PART 1: MULTIPLE CHOICE. (60 points) Questions 1 - 20 are multiple choice, worth 3 points each. Mark the letter corresponding to the one correct answer on your scantron sheet.

1) The decomposition of dinitrogen pentoxide is described by the chemical equation

2 N₂O₅(g) ® 4 NO₂(g) + O₂(g)

If the rate of disappearance of N₂O₅ is equal to 1.40 mol/min at a particular moment, what is the rate of appearance of NO₂ at that moment?

A) 0.700 mol/min

B) 2.80 mol/min

C) 1.40 mol/min

D) 5.60 mol/min

2) For a reaction that follows the general rate law, Rate = k[A][B]², what will happen to the rate of reaction if the concentration of B is increased by a factor of 2.00? The rate will

A) decrease by a factor of 1/2.00.

B) decrease by a factor of 1/4.00.

C) increase by a factor of 2.00.

D) increase by a factor of 4.00.

3) What is the overall reaction order for the reaction that has the rate law: Rate = k[H₂][NO]²?

A) zero order

B) first order

C) second order

D) third order

4) The following set of data was obtained by the method of initial rates for the reaction:

2 HgCl₂(aq) + C₂O₄^2-(aq) ® 2 Cl^-(aq) + 2 CO₂(g) + Hg₂Cl₂(s)

What is the rate law for the reaction?

[HgCl2], M	[C2O4^2-], M	Rate, M/s
0.10	0.10	1.3 x 10^-7
0.10	0.20	5.2 x 10^-7
0.20	0.20	1.0 x 10^-6

A) Rate = k[HgCl₂]²[C₂O₄^2-]

B) Rate = k[HgCl₂][C₂O₄^2-]^-1

C) Rate = k[HgCl₂][C₂O₄^2-]²

D) Rate = k[HgCl₂][C₂O₄^2-]^-2

5) The first-order reaction, SO₂Cl₂ ® SO₂ + Cl₂, has a half-life of 8.75 hours at 593 K. How long will it take for the concentration of SO₂Cl₂ to fall to 12.5% of its initial value?

A) 6.06 hr

B) 3.22 hr

C) 0.165 hr

D) 26.2 hr

6) A plot of 1/[BrO^-] vs time is linear for the reaction:

3 BrO^-(aq) ®BrO₃^-(aq) + 2 Br^-(aq)

What is the order of the reaction with respect to the hypobromite ion, BrO^-?

A) 0

B) 1

C) 2

D) 3

7) The elementary reaction: 2 HI ® H₂ + I₂, is an example of a ________ reaction.

A) termolecular

B) bimolecular

C) tetramolecular

D) unimolecular

8) A mechanism for a naturally occurring reaction that destroys ozone is:

Step 1: O₃(g) + HO(g) ® HO₂(g) + O₂(g)

Step 2: HO₂(g) + O(g) ® HO(g) + O₂(g)

Which species is an intermediate?

A) O

B) HO

C) HO₂

D) O₃

9) When the concentration of A is doubled, the rate for the reaction: 2 A + B ® 2 C quadruples.

When the concentration of B is doubled the rate remains the same. Which mechanism below is consistent with the experimental observations?

A) Step 1: 2 A = D (fast equilibrium)

Step 2: B + D ® E (slow)

Step 3: E ® 2 C (fast)

B) Step 1: A + B ®D (slow)

Step 2: A + D ® 2 C (fast equilibrium)

C) Step 1: 2 A ® D (slow)

Step 2: B + D ® E (fast)

Step 3: E ® 2 C (fast)

D) Step 1: A + B = D (fast equilibrium)

Step 2: A + D ® 2 C (slow)

10) What factor affects the rate of a chemical reaction?

A) collision frequency

B) orientation of molecules

C) fraction of collisions with sufficient energy

D) all of the above

11) The decomposition of hydrogen peroxide is given by the following reaction:

2 H₂O₂(aq) ® 2 H₂O(l) + O₂(g)

In the presence of KI the reaction is thought to occur by the following mechanism:

Step 1: H₂O₂ + I^- ® H₂O + IO^-

Step 2: IO^- + H₂O₂ ® H₂O + O₂ + I^-

What is the role of I^- in this mechanism?

A) transition state

B) intermediate

C) frequency factor

D) catalyst

Shown is a concentration versus time plot for a reaction involving gases A, B, and C.

