CHM 152 Practice test#2

 

Conceptual questions:

 

1. A rate equation for a chemical reaction was determined to Rate = k [A]2.

How will the reaction rate change if

(a)     the concentration is tripled?

(b)     the concentration of [A] is halved?

 

2. Name four factors that influence the rate of a chemical reaction.

 

3. Of the four factors in question #2 choose one factor and explain how and why it affects the rate of a chemical reaction.

 

4. For a reaction with a given activation energy (Ea), how does an increase in temperature (T) affect the rate?

 

5. For a reaction with a given T, how does a decrease in Ea affect the reaction rate?

 

6. Explain how a catalyst works. Include a diagram of an energy profile.

 

Numerical questions:

 

1. Phosgene is a toxic gas prepared by the reaction of carbon monoxide with chlorine.

CO(g) + Cl2(g) COCl2(g)

 

The following data were obtained in a kinetic study of its formation:

Trial

Initial [CO]

(mol/L)

Initial [Cl2]

(mol/L)

Initial rate

(mol/L s)

1

1.00

0.100

1.29x10-29

2

0.100

0.100

1.33x10-30

3

0.100

1.00

1.30x10-29

4

0.100

0.0100

1.32x10-31

 

(a)    What is the order of the reaction with respect to carbon monoxide? First order

(b)     What is the order of the reaction with respect to chlorine? First order

(c)    What is the rate law for the reaction? Rate = k [CO] [Cl2]

(d) Determine the rate constant k. (1.28 x 10-28 L/mol s)

 

2. A biochemist studying the breakdown in soil of the insecticide "ABC" finds that is decays by a first order reaction. The rate constant k for the reaction 3.45x10-6 s-1.

(a) Calculate the half-life of the decomposition reaction: 2.0 x 105 s

(b) How long ( in seconds and hours) does it take for the insecticide to drop to one-tenth of its initial value? 6.6x105 s

 

For the following questions provide a graph, indicate clearly on the graph how you determined your values, and write the final answers in the spaces on the testing sheets.

 

3. For the decomposition reaction of the reactant "X" the following data were collected:

time [h] conc X[mol/L]

15 0.835

30 0.680

80 0.350

120 0.220

(a) Draw a diagram concentration versus time for the reaction (on graphing paper):

(b) From your graph determine the average rate for the overall reaction between 0 and 80 hours

(c) From your graph determine the instantaneous rate at t= 40h.

(d) From your graph estimate the initial rate.

(e) Graphically determine the order of the reaction and state which graph yields a straight line. First order

(f) What is the rate law for the reaction? Rate = k [X]

(g) What is the rate constant k? 0.0128 L/mol s

 

 

5. Using the experimental data shown below, determine the activation energy Ea for the raction

2 AB 2 A + 2 B

Temperature k 1/T ln k

K [L/mol s] K-1

288 0.0521 ______ _____

298 0.101 ______ _____

308 0184 ______ _____

318 0.332 ______ _____

 

a. Complete the table above.

b. Draw an appropriate diagram and determine the activation energy from the slope of the graph. (slope = -5638.3 K, Ea = 46.8 kJ)

 

6. Calculate the activation energy, Ea, for the reaction

N2O5(g) 2 NO2 (g) + 1/2 O2(g)

From the observed rate constants:

k at 25oC = 3.46x10-5 s-1, and k at 55oC = 1.5 x 10-3 s-1.(59.9 kJ)