)The energy absorbed as the bonds in the reactants are broken is greater than the energy released as the bonds in the product formed. (B) The energy released as the bonds in the reactants are broken is greater than the energy absorbed as the bonds in the product are formed. (C) The energy absorbed as the bonds in the reactants are broken is less than the energy released as the bonds in the product are formed. (D)The energy released as the bonds in the reactants are broken is less than the energy absorbed as the bonds in the product are formed.

X2 + Y2  X2Y2 rate = k [X2] 21. A reaction and its experimentally determined rate law are represented above. A chemist proposes two different possible mechanisms for the reaction, which are given below:

Mechanism 1 Mechanism 2 X2  2X (slow) X2  2X (slow) X + Y2  XY2 (fast) X + Y2  XY + Y (fast) X + XY  X2Y2 (fast) X + XY  X2Y (fast) X2Y + Y  X2Y2 (fast)

Based on the information above, which of the following is true? (A)Only mechanism 1 is consistent with the rate law and valid. (B) Only mechanism 2 is consistent with the rate law and valid. (C) Both mechanism 1 and mechanism 2 are consistent with the rate law and valid (D)Neither mechanism 1 nor mechanism 2 is consistent with the rate law nor valid. - + 2+ 5H2O2(aq) + 2MnO4 (aq) + 6H (aq)  2Mn (aq) + 8H2O (l) + 5O2 (g)

- In a titration experiment, H2O2 (aq) reacts with aqueous MnO4 (aq) as represented by the equation above. The dark purple KMNO4 solution is added from a buret to a colorless, acidified solution of H2O2 (aq) in an Erlenmeyer flask. (Note: At the end point of the titration, the solution is a pale pink color.)

-1 -3 22. At a certain time during the experiment, the rate of disappearance of MnO4 (aq) was 2.0 x 10 mol/L s. What was the rate of appearance of O2 at the same time? (A) 2.5 x 10-3 mol/L s (B) 5.0 x 10-3 mol/L s (C) 2.0 x 10-3 mol/L s (D) 1.6 x 10-3 mol/L s

23. Consider the reaction represented by the equation 2B + 2A  B2A2. During a reaction in which a large excess of reactant B was present, the concentration of reaction A was monitored over time. A plot of the natural logarithm of the concentration of A versus time is shown in the figure above. The order of the reaction with respect to reactant A is (A) zero order (B) first order (C) second order (D) third order

NO (g) + NO3 (g)  2NO2 (g) rate = k[NO][NO3] 24. The reaction represented above occurs in a single step that involves a collision between a particle of NO and a particle of NO3. A scientist correctly calculates the rate of collisions between NO and NO3 that have sufficient energy to overcome the activation energy. The observed reaction rate is only a small fraction of the calculated collision rate. Which of the following best explains the discrepancy? (A) The energy of collisions between two reactant particles is frequently absorbed by collision with a third particle. (B) The two reactant particles must collide with a particular orientation in order to react. (C) The activation energy for a reaction is dependent on the concentrations of the reactant particles. (D) The activation energy for a reaction is dependent on temperature. Section II: Free Response (~50%)

Long Response: Calculator may be used. CLEARLY SHOW THE METHOD USED AND STEPS INVOLVED IN ARRIVING AT YOUR ANSWERS. It is to your advantage to do this, because you may earn partial credit if you do and you will receive little or no credit if you do not. Attention should be paid to significant figures. Be sure to write all your answers to the questions on the lined pages following each question in this booklet.

1. C6H5OH(s) + 7 O2(g)  6 CO2(g) + 3 H2O(l)

When a 2.000-gram sample of pure phenol, C6H5OH(s), is completely burned according to the equation above, 64.98 kilojoules of heat is released. Use the information in the table below to answer the questions that follow. Standard Heat of Absolute Entropy,

Formation, Hf; S, at 25C (J/mol Substance at 25C (kJ/mol) K) C(graphite) 0.00 5.69

CO2(g) -393.5 213.6

H2(g) 0.00 130.6

H2O(l) -285.85 69.91

O2(g) 0.00 205.0

C6H5OH(s) ? 144.0

(a) Calculate the molar heat of combustion of phenol in kilojoules per mole at 25C.