12) Which equation best represents the reaction?

A) 2C(g) + B(g) ® 4A(g)

B) 4A(g) ® B(g) + 2C(g)

C) 2C(g) ® 4A(g) + B(g)

D) 4A(g) + B(g) ® 2C(g)

13) Which of the following statements is false regarding the equilibrium constant, K_c?

A) The numerical value of K_c depends on the form of the balanced equation.

B) K_c for a reaction at a particular temperature always has the same value.

C) K_c for the reverse reaction is the negative of K_c for the forward reaction.

D) When quoting K_c it is customary to omit units.

14) Given the reaction at a certain temperature: 2 HI(g) = H₂(g) + I₂(g). At equilibrium, the partial pressure of HI is 1.8 x 10^-3 atm, and the partial pressures for H₂ and I₂ are 0.10 atm each. Find K_p at that temperature.

A) 3.1 x 10³

B) 3.1 x 10⁴

C) 3.2 x 10^-4

D) 5.6 x 10¹

15) What is the equilibrium constant, K_c, for the reaction: 2 Hg(l) + O₂(g) = 2 HgO(s) if the amounts of reactants and products at equilibrium are: 1.00 g of HgO(s), 2.00 mL of Hg(l), and 1.60 M O₂(g)? (The molar mass and density of HgO(s) is 217 g/mol, and 1.10 g/cm³. The molar mass and density of Hg(l) is 201 g/mol, and 13.6 g/mL.)

A) 6.85 x 10^-4

B) 0.625

C) 1.46 x 10³

D) 4.66 x 10^-2

16) Which statement is true for a reaction with K_c equal to 2.43 x 10^-12?

A) The reaction proceeds nearly all the way to completion.

B) The reaction proceeds hardly at all towards completion.

C) There are appreciable concentrations of both reactants and products.

D) Increasing the temperature will not change the value of K_c.

17) The dissolution of calcium hydroxide is exothermic.

Ca(OH)₂(s) = Ca²⁺(aq) + 2 OH^-(aq) What happens when the solution of Ca(OH)₂ is heated?

A) The amount of Ca(OH)₂(s) increases.

B) The amount of Ca(OH)₂(s) remains unchanged.

C) The amount of Ca(OH)₂(s) decreases.

D) The Ca(OH)₂(s) completely dissolves.

18) Which change in the system will drive equilibrium to the left in the reaction below?

N₂O₅(g) = NO₂(g) + NO₃(g)

A) increase the amount of N₂O₅

B) increase the volume

C) increase the pressure

D) decrease the amount of NO₃

19) Shown below is a concentration vs. time plot for the reaction A = B. For this reaction the value of the equilibrium constant is

A) K_c < 1.

B) K_c > 1.

C) K_c = 1.

D) K_c = 0.

20) A catalyst increases the rate of a chemical reaction by providing a lower-energy mechanism for the reaction. When this occurs, which one of the following is not affected?

A) equilibrium constant

B) activation energy for the forward reaction

C) rate of the reverse reaction

D) activation energy for the reverse reaction

PART 2: Problems (21 -23) You must show all of your work clearly in the following problems to obtain full credit.

21) (13) Hydrogen iodide decomposes according to the second order reaction:

2HI(g) ® H₂(g) + I₂(g). the rate constant is 5.133 x 10^-4 M^-1 s^-1 at 410^oC. If the initial concentration of HI is 0.500 M, how long will it take for its concentration to decrease to 0.100 M?

22) (13) For the reaction 2 NO₂(g) ® 2 NO(g) + O₂(g) the rate constants are 0.77 M^-1 s^-1 at 603 K and 4 .1 M^-1 s^-1 at 651 K. What is the activation energy for this reaction in kJ/mol?

23) (14) The air pollutant NO is produced in automobile engines from the high temperature reaction: N₂(g) + O₂(g) = 2 NO(g); K_c = 1.7 x 10^-3 at 2300 K. If the initial concentrations are: [N₂] = 0.200 M, [O₂] = 0.200 M, and [NO] = 5.0 x 10^-3 M,

a) in which direction will the reaction proceed to reach equilibrium?

b) what are the equilibrium concentrations of N₂, O₂, and NO(g)?

c) What is the value of K_p at 2300 K?