Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 2 Tuesday, April 3, 2007 Page 1 of 6

Exam 2 Name: ______________________________ Recitation Section Lenny.: 6:30 7:30 (circle one): Greg.: 6:30 7:30 Andrew ID: _____________________ Nic.: 6:30 7:30 Please show your work and put your answers in the spaces provided.

1) (20 pts total) Consider the reaction: 2 NO (g) + O2 (g) 2 NO2 (g)

The initial rate of this reaction was measured at 250C for several different concentrations of the reactants. The results are shown below.

experiment [NO] (moles/liter)

[O2] (moles/liter)

initial rate (moles/liter)/sec

I 1.0 x 10-4 1.0 x 10-4 2.8 x 10-6

II 1.0 x 10-4 3.0 x 10-4 8.4 x 10-6 III 2.0 x 10-4 2.0 x 10-4 2.2 x 10-5

a) (5 pts) For experiment I, what is the concentration of NO, [NO], 10 sec into the reaction? (i.e. A mixture is created that initially has [NO] = 1.0 x 10-4 M and [O2] = 1.0 x 10-4 M. Based on the initial rate in the above table, what is [NO] 10 sec after the reaction begins?)

[NO] = ___________________________M b) (7pts) What is the order of the reaction with respect to [O2]?

Order with respect to [O2] = ___________________________

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 2 Tuesday, April 3, 2007 Page 2 of 6

c) (8pts) What is the order of the reaction with respect to [NO]?

Order with respect to [NO] = ___________________________

2) (20 pts total)

Consider the reaction, 2 NO2Cl 2 NO2 + Cl2 Derive the rate law for each of the following proposed mechanisms. Mechanism A: (2pts) NO2Cl NO2 + Cl (slow) NO2Cl + Cl NO2 + Cl2 (fast) Rate = ________________________________ Mechanism B: (5pts) NO2Cl NO2 + Cl (fast equilibrium) NO2Cl + Cl NO2 + Cl2 (slow) Rate = ________________________________

k1 k-1

k2

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 2 Tuesday, April 3, 2007 Page 3 of 6

Mechanism C: (9pts) NO2Cl NO2 + Cl (fast equilibrium) NO2Cl + Cl NO2 + Cl2 (fast) Rate = ________________________________ (2pts) The rate predicted by mechanism C reduces to that predicted by mechanism A when: k1 << k2 k1 << k -1 k -1 << k2 k1 >> k2 k1 >> k -1 k -1 >> k2 (circle one) (2pts) The rate predicted by mechanism C reduces to that predicted by mechanism B when: k1 << k2 k1 << k -1 k -1 << k2 k1 >> k2 k1 >> k -1 k -1 >> k2 (circle one)

k1

k-1

k2

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 2 Tuesday, April 3, 2007 Page 4 of 6

3) (20pts) Consider the following reaction:

2 A + B ⇌ 4 C + 2 D K = 2.4 x 10-12

A solution is prepared by mixing the following solutions together: 25.0 ml of 4.0 M A 25.0 ml of 4.0 M B 25.0 ml of 4.0 M C 25.0 ml of 4.0 M D What are the concentrations of A, B, C and D when the system reaches equilibrium?

[A] = ____________M [B] =_________________M [C]= _______________M [D]= _______________M

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 2 Tuesday, April 3, 2007 Page 5 of 6

4) (10pts) What is the pH resulting from mixing the following two solutions together?

150 ml 0.10M HCl + 50 ml 0.25M NaOH

pH = ___________________________

5) (30 pts total)

Silver chloride is a slightly-soluble salt that dissolves in water according to the equilibrium reaction: AgCl(s) Ag+ (aq) + Cl-

(aq)

Consider a solution containing some AgCl salt in water.

(3 pts) If additional AgCl(s) is added to the solution, the concentration [Ag+]

remains the same increases decreases (circle one)

(3 pts) When the solution is heated, the amount of AgCl(s) decreases. The above reaction is therefore:

endothermic exothermic (circle one)

(3 pts) If the volume of the solution is decreased by evaporating half of the water, the concentration [Ag+]

remains the same increases decreases (circle one)

Chemistry 09-106: Modern Chemistry II Carnegie Mellon University

Exam 2 Tuesday, April 3, 2007 Page 6 of 6

The following reaction is endothermic: CaSO3 (s) CaO (s) + SO2 (g) (3pts) Some solid CaSO3 is placed in a sealed container and allowed to come to equilibrum. If the temperature is increased, what happens to the partial pressure of SO2 (g) in the container? remains the same increases decreases (circle one)

(3pts) If additional SO2 (g) is introduced into the container (keeping the volume fixed), what happens to the amount of solid CaO? remains the same increases decreases (circle one)

(3pts) If the volume of the container is cut in half, what happens to the amount of solid CaO? remains the same increases decreases (circle one)

(3pts) In which gas reaction would a change in pressure have no appreciable effect upon the composition of the equilibrium mixture? ____ H2 (g) + I2 (g) 2HI (g) ____ 2 SO2 (g) + O2 (g) 2SO3 (g) ____ 4 HCl (g)+ O2 (g) 2Cl2 (g) + 2H2O (g) (check one) ____ N2 (g) + 3H2 (g) 2NH3 (g) ____ 2 NO (g) + O2 (g) 2NO2 (g) (5pts) For this problem, consider the following reaction: 2NO (g) + O2 (g) ⇌ 2 NO2 (g) At 750K, the equilibrium constant, K, for the above reaction is 0.5. A sealed vessel at 750K contains NO(g), O2(g) and NO2(g) with partial pressures: PNO = 2.0 atm PO2= 0.5 atm PNO2 = 2.0 atm ____ This system is at equilibrium ____ This system is not at equilibrium, and will shift towards products ( ) to reach equilibrium ____ This system is not at equilibrium, and will shift towards reactants () to reach equilibrium (4pts) Consider the following diagram showing the free energy profile for a reaction: reac prod

The equilibrium constant for this reaction is: ____ less than 1 (check one) ____ greater than 1 ____ we need to know the temperature to determine this

E

reac prod For official use only

1) ____________/20

2) ____________/20

3) ____________/20

4) ____________/10

5) ____________/30

Total ___________/100

G