(b) Calculate the standard heat of formation, Hf, of phenol in kilojoules per mole at 25C.

(c) Calculate the value of the standard free-energy change, G, for the combustion of phenol at 25C.

(d) If the volume of the combustion container is 10.0 liters, calculate the final pressure in the container when the temperature is changed to 110.C. (Assume no oxygen remains unreacted and that all products are gaseous.) 2. C2H2(g) + 2 H2(g)  C2H6(g) Information about the substances involved in the reaction represented above is summarized in the following tables.

Substance S (J/molK) Hf (kJ/mol)

C2H2(g) 200.9 226.7

H2(g) 130.7 0

C2H6(g) - - - - -84.7

Bond Bond Energy (kJ/mol) C-C 347 C=C 611 C-H 414 H-H 436

(a) If the value of the standard entropy change, ΔS, for the reaction is -232.7 joules per moleKelvin, calculate the standard molar entropy, S, of C2H6 gas.

(b) Calculate the value of the standard free-energy change, ΔG, for the reaction. What does the sign of G indicate about the reaction above?

(d) Calculate the value of the CC bond energy in C2H2 in kilojoules per mole. 3. Nitrogen monoxide gas, a product of the reaction above, can react with oxygen to produce nitrogen dioxide gas, as represented below.

2 NO(g) + O2(g)  2 NO2(g)

Initial Concentration of Initial Concentration of O2 Initial Rate of Formation Experiment -1 -1 -1 -l NO (mol L ) (mol L ) of NO2 (mol L s )

1 0.0200 0.0300 8.52  10-2 2 0.0200 0.0900 2.56  10-1 3 0.0600 0.0300 7.67  10-1

(d) Determine the order of the reaction with respect to each of the following reactants. Give details of your reasoning, clearly explaining or showing how you arrived at your answers. (i) NO

(ii) O2

(e) Write the expression for the rate law for the reaction as determined from the experimental data.

(f) Determine the value of the rate constant for the reaction, clearly indicating the units.

4. The following mechanism has been proposed for the reaction of methane gas with chlorine gas. All species are in the gas phase.

Step 1 Cl2  2 Cl fast equilibrium

Step 2 CH4 + Cl  CH3 + HCl slow

Step 3 CH3 + Cl2  CH3Cl + Cl fast

Step 4 CH3Cl + Cl  CH2Cl2 + H fast Step 5 H + Cl  HCl fast

(d) In the mechanism, is CH3Cl a catalyst, or is it an intermediate? Justify your answer.

(e) Identify the order of the reaction with respect to the CH4(g), according to the mechanism. Justify your answer. Unit 4 Equations Practice: (a) A 0.1M solution of potassium iodide is mixed with a 0.1M solution of lead II nitrate. (i) Balanced equation:

(ii) What evidence of a reaction will be observed? Explain.

(b) Sulfur dioxide is bubbled into water. (i) Balanced equation:

(ii) Will the resulting product have a pH < 7, pH > 7, or pH = 7? Explain.

(c) The complete combustion of hydrogen. (i) Balanced equation:

(ii) Is this a spontaneous reaction? Explain.

2. BCl3(g) + NH3(g)  Cl3BNH3(s) The reaction represented above is a reversible reaction. (a) Predict the sign of the entropy change, S, as the reaction proceeds to the right. Explain your prediction.

(b) If the reaction spontaneously proceeds to the right, predict the sign of the enthalpy change, H. Explain your prediction.

(c) The direction in which the reaction spontaneously proceeds changes as the temperature is increased above a specific temperature. Explain.