1. Chapter 13 Chemical Kinetics
Presented by: Sudhir Kumar Maingi PGT (Chemistry)
K.V. No. 1 PATHANKOT, AIR FORCE STN, JAMMU REGION

2. Kinetics
kinetics is the study of the factors that affect the speed of a reaction and the mechanism by which a reaction proceeds.
experimentally it is shown that there are 4 factors that influence the speed of a reaction:
nature of the reactants,
temperature,
catalysts,
concentration

3. Defining Rate
rate is how much a quantity changes in a given period of time
the speed you drive your car is a rate – the distance your car travels (miles) in a given period of time (1 hour)
so the rate of your car has units of mi/hr

4. Defining Reaction Rate
the rate of a chemical reaction is generally measured in terms of how much the concentration of a reactant decreases in a given period of time
or product concentration increases
for reactants, a negative sign is placed in front of the definition

5. Reaction Rate Changes Over Time
as time goes on, the rate of a reaction generally slows down
because the concentration of the reactants decreases.
at some time the reaction stops, either because the reactants run out or because the system has reached equilibrium.

6. at t = 0
[A] = 8
[B] = 8
[C] = 0
at t = 0
[X] = 8
[Y] = 8
[Z] = 0
at t = 16
[A] = 4
[B] = 4
[C] = 4
at t = 16
[X] = 7
[Y] = 7
[Z] = 1

7. at t = 16
[A] = 4
[B] = 4
[C] = 4
at t = 16
[X] = 7
[Y] = 7
[Z] = 1
at t = 32
[A] = 2
[B] = 2
[C] = 6
at t = 32
[X] = 6
[Y] = 6
[Z] = 2

8. at t = 32
[A] = 2
[B] = 2
[C] = 6
at t = 32
[X] = 6
[Y] = 6
[Z] = 2
at t = 48
[A] = 0
[B] = 0
[C] = 8
at t = 48
[X] = 5
[Y] = 5
[Z] = 3

9. Hypothetical Reaction Red  Blue
Time (sec)
Number Red
Number Blue
100 5 84 16 10 71 29 15 59 41 20 50 25 42 58 30 35 65 70 40 75 45 21 79 18 82
in this reaction,
one molecule of Red turns
into one molecule of Blue
the number of molecules
will always total 100
the rate of the reaction can
be measured as the speed of
loss of Red molecules
over time, or the speed of
gain of Blue molecules
over time

10. Hypothetical Reaction Red  Blue

11. Hypothetical Reaction Red  Blue
ro, Chemistry: A Molecular Approach

12. Reaction Rate and Stoichiometry
in most reactions, the coefficients of the balanced equation are not all the same
H2 (g) + I2 (g)  2 HI(g)
for these reactions, the change in the number of molecules of one substance is a multiple of the change in the number of molecules of another
for the above reaction, for every 1 mole of H2 used, 1 mole of I2 will also be used and 2 moles of HI made
therefore the rate of change will be different
in order to be consistent, the change in the concentration of each substance is multiplied by 1/coefficient

13. Average Rate
the average rate is the change in measured concentrations in any particular time period
linear approximation of a curve
the larger the time interval, the more the average rate deviates from the instantaneous rate

14. Hypothetical Reaction Red  Blue
Avg. Rate
Time (sec)
Number Red
Number Blue
(5 sec intervals)
(10 sec intervals)
(25 sec intervals)
100 5 84 16
3.2 10 71 29
2.6
2.9 15 59 41
2.4 20 50
1.8
2.1 25 42 58
1.6
2.3 30 35 65
1.4
1.5 70 1 40 75 45 21 79
0.8 18 82
0.6
0.7

