UCI General Chemistry Peer TutoringChem 1C - Professor Law

https://sites.uci.edu/gcptutoring/law/

Midterm 1 Review Packet ANSWER KEY

Kinetics

1. Consider this balanced chemical equation:

H2O2 (aq) + 3I- (aq) + 2H+ (aq) → I3- (aq) + 2H2O (l)

In the first 10.0 seconds of the reaction, the concentration of I- drops from 1.000M to0.868M.

(a) Calculate the average rate of this reaction in this time interval.1. Write the expression signifying the rate of change of concentration using the reactant I-

𝑟𝑎𝑡𝑒 = − 13

∆[𝐼−]∆𝑡

2. Substitute the equation for average rate of reaction into this rate expression for I-

𝑟𝑎𝑡𝑒 = − 13

∆[𝐼−]𝑓𝑖𝑛𝑎𝑙 −∆[𝐼−]𝑖𝑛𝑖𝑡𝑖𝑎𝑙

𝑡𝑓𝑖𝑛𝑎𝑙

−𝑡𝑖𝑛𝑖𝑡𝑖𝑎𝑙

3. 𝑟𝑎𝑡𝑒 = − 13

(0.868𝑀) −(1.000𝑀)10.0𝑠−0.0𝑠

𝑟𝑎𝑡𝑒 = 4. 40 𝑥 10−3 𝑀/𝑠

(b) Determine the rate of change in the concentration of H+ (that is, ∆[H+]/∆t) duringthis time interval.

1. Write the expression signifying the rate of change of concentration using the reactant H-

𝑟𝑎𝑡𝑒 = − 12

∆[𝐻+]∆𝑡

2. Solve for ∆[𝐻+]∆𝑡

𝑟𝑎𝑡𝑒 = − 12

∆[𝐻+]∆𝑡

− 2(𝑟𝑎𝑡𝑒) = ∆[𝐻+]∆𝑡

− 2(4. 40 𝑥 10−3 𝑀/𝑠) = ∆[𝐻+]∆𝑡

𝑟𝑎𝑡𝑒 = − 8. 80 𝑥 10−3 𝑀/𝑠

2. Consider the equation:CHCl3 (g) + Cl2 (g) → CCl4 (g) + HCl (g)

The initial rate of reaction is measured at several different concentrations of the reactants withthe following results:

[CHCl3] (M) [Cl2] (M) Initial Rate (M/s)

0.010 0.010 0.0035

0.020 0.010 0.0069

0.020 0.020 0.0098

0.040 0.040 0.027

From the data, determine the rate law for the reaction.

3. Consider the combustion of propane:

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)

When O2 is reacting at a rate of 0.3 M/s, at what rate is CO2 being formed?(a) 0.5 M/s(b) 0.18 M/s(c) 0.3 M/s(d) 3 M/s

4. The reaction is first-order in cyclopropene and has a measured rate constant of 3.36 x 10-5 s-1 at720K. If the initial cyclopropane concentration is 0.0445M, what is the cyclopropaneconcentration after 235.0 minutes?

5. A certain first-order reaction has a half-life of 1.2 x 103 s.(a) Calculate the rate constant for this reaction

(b) How much time is required for this reaction to be 75% complete?

6. Consider the reaction between nitrogen dioxide and carbon monoxide:

NO2 (g) + CO (g) → NO (g) + CO2 (g)

The rate constant at 701K is measured as 2.57 M-1s-1, and that at 895K is measured as 567 M-1s-1.Find the activation energy for the reaction in kJ/mol.

7. Which statement most accurately describes the behavior of a catalyst?a. A catalyst increases the ΔG of a reaction and hence the forward rate.b. A catalyst reduces the ΔH of a reaction and hence the temperature needed to

produce products.c. A catalyst reduces the activation energy for a reaction and increases the rate of a

reaction.d. A catalyst increases the equilibrium constant and final product concentrations.

