Chemistry 104 Study Groups
KINETICS
 

CONCEPTS

  Kinetics offers information about the rate and mechanism of chemical reactions. The rate of a reaction may be described by a mathematical expression called the rate law. For the reaction

A + B ——> products

the rate law will might have the form:

rate = k[A]x[B]y

The exponents x and y must be determined experimentally. When x and y are integers, the overall order of the reaction is given by x+y. The constant k is the rate constant which must also be determined experimentally.

 
 

Interesting tidbits about kinetics

  • The units of the rate constant k will vary depending on the overall order of the reaction.

  • The rate of a reaction represents the change in concentration of one of the reactans as a function of time.

  • Many reactions have an overall order of either one or two (first or second order reaction).

  • The rate law for all first second order reactions of the form

    A ——> products

    may be easily integrated, resulting in an equation which can be used to calculate the concentration of A as a function of time.

  • The half-life, the amount of time for the concentration of A to fall to one-half its initial value may be calculated in a straightforward manner for reactions of this type.

 
 
 

Some Useful Equations

  First Order
reaction 		Second Order
reaction
 
 
  rate=k[A]

 rate=k[A]2

 
 
 

Exercises

  The initial rate of the reaction A+2B——> C is determined for the following initial conditions:

Experiment

[A] (M)

[B] (M)

Rateo (M/s)

1

0.3

0.2

4.2 x 10–2

2

0.3

0.4

1.67 x 10-1

3

0.6

0.2

8.4 x 10-2

4

0.3

0.1

 

Determine:
  1. the rate law for the reaction
  2. the overall order of the reaction
  3. the rate for experiment 4
  4. the units for the rate constant

 
  Consider the following data for three different reactions of the form A ——> products

 

Reaction I

Reaction II

Reaction III

Time (s)

[A] (M)

Time (s)

[A] (M)

Time (s)

[A] (M)

0

1.00

0

1.00

0

1.00

25

0.78

25

0.75

25

0.80

50

0.61

50

0.50

50

0.67

75

0.47

75

0.25

75

0.57

100

0.37

100

0.00

100

0.50

150

0.22

 

 

150

0.40

200

0.14

 

 

200

0.33

250

0.08

 

 

250

0.29

Assign either zeroth, first, or second order to each of these reactions.

 
  The rate constant for the reaction A ——> products is 0.23 s-1. Calculate the time required for [A] to decrease to:
  1. one quarter [A]o
  2. one sixteenth [A]o
  3. one third [A]o

 
  The reaction A2 + B2 -----> 2AB was studied at 500oC and 550 oC. The rate constants were determined to be 8.23 x 10-3 M-1s-1 at 500 oC and 3.81 x 10-2 M-1s-1 at 550 oC. Find the activation energy for this reaction and find the temperature where the rate constant will be 1.64 x 10-2 M-1s-1.

 
  The reaction 2AB + B2 ----->2AB2 has rate = k[AB]2[B2]. Which of the following mechanism is consistent with that rate law?

  1. AB + B2 <-----> AB3(fast, equilibrium)

    AB + AB3 -----> 2AB2(slow)

  2. 2AB -----> A2B2(slow)

    B2 + A2B2 ----->A2B4 (fast)

    A2B2 -----> 2AB2(fast)

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