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11.1 11.1 REACTION RATEREACTION RATE
C H E M I S T RY U N I TC H E M I S T RY U N I T
S K 0 2 7S K 0 2 7
OBJECTIVES:OBJECTIVES:
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
1. define rate laws, order of reaction and
rate constant
2. write the rate law / rate equation
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
2. write the rate law / rate equation
3. calculate the order with respect to a
certain reactant and the overall order of
a reaction from experimental data.
4. calculate the value and determine the
units of rate constants, k.
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
THE RATE LAWTHE RATE LAW
�The rate law for a chemical reaction relates the rate of reaction to the concentrations of reactants.
�At constant T & P, rate of chemical reaction directly proportional to [reactants].
Rate = k [A]m[B]n
directly proportional to [reactants].
Rate ∝ [reactants]
Consider this equation,
aA + bB → cC
Rate ∝ [A]m [B]n
k = rate constant
n = reaction order wrt B
m = reaction order with
respect to A
m + n= overall rxn order
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
� The exponents in a rate law / reaction order
must be determined by experiment.
They are not derived from the stoichiometric
coefficients in an overall chemical equation.
They are generally small, positive integers They are generally small, positive integers
(0,1,2…).
� The rate constant, k and its value depends on
specific rxn, T & the presence of catalyst.
The larger k, the faster the reaction proceeds.
Units of k depend on the values of m, n, …
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
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11.1 REACTION RATE11.1 REACTION RATE
Example 1:
Determining Reaction Order from Rate Laws
2NO(g) + O2(g) 2NO2(g)
rate = k [NO]2 [O ]
The overall reaction order = 3
The reaction order with respect to NO = 2
The reaction order with respect to O2 = 1
rate = k [NO]2 [O2]
Solution:
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
S2O82- + 3I- 2SO4
2- + I3-
The above reaction is first order with respect to Iodide ion and
to thiosulphate ion.
a) Write the rate equation for the reaction
Example 2 :
a) Write the rate equation for the reaction
b) What is the unit of rate constant, k?
Solution :
a) Rate = k [S2O82-]1[I-]1
b) Rate = k [S2O82-]1[I-]1
k = rate
[S2O82-]1[I-]1
=Ms-1
M2= M-1s-1
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
Determine rate law: Method Of Initial Rates
• The method of initial rates involves a series of
expt in which the initial [ ] of some reactants are
held constant and others are varied in order to
determine the rate law for that reaction.determine the rate law for that reaction.
• The effects of doubling one initial concentration:
– For zero-order reactions, no effect on rate.
– For first-order reactions, the rate doubles.
– For second-order reactions, the rate quadruples.
– For third-order reactions, the rate increases
eightfold.S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
Method Of Initial Rates
Initial Rates of the Reaction:
2NO(g) + Cl2(g) → 2NOCl(g)
Exp. Initial [NO] Initial [Cl2] Initial rate, Ms-1
1 0.0125 0.0255 2.27 × 10-5
2 0.0125 0.0510 4.55 × 10-5
3 0.0250 0.0255 9.08 × 10-5
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
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11.1 REACTION RATE11.1 REACTION RATE
Method Of Initial Rates (continue..)
Compare 2 experiments, of [one reactant] varies and the
[ other reactant] constant.
2NO(g) + Cl2(g) → 2NOCl(g)
The rate law in general is; Rate = k [NO]m [Cl2]n
[ other reactant] constant.
=exp1 rate
exp2 rate
nm
nm
(0.0255)k(0.0125)
(0.0510)k(0.0125)
n
exp12
m
exp1
n
exp22
m
exp2
][Clk[NO]
][Cl k[NO]
=×
×
5-
-5
102.27
104.55
2 = 2n Thus, n, order of rxn wrt Cl2 = 1S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
Method Of Initial Rates (continue..)
=exp1 rate
exp3 raten
exp12
m
exp1
n
exp32
m
exp3
][Cl k[NO]
][Cl k[NO]
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
nm
nm
(0.0255)k(0.0125)
(0.0255)k(0.0250)=
×
×
−
−
5
5
1027.2
1008.9
4 = 2m
Thus, m, order of rxn wrt NO = 2
Overall order of reaction = m + n = 2 + 1 = 3
Thus, rate law is Rate = k [NO]2 [Cl2]
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
The order of reaction
For Reaction,
A Products
Rate = k [A]xRate = k [A]x
Rate is not dependent on [A]
Rate = k [A]0
Rate = k
Thus, this reaction is zero order with respect to A
i) If x = 0
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
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11.1 REACTION RATE11.1 REACTION RATE
The order of reaction
ii) Rate = k [A]1
A Products
Rate = k [A]x
If x = 1ii) Rate = k [A]
If the [A] is doubled, A 2A
Double the [A] will double the rate of reaction.
Thus, this reaction is first order with respect to A
Rate = k[2A]1
Rate = 2k[A]1
If x = 1
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
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11.1 REACTION RATE11.1 REACTION RATE
The order of reaction
A Products
iii) Rate = k[A]2If x = 2
Rate = k [A]x
iii) Rate = k[A]2
If the [A] is doubled, A 2A
Rate = 4k[A]2
Doubling the [A], the rate increase by a factor of 4.
Thus, the reaction is second order with respect to A
Rate = k[2A]2
If x = 2
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
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11.1 REACTION RATE11.1 REACTION RATE
The results of the kinetic studies are given below.
Exercise:
exp [ClO2] / M [OH-] / M Initial rate / Ms-1
1 0.0421 0.0185 8.21 ×10-3
2 0.0522 0.0185 1.26 ×10-2
ClO2(aq) + 2OH- (aq) → products
2 0.0522 0.0185 1.26 ×10-2
3 0.0421 0.0285 1.26 ×10-2
a) Explain what is meant by the order of reaction
b) Referring to the data determine
(i) rate law /rate equation
(ii) rate constant, k
(iii) the reaction rate if the concentration of both ClO2and OH- = 0.05 M
rate = k [ClO2]2[OH-]
k = 250 M-2s-1
rate = 3.12 ×10-2 M/sS K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
C H A P T E R 1 1 : R E A C T ION K INE T IC SC H A P T E R 1 1 : R E A C T ION K INE T IC S
11.1 REACTION RATE11.1 REACTION RATE
Exercise:
i) When [A] is double, rate also double. But double the
[B] has no effect on rate.
A + B → C
Write rate