Remember that catalysts affect the kinetic quantities of a reaction, but do not affect thethermodynamic quantities, such as ΔG and ΔH. Catalysts lower the activation energy of areaction, increasing the rate.

8. A proposed mechanism for a reaction is

(slow)𝐶4𝐻

9𝐵𝑟 → 𝐶

4𝐻

9+ + 𝐵𝑟−

(fast)𝐶4𝐻

9+ + 𝐻

2𝑂 →𝐶

4𝐻

9𝑂𝐻

2

+

(fast)𝐶4𝐻

9𝑂𝐻

2

+ +𝐻2𝑂 →𝐶

4𝐻

9𝑂𝐻 + 𝐻

3𝑂+

Write the rate law expected for this mechanism. What is the overall balanced equation forthe reaction? What are the intermediates in the proposed mechanism?

9. Consider the reaction

𝐶𝑙2(𝑎𝑞) + 𝐻

2𝑆 (𝑎𝑞) → 𝑆(𝑠) + 2𝐻+(𝑎𝑞) + 2𝐶𝑙−(𝑎𝑞)

The rate equation for this reaction isRate = k [ ] [ ]𝐶𝑙

2𝐻

2𝑆

Which of these mechanisms is (or are) consistent with this rate equation?

I. (slow)𝐶𝑙2 + 𝐻

2𝑆 → 𝐻+ + 𝐶𝑙− + 𝐶𝑙

++ 𝐻𝑆−

(fast)𝐶𝑙+ + 𝐻𝑆− → 𝐻+ + 𝐶𝑙− + 𝑆

II. ⇌ (fast equilibrium)𝐻2𝑆 𝐻+ + 𝐻𝑆−

+ (slow)𝐶𝑙2

𝐻𝑆−→ 2𝐶𝑙− + 𝐻+ + 𝑆

a. I onlyb. II onlyc. Both I and IId. Neither I or II

10. Nitrogen monoxide can be reduced with hydrogen gas to give nitrogen and water vapor.2NO (g) + (overall equation)2𝐻

2(𝑔) → 𝑁

2(𝑔) + 2𝐻

2𝑂 (𝑔)

A proposed mechanism is

What rate law is predicted by this mechanism?

11. Identify the molecularity of each of the following elementary reactions.e. NO + 𝑂

3→ 𝑁𝑂

2+ 𝑂

2

f. 𝑁𝑂𝐶𝑙2

+ 𝑁𝑂 → 2𝑁𝑂𝐶𝑙

g. 𝑂3

→ 𝑂2

+ 𝑂

h. H+H+𝑁2

→ 𝐻2

+ 𝑁2

Equilibrium

12. Write the expression for the equilibrium constant Kc for each of the following equations:a. C(s) + (g) ⇌ 2CO(g)𝐶𝑂

2

b. FeO(s) + CO(g) ⇌ Fe(s) + (g)𝐶𝑂2

c. (s) + + ⇌ (g)𝑁𝑎2𝐶𝑂

3𝑆𝑂

2(𝑔) 1

2 𝑂2(𝑔) 𝑁𝑎

2𝑆𝑂

4 (𝑠) +𝐶𝑂

2

d. (s) ⇌ (aq) + 2 (aq)𝑃𝑏𝐼2

𝑃𝑏2+ 𝐼−

13. A mixture containing nitrogen, hydrogen, and iodine established the following equilibrium at400℃:

2 ⇌𝑁𝐻3(𝑔) + 3𝐼

2(𝑔) 𝑁

2(𝑔) + 6𝐻𝐼(𝑔)

Use the information below to calculate Kc for this reaction.

+ 3 ⇌ 2 Kc1 = 0.50 at 400℃𝑁2(𝑔) 𝐻

2(𝑔) 𝑁𝐻

3(𝑔)

+ ⇌ Kc2 = 50 at 400℃𝐻2(𝑔) 𝐼

2(𝑔) 2𝐻𝐼(𝑔)