15. H2 I2 HI Avg. Rate, M/s Time (s) [H2], M [HI], M -D[H2]/Dt
1/2 D[HI]/Dt
0.000
1.000
10.000
0.819
0.362
0.0181
20.000
0.670
0.660
0.0149
30.000
0.549
0.902
0.0121
40.000
0.449
1.102
0.0100
50.000
0.368
1.264
0.0081
60.000
0.301
1.398
0.0067
70.000
0.247
1.506
0.0054
80.000
0.202
1.596
0.0045
90.000
0.165
1.670
0.0037
0.135
1.730
0.0030
Avg. Rate, M/s
Time (s)
[H2], M
[HI], M
-D[H2]/Dt
1/2 D[HI]/Dt
0.000
1.000
10.000
0.819
20.000
0.670
30.000
0.549
40.000
0.449
50.000
0.368
60.000
0.301
70.000
0.247
80.000
0.202
90.000
0.165
0.135
Avg. Rate, M/s
Time (s)
[H2], M
[HI], M
-D[H2]/Dt
0.000
1.000
10.000
0.819
0.362
0.0181
20.000
0.670
0.660
0.0149
30.000
0.549
0.902
0.0121
40.000
0.449
1.102
0.0100
50.000
0.368
1.264
0.0081
60.000
0.301
1.398
0.0067
70.000
0.247
1.506
0.0054
80.000
0.202
1.596
0.0045
90.000
0.165
1.670
0.0037
0.135
1.730
0.0030
Avg. Rate, M/s
Time (s)
[H2], M
[HI], M
-D[H2]/Dt
1/2 D[HI]/Dt
0.000
1.000
10.000
0.819
0.362
20.000
0.670
0.660
30.000
0.549
0.902
40.000
0.449
1.102
50.000
0.368
1.264
60.000
0.301
1.398
70.000
0.247
1.506
80.000
0.202
1.596
90.000
0.165
1.670
0.135
1.730
Stoichiometry tells us that for every 1 mole/L of H2 used,
2 moles/L of HI are made.
The average rate is the change in the concentration in a given time period.
Assuming a 1 L container, at 10 s, we used moles of H2. Therefore the amount of HI made is 2(0.181 moles) = moles
In the first 10 s, the D[H2] is M, so the rate is
At 60 s, we used moles of H2. Therefore the amount of HI made is 2(0.699 moles) = moles

16. average rate in a given time period =  slope of the line connecting the [H2] points; and ½ +slope of the line for [HI]
the average rate for the first 80 s is M/s
the average rate for the first 10 s is M/s
the average rate for the first 40 s is M/s

17. Instantaneous Rate
the instantaneous rate is the change in concentration at any one particular time
slope at one point of a curve
determined by taking the slope of a line tangent to the curve at that particular point
first derivative of the function
for you calculus fans

18. H2 (g) + I2 (g)  2 HI (g)
Using [H2], the instantaneous rate at 50 s is:
Using [HI], the instantaneous rate at 50 s is:

19. Ex 13. 1 - For the reaction given, the [I] changes from 1. 000 M to 0
Ex For the reaction given, the [I] changes from M to M in the first 10 s. Calculate the average rate in the first 10 s and the D[H+]. H2O2 (aq) + 3 I(aq) + 2 H+(aq)  I3(aq) + 2 H2O(l)
Solve the equation for the Rate (in terms of the change in concentration of the Given quantity)
Solve the equation of the Rate (in terms of the change in the concentration for the quantity to Find) for the unknown value

20. Measuring Reaction Rate
in order to measure the reaction rate you need to be able to measure the concentration of at least one component in the mixture at many points in time
there are two ways of approaching this problem (1) for reactions that are complete in less than 1 hour, it is best to use continuous monitoring of the concentration, or (2) for reactions that happen over a very long time, sampling of the mixture at various times can be used
when sampling is used, often the reaction in the sample is stopped by a quenching technique

21. Continuous Monitoring
polarimetry – measuring the change in the degree of rotation of plane-polarized light caused by one of the components over time
spectrophotometry – measuring the amount of light of a particular wavelength absorbed by one component over time
the component absorbs its complimentary color
total pressure – the total pressure of a gas mixture is stoichiometrically related to partial pressures of the gases in the reaction

22. Sampling
gas chromatography can measure the concentrations of various components in a mixture
for samples that have volatile components
separates mixture by adherence to a surface
drawing off periodic aliquots from the mixture and doing quantitative analysis
titration for one of the components
gravimetric analysis

23. Factors Affecting Reaction Rate Nature of the Reactants
nature of the reactants means what kind of reactant molecules and what physical condition they are in.
small molecules tend to react faster than large molecules;
gases tend to react faster than liquids which react faster than solids;
powdered solids are more reactive than “blocks”
more surface area for contact with other reactants
certain types of chemicals are more reactive than others
e.g., the activity series of metals
ions react faster than molecules
no bonds need to be broken

24. Factors Affecting Reaction Rate Temperature
increasing temperature increases reaction rate
chemist’s rule of thumb - for each 10°C rise in temperature, the speed of the reaction doubles
for many reactions
there is a mathematical relationship between the absolute temperature and the speed of a reaction discovered by Svante Arrhenius which will be examined later

25. Factors Affecting Reaction Rate Catalysts
catalysts are substances which affect the speed of a reaction without being consumed.
most catalysts are used to speed up a reaction, these are called positive catalysts
catalysts used to slow a reaction are called negative catalysts.
homogeneous = present in same phase
heterogeneous = present in different phase
how catalysts work will be examined later

26. Factors Affecting Reaction Rate Reactant Concentration
generally, the larger the concentration of reactant molecules, the faster the reaction
increases the frequency of reactant molecule contact
concentration of gases depends on the partial pressure of the gas
higher pressure = higher concentration
concentration of solutions depends on the solute to solution ratio (molarity)

27. The Rate Law
the Rate Law of a reaction is the mathematical relationship between the rate of the reaction and the concentrations of the reactants
and homogeneous catalysts as well
the rate of a reaction is directly proportional to the concentration of each reactant raised to a power
for the reaction aA + bB  products the rate law would have the form given below
n and m are called the orders for each reactant
k is called the rate constant

28. Reaction Order
the exponent on each reactant in the rate law is called the order with respect to that reactant
the sum of the exponents on the reactants is called the order of the reaction
The rate law for the reaction:
2 NO(g) + O2(g) ® 2 NO2(g)
is Rate = k[NO]2[O2]
The reaction is
second order with respect to [NO],
first order with respect to [O2],
and third order overall

29. The reaction is autocatalytic, because a product affects the rate.
Sample Rate Laws
The reaction is autocatalytic, because a product affects the rate.
Hg2+ is a negative catalyst, increasing its concentration slows the reaction.

30. Reactant Concentration vs. Time A  Products

31. Half-Life
the half-life, t1/2, of a reaction is the length of time it takes for the concentration of the reactants to fall to ½ its initial value
the half-life of the reaction depends on the order of the reaction

32. Zero Order Reactions [A] Rate = k[A]0 = k constant rate reactions
[A] = -kt + [A]0
graph of [A] vs. time is straight line with slope = -k and y-intercept = [A]0
t ½ = [A0]/2k
when Rate = M/sec, k = M/sec
[A]0
[A]
time
slope = - k

33. First Order Reactions Rate = k[A] ln[A] = -kt + ln[A]0
graph ln[A] vs. time gives straight line with slope = -k and y-intercept = ln[A]0
used to determine the rate constant
t½ = 0.693/k
the half-life of a first order reaction is constant
the when Rate = M/sec, k = sec-1

34. ln[A]0
ln[A]
time
slope = −k

35. Half-Life of a First-Order Reaction Is Constant

36. Rate Data for C4H9Cl + H2O ® C4H9OH + HCl
Time (sec)
[C4H9Cl], M
0.0
0.1000
50.0
0.0905
100.0
0.0820
150.0
0.0741
200.0
0.0671
300.0
0.0549
400.0
0.0448
500.0
0.0368
800.0
0.0200
0.0000

37. C4H9Cl + H2O  C4H9OH + 2 HCl

38. C4H9Cl + H2O  C4H9OH + 2 HCl

39. C4H9Cl + H2O  C4H9OH + 2 HCl
slope =
-2.01 x 10-3
k =
2.01 x 10-3 s-1

40. Second Order Reactions
Rate = k[A]2
1/[A] = kt + 1/[A]0
graph 1/[A] vs. time gives straight line with slope = k and y-intercept = 1/[A]0
used to determine the rate constant
t½ = 1/(k[A0])
when Rate = M/sec, k = M-1∙sec-1

41. slope = k
1/[A]
l/[A]0
time

42. Partial Pressure NO2, mmHg
Rate Data For 2 NO2 ® 2 NO + O2
Time (hrs.)
Partial Pressure NO2, mmHg
ln(PNO2)
1/(PNO2)
100.0
4.605 30
62.5
4.135 60
45.5
3.817 90
35.7
3.576
120
29.4
3.381
150
25.0
3.219
180
21.7
3.079
210
19.2
2.957
240
17.2
2.847

43. Rate Data Graphs For 2 NO2 ® 2 NO + O2
Tro, Chemistry: A Molecular Approach

44. Rate Data Graphs For 2 NO2 ® 2 NO + O2

45. Rate Data Graphs For 2 NO2 ® 2 NO + O2

46. Determining the Rate Law
can only be determined experimentally
graphically
rate = slope of curve [A] vs. time
if graph [A] vs time is straight line, then exponent on A in rate law is 0, rate constant = -slope
if graph ln[A] vs time is straight line, then exponent on A in rate law is 1, rate constant = -slope
if graph 1/[A] vs time is straight line, exponent on A in rate law is 2, rate constant = slope
initial rates
by comparing effect on the rate of changing the initial concentration of reactants one at a time

47. Tro, Chemistry: A Molecular Approach

48. Practice - Complete the Table and Determine the Rate Equation for the Reaction A ® 2 Prod

49. Practice - Complete the Table and Determine the Rate Equation for the Reaction A ® 2 Prod

53. Practice - Complete the Table and Determine the Rate Equation for the Reaction A ® 2 Prod
-D[A] Dt
= 0.1 [A]2
the reaction is second order,

54. Ex – The reaction SO2Cl2(g)  SO2(g) + Cl2(g) is first order with a rate constant of 2.90 x 10-4 s-1 at a given set of conditions. Find the [SO2Cl2] at 865 s when [SO2Cl2]0 = M
Given:
Find:
[SO2Cl2]0 = M, t = 865, k = 2.90 x 10-4 s-1
[SO2Cl2]
Concept Plan:
Relationships:
[SO2Cl2]
[SO2Cl2]0, t, k
Solution:
Check:
the new concentration is less than the original, as expected

55. Initial Rate Method
another method for determining the order of a reactant is to see the effect on the initial rate of the reaction when the initial concentration of that reactant is changed
for multiple reactants, keep initial concentration of all reactants constant except one
zero order = changing the concentration has no effect on the rate
first order = the rate changes by the same factor as the concentration
doubling the initial concentration will double the rate
second order = the rate changes by the square of the factor the concentration changes
doubling the initial concentration will quadruple the rate

56. Comparing Expt #1 and Expt #2, the [NO2] changes but the [CO] does not
Ex 13.2 – Determine the rate law and rate constant for the reaction NO2(g) + CO(g)  NO(g) + CO2(g) given the data below.
Write a general rate law including all reactants
Examine the data and find two experiments in which the concentration of one reactant changes, but the other concentrations are the same
Expt.
Number
Initial [NO2], (M)
Initial
[CO], (M)
Initial Rate
(M/s) 1.
0.10
0.0021 2.
0.20
0.0082 3.
0.0083 4.
0.40
0.033
Expt.
Number
Initial [NO2], (M)
Initial
[CO], (M)
Initial Rate
(M/s) 1.
0.10
0.0021 2.
0.20
0.0082 3.
0.0083 4.
0.40
0.033
Comparing Expt #1 and Expt #2, the [NO2] changes but the [CO] does not

57. Ex 13.2 – Determine the rate law and rate constant for the reaction NO2(g) + CO(g)  NO(g) + CO2(g) given the data below.
Expt.
Number
Initial [NO2], (M)
Initial
[CO], (M)
Initial Rate
(M/s) 1.
0.10
0.0021 2.
0.20
0.0082 3.
0.0083 4.
0.40
0.033
Determine by what factor the concentrations and rates change in these two experiments.

58. Ex 13.2 – Determine the rate law and rate constant for the reaction NO2(g) + CO(g)  NO(g) + CO2(g) given the data below.
Expt.
Number
Initial [NO2], (M)
Initial
[CO], (M)
Initial Rate
(M/s) 1.
0.10
0.0021 2.
0.20
0.0082 3.
0.0083 4.
0.40
0.033
Determine to what power the concentration factor must be raised to equal the rate factor.

59. Ex 13.2 – Determine the rate law and rate constant for the reaction NO2(g) + CO(g)  NO(g) + CO2(g) given the data below.
Expt.
Number
Initial [NO2], (M)
Initial
[CO], (M)
Initial Rate
(M/s) 1.
0.10
0.0021 2.
0.20
0.0082 3.
0.0083 4.
0.40
0.033
Expt.
Number
Initial [NO2], (M)
Initial
[CO], (M)
Initial Rate
(M/s) 1.
0.10
0.0021 2.
0.20
0.0082 3.
0.0083 4.
0.40
0.033
Repeat for the other reactants

60. Ex 13.2 – Determine the rate law and rate constant for the reaction NO2(g) + CO(g)  NO(g) + CO2(g) given the data below.
Expt.
Number
Initial [NO2], (M)
Initial
[CO], (M)
Initial Rate
(M/s) 1.
0.10
0.0021 2.
0.20
0.0082 3.
0.0083 4.
0.40
0.033
Substitute the exponents into the general rate law to get the rate law for the reaction
n = 2, m = 0

61. Ex 13.2 – Determine the rate law and rate constant for the reaction NO2(g) + CO(g)  NO(g) + CO2(g) given the data below.
Expt.
Number
Initial [NO2], (M)
Initial
[CO], (M)
Initial Rate
(M/s) 1.
0.10
0.0021 2.
0.20
0.0082 3.
0.0083 4.
0.40
0.033
Substitute the concentrations and rate for any experiment into the rate law and solve for k

62. Practice - Determine the rate law and rate constant for the reaction NH4+1 + NO2-1 ® N2 + 2 H2O given the data below.
Expt.
No.
Initial [NH4+], M
Initial [NO2-], M
Initial Rate,
(x 10-7), M/s 1
0.0200
0.200
10.8 2
0.0600
32.3 3
0.0202 4
0.0404
21.6

63. Practice - Determine the rate law and rate constant for the reaction NH4+1 + NO2-1 ® N2 + 2 H2O given the data below.
Rate = k[NH4+]n[NO2]m
Expt.
No.
Initial [NH4+], M
Initial [NO2-], M
Initial Rate,
(x 10-7), M/s 1
0.0200
0.200
10.8 2
0.0600
32.3 3
0.0202 4
0.0404
21.6

64. The Effect of Temperature on Rate
changing the temperature changes the rate constant of the rate law
Svante Arrhenius investigated this relationship and showed that:
where T is the temperature in kelvins
R is the gas constant in energy units, J/(mol∙K)
A is a factor called the frequency factor
Ea is the activation energy, the extra energy needed to start the molecules reacting

66. Activation Energy and the Activated Complex
energy barrier to the reaction
amount of energy needed to convert reactants into the activated complex
aka transition state
the activated complex is a chemical species with partially broken and partially formed bonds
always very high in energy because partial bonds

67. Isomerization of Methyl Isonitrile
methyl isonitrile rearranges to acetonitrile
in order for the reaction to occur, the H3C-N bond must break; and a new H3C-C bond form

68. Energy Profile for the Isomerization of Methyl Isonitrile
As the reaction begins, the C-N bond weakens enough for the CN group to start to rotate
the activated complex is a chemical species with partial bonds
the collision frequency is the number of molecules that approach the peak in a given period of time
the activation energy is the difference in energy between the reactants and the activated complex

69. The Arrhenius Equation: The Exponential Factor
the exponential factor in the Arrhenius equation is a number between 0 and 1
it represents the fraction of reactant molecules with sufficient energy so they can make it over the energy barrier
the higher the energy barrier (larger activation energy), the fewer molecules that have sufficient energy to overcome it
that extra energy comes from converting the kinetic energy of motion to potential energy in the molecule when the molecules collide
increasing the temperature increases the average kinetic energy of the molecules
therefore, increasing the temperature will increase the number of molecules with sufficient energy to overcome the energy barrier
therefore increasing the temperature will increase the reaction rate

71. Arrhenius Plots
the Arrhenius Equation can be algebraically solved to give the following form:
this equation is in the form y = mx + b
where y = ln(k) and x = (1/T)
a graph of ln(k) vs. (1/T) is a straight line
( J/mol∙K)(slope of the line) = Ea, (in Joules)
ey-intercept = A, (unit is the same as k)

72. Ex Determine the activation energy and frequency factor for the reaction O3(g)  O2(g) + O(g) given the following data:
Temp, K
k, M-1∙s-1
600
3.37 x 103
1300
7.83 x 107
700
4.83 x 104
1400
1.45 x 108
800
3.58 x 105
1500
2.46 x 108
900
1.70 x 106
1600
3.93 x 108
1000
5.90 x 106
1700
5.93 x 108
1100
1.63 x 107
1800
8.55 x 108
1200
3.81 x 107
1900
1.19 x 109

73. Ex Determine the activation energy and frequency factor for the reaction O3(g)  O2(g) + O(g) given the following data:
use a spreadsheet to graph ln(k) vs. (1/T)

74. Ex Determine the activation energy and frequency factor for the reaction O3(g)  O2(g) + O(g) given the following data:
Ea = m∙(-R)
solve for Ea
A = ey-intercept
solve for A

75. Arrhenius Equation: Two-Point Form
if you only have two (T,k) data points, the following forms of the Arrhenius Equation can be used:

76. Ex – The reaction NO2(g) + CO(g)  CO2(g) + NO(g) has a rate constant of 2.57 M-1∙s-1 at 701 K and 567 M-1∙s-1 at 895 K. Find the activation energy in kJ/mol
Given:
Find:
T1 = 701 K, k1 = 2.57 M-1∙s-1, T2 = 895 K, k2 = 567 M-1∙s-1
Ea, kJ/mol
Concept Plan:
Relationships: Ea
T1, k1, T2, k2
Solution:
Check:
most activation energies are tens to hundreds of kJ/mol – so the answer is reasonable

77. Collision Theory of Kinetics
for most reactions, in order for a reaction to take place, the reacting molecules must collide into each other.
once molecules collide they may react together or they may not, depending on two factors -
whether the collision has enough energy to "break the bonds holding reactant molecules together";
whether the reacting molecules collide in the proper orientation for new bonds to form.

78. Effective Collisions
collisions in which these two conditions are met (and therefore lead to reaction) are called effective collisions
the higher the frequency of effective collisions, the faster the reaction rate
when two molecules have an effective collision, a temporary, high energy (unstable) chemical species is formed - called an activated complex or transition state

79. Effective Collisions Kinetic Energy Factor
for a collision to lead to overcoming the energy barrier, the reacting molecules must have sufficient kinetic energy so that when they collide it can form the activated complex

80. Effective Collisions Orientation Effect

81. Collision Theory and the Arrhenius Equation
A is the factor called the frequency factor and is the number of molecules that can approach overcoming the energy barrier
there are two factors that make up the frequency factor – the orientation factor (p) and the collision frequency factor (z)

82. Orientation Factor
the proper orientation results when the atoms are aligned in such a way that the old bonds can break and the new bonds can form
the more complex the reactant molecules, the less frequently they will collide with the proper orientation
reactions between atoms generally have p = 1
reactions where symmetry results in multiple orientations leading to reaction have p slightly less than 1
for most reactions, the orientation factor is less than 1
for many, p << 1
there are some reactions that have p > 1 in which an electron is transferred without direct collision

83. Reaction Mechanisms
we generally describe chemical reactions with an equation listing all the reactant molecules and product molecules
but the probability of more than 3 molecules colliding at the same instant with the proper orientation and sufficient energy to overcome the energy barrier is negligible
most reactions occur in a series of small reactions involving 1, 2, or at most 3 molecules
describing the series of steps that occur to produce the overall observed reaction is called a reaction mechanism
knowing the rate law of the reaction helps us understand the sequence of steps in the mechanism

84. An Example of a Reaction Mechanism
Overall reaction:
H2(g) + 2 ICl(g)  2 HCl(g) + I2(g)
Mechanism:
H2(g) + ICl(g)  HCl(g) + HI(g)
HI(g) + ICl(g)  HCl(g) + I2(g)
the steps in this mechanism are elementary steps, meaning that they cannot be broken down into simpler steps and that the molecules actually interact directly in this manner without any other steps

85. Elements of a Mechanism Intermediates
H2(g) + 2 ICl(g)  2 HCl(g) + I2(g)
H2(g) + ICl(g)  HCl(g) + HI(g)
HI(g) + ICl(g)  HCl(g) + I2(g)
notice that the HI is a product in Step 1, but then a reactant in Step 2
since HI is made but then consumed, HI does not show up in the overall reaction
materials that are products in an early step, but then a reactant in a later step are called intermediates

86. Molecularity
the number of reactant particles in an elementary step is called its molecularity
a unimolecular step involves 1 reactant particle
a bimolecular step involves 2 reactant particles
though they may be the same kind of particle
a termolecular step involves 3 reactant particles
though these are exceedingly rare in elementary steps

87. Rate Laws for Elementary Steps
each step in the mechanism is like its own little reaction – with its own activation energy and own rate law
the rate law for an overall reaction must be determined experimentally
but the rate law of an elementary step can be deduced from the equation of the step
H2(g) + 2 ICl(g)  2 HCl(g) + I2(g)
H2(g) + ICl(g)  HCl(g) + HI(g) Rate = k1[H2][ICl]
HI(g) + ICl(g)  HCl(g) + I2(g) Rate = k2[HI][ICl]

88. Rate Laws of Elementary Steps

89. Rate Determining Step
in most mechanisms, one step occurs slower than the other steps
the result is that product production cannot occur any faster than the slowest step – the step determines the rate of the overall reaction
we call the slowest step in the mechanism the rate determining step
the slowest step has the largest activation energy
the rate law of the rate determining step determines the rate law of the overall reaction

90. Another Reaction Mechanism
NO2(g) + CO(g)  NO(g) + CO2(g) Rateobs = k[NO2]2
NO2(g) + NO2(g)  NO3(g) + NO(g) Rate = k1[NO2]2 slow
NO3(g) + CO(g)  NO2(g) + CO2(g) Rate = k2[NO3][CO] fast
The first step is slower than the second step because its activation energy is larger.
The first step in this mechanism is the rate determining step.
The rate law of the first step is the same as the rate law of the overall reaction.

91. Validating a Mechanism
in order to validate (not prove) a mechanism, two conditions must be met:
the elementary steps must sum to the overall reaction
the rate law predicted by the mechanism must be consistent with the experimentally observed rate law

92. Mechanisms with a Fast Initial Step
when a mechanism contains a fast initial step, the rate limiting step may contain intermediates
when a previous step is rapid and reaches equilibrium, the forward and reverse reaction rates are equal – so the concentrations of reactants and products of the step are related
and the product is an intermediate
substituting into the rate law of the RDS will produce a rate law in terms of just reactants

93. An Example 2 NO(g)  N2O2(g) Fast
H2(g) + N2O2(g)  H2O(g) + N2O(g) Slow Rate = k2[H2][N2O2]
H2(g) + N2O(g)  H2O(g) + N2(g) Fast k1
k-1
2 H2(g) + 2 NO(g)  2 H2O(g) + N2(g) Rateobs = k [H2][NO]2
for Step 1 Rateforward = Ratereverse

94. Ex Show that the proposed mechanism for the reaction 2 O3(g)  3 O2(g) matches the observed rate law Rate = k[O3]2[O2]-1
O3(g)  O2(g) + O(g) Fast
O3(g) + O(g)  2 O2(g) Slow Rate = k2[O3][O] k1
k-1
for Step 1 Rateforward = Ratereverse

95. Catalysts
catalysts are substances that affect the rate of a reaction without being consumed
catalysts work by providing an alternative mechanism for the reaction
with a lower activation energy
catalysts are consumed in an early mechanism step, then made in a later step
mechanism without catalyst
O3(g) + O(g)  2 O2(g) V. Slow
mechanism with catalyst
Cl(g) + O3(g)  O2(g) + ClO(g) Fast
ClO(g) + O(g)  O2(g) + Cl(g) Slow

96. Ozone Depletion over the Antarctic

97. Energy Profile of Catalyzed Reaction
polar stratospheric clouds contain ice crystals that catalyze reactions that release Cl from atmospheric chemicals

98. Catalysts
homogeneous catalysts are in the same phase as the reactant particles
Cl(g) in the destruction of O3(g)
heterogeneous catalysts are in a different phase than the reactant particles
solid catalytic converter in a car’s exhaust system

99. Types of Catalysts

100. Catalytic Hydrogenation H2C=CH2 + H2 → CH3CH3

101. Ethene to Ethane animation

102. Enzymes
because many of the molecules are large and complex, most biological reactions require a catalyst to proceed at a reasonable rate
protein molecules that catalyze biological reactions are called enzymes
enzymes work by adsorbing the substrate reactant onto an active site that orients it for reaction

103. Enzyme-Substrate Binding Lock and Key Mechanism

104. Enzymatic Hydrolysis of Sucrose

105. Thank You Presented by: Sudhir Kumar Maingi PGT CHEMISTRY
K.V. No. 1 PATHANKOT, JAMMU REGION
Phone:
Acknowledgements : Prentice Hall Publications
Steven S. Zumdahl