CHAPTERS 14 AND 15

CHAPTER 14Reaction Rates

Factors That Affect Reaction Rates

Instantaneous Rate

Reaction Rates and Stoichiometry

How is the rate at which ozone disappears related to the rate at which oxygen appears in the reaction 2 O3(g) 3 O2(g)?

If the rate at which O2 appears is 6.0 x 10-

5 M/s, at a particular instant, at what rate is O3 disappearing at this time?

The Effect of Concentration on Rate

NH4+(aq) + NO2

-(aq) N2(g) + 2 H2O(l)

Experiment

Number

Initial [NH4

+]Initial [NO2

-]Observed

Rate

1 0.010 0.20 5.4 x 10-7

2 0.020 0.20 10.8 x 10-7

3 0.040 0.20 21.5 x 10-7

4 0.20 0.0202 10.8 x 10-7

5 0.20 0.0404 21.6 x 10-7

6 0.20 0.0808 43.3 x 10-7

Using Initial Rates to Determine Rate Laws

Experiment Number

[A] (M) [B] (M) Initial Rate (M/s)

1 0.100 0.100 4.0 x 10-5

2 0.100 0.200 4.0 x 10-5

3 0.200 0.100 16.0 x 10-5

First Order Reactions

The decomposition of a certain insecticide in water follows first-order kinetics with a rate constant of 1.45 yr-1 at 12oC. A quantity of this insecticide is washed into a lake on June 1, leading to a concentration of 5.0 x 10-7 g/cm3. Assume that the average temperature of the lake is 12oC. (a) What is the concentration of the insecticide on June 1 of the following year? (b) How long will it take for the concentration of the insecticide to decrease to 3.0 x 10-7 g/cm3?

Second Order Reactions

2nd Order Reactions vs 1st Order

0 100200300400

-6

-5

-4

-3

-2

-1

0

ln[No2] vs Time

Y-Value 1

010

020

030

040

00

50

100

150

200

250

300

1/[NO2] vs Time

Y-Value 1

Half Life

Temperature and Rate

Determination of Activation Energy

Temp (K)

k(s-1) 1/T lnk

462.9 2.52 x 10-5

2.16 x 10-3

-10.589

472.1 5.25 x 10-5

2.11 x 10-3

-9.855

503.5 6.30 x 10-4

1.986 x 10-3

-7.370

524.4 3.16 x 10-3

1.907 x 10-3

-5.757

1.80

E-03

2.00

E-03

2.20

E-03

-12

-10

-8

-6

-4

-2

0

f(x) = − 19079.703 x + 30.582313

lnk vs 1/T

Y-Value 1Linear (Y-Value 1)

Reaction Mechanism

Example Problems

The reaction between nitrogen monoxide and oxygen is shown below.

2 NO(g) + O2(g) 2 NO2(g)

One proposed mechanism is the following:Step I: NO(g) + O2(g) NO2(g) + O(g) (Slow)

Step II: NO(g) + O(g) NO2(g) (Fast)

Which of the following rate expressions agrees best with this possible mechanism?

a) Rate = k[NO][O]b) Rate = k[NO][O2]

c) Rate = k[NO]2[O2]

d) Rate = k[NO]/[O2]

e) Rate = k[NO]/[O]

C6H6(l) + 15/2 O2(g) 6 CO2(g) + 3 H2O(l)

The above reaction represents the combustion of benzene. If a sample of benzene is burning at 0.25 mol/L*s, which of the following is the rate at which CO2(g) is being produced.

a) 0.25 mol/L*sb) 0.75 mol/L*sc) 1.5 mol/L*sd) 2.5 mol/L*se) 3.0 mol/L*s

2 N2O5(g) 4 NO2(g) + O2(g)

A sample of N2O5 was placed in an evacuated container, and the reaction represented above occurred. The partial pressure of N2O5(g), was measured during the reaction and recorded in the table.

Which of the following correctly describes the reaction?

a) Zero-order reactionb) 1st order reactionc) 2nd order reactiond) 3rd order reaction

T (min) PN2O5 ln(PN2O5

)1/PN2O5

0 150 5.0 0.0067

100 75 4.3 0.013

200 38 3.6 0.027

300 19 2.9 0.053

CHAPTER 15Equilibrium

Equilibrium

N2O4(g) 2 NO2

Experiment

Initial [N2O4]

Initial [NO2]

Equilibrium [N2O4]

Equilibrium [NO2]

kc

1 0.0 0.02 0.0014 0.0172 0.211

2 0.0 0.03 0.0028 0.0243 0.211

3 0.0 0.04 0.00452 0.031 0.211

4 0.02 0.0 0.00452 0.031 0.211

Equilibrium Constants and Pressure

The synthesis of ammonia from hydrogen and nitrogen has a kc of 9.6 at 300oC. Calculate kp for this reaction at this temperature

Using Equilibrium Constants

Heterogeneous Equalibria

Calculating the Value of kc

Applications of Equilibrium Constants

Le Chatelier’s Principle

Effects of Volume and Pressure Changes

Effect of Temperature Change

Example Questions

Iodine and bromine are combined at high pressure and high temperature and react as follows:

I2 + Br2 2 IBr

The equilibrium constant for the above reaction is 280 at 250oC. If the equilibrium concentrations of IBr and Br2 are 1.3 M and 0.23 M respectively, what is the equilibrium concentration of I2 at 250oC?

a) 1.3 x 10-3 Mb) 8.4 x 10-3 Mc) 1.2 x 10-2 Md) 2.6 x 10-2 Me) 6.7 x 10-2 M

H2(g) + I2(g) 2 HI

At 450oC, 2.0 moles each of H2(g), I2(g) and HI(g) are combined in a 1.0 L rigid container. The value of Kc at 450oC is 50. Which of the following will occur as the system moves toward equilibrium?

a) More H2(g) and I2(g) will form

b) More HI(g) will formc) The total pressure will decreased) No net reaction will occur, because the

number of molecules is the same on both sides of the equation.

The following components are at equilibrium in a 1 L flask at 25oC:

P4(g) + 5 O2(g) P4O10

If 0.25 mol of gaseous chlorine is added to the flask, which of the following is true?

I. [P4] will increase

II. [P4O10] will increase

III. [O2] will decrease

a) I onlyb) II onlyc) I and II onlyd) II and III onlye) I, II and III

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

COCl2(g) decomposes according to the equation above. When pure COCl2(g) is injected into a rigid, previously evacuated flask at 690 K, the pressure in the flask is initially 1.0 atm. After the reaction reaches equilibrium at 690 K, the total pressure in the flask is 1.2 atm. What is the value of Kp for the reaction at 690 K?

a) 0.040b) 0.050c) 0.80d) 1.0

A 4.32 g sample of liquid SO2Cl2 is placed in a ridged, evacuated 1.50 L reaction vessel. As the container is heated to 400 K, the sample vaporizes completely and starts decomposing according to the equation below. The decomposition reaction is endothermic.

SO2Cl2(g) SO2(g) + Cl2(g)

a) If no decomposition occurred, what would be the pressure, in atm, of the SO2Cl2 in the vessel at 400 K.

b) When the system has reached equilibrium at 400 K, the total pressure in the container is 1.26 atm. Calculate the partial pressures, in atm, of SO2Cl(g), SO2(g) and Cl2(g) in the container at 400 K.

c) For the decomposition reaction at 400 K,I. Write the equilibrium expression for Kp for the reaciont and

II. Calculate the value of the equilibrium constant Kp

d) The temperature of the equilibrium mixture is increased to 425 K. Will the value of Kp increase, decrease or remain the same? Justify your prediction.

e) In another experiment, the original partial pressuresof SO2Cl2(g), SO2(g), and Cl2(g) are 1.0 atm each at 400 K. Predict whether the amount of SO2Cl2(g) in the container will increase, decrease or remain the same. Justify your prediction.

2 NO(g) + Br2(g) 2 NOBr(g)

NO reacts with Br2 according to the equation above. An experiment was performed to study the rate of the reaction at 546 K. Data from three trials are shown in the table below.

a) Using the data above determine the order of the reaction with respect to both NO(g) and Br2(g).

b) Write the rate law for the reaction.c) Determine the value of the rate constant, k, for the

reaction, include its units.d) At a later time during trial 2, the concentration of Br2(g) is

determined to be 0.16 M.I. Calculate the concentration of NO(g) at that time.II. Calculate the rate of consumption of Br2 at that time.

e) A two step mechanism is proposed for this reaction.Step 1: NO + Br2 NOBr2 Slow

Step 2: NO + NOBr2 2 NOBrFast

is the proposed mechanism consistent with the rate law determined in part (b)? Justify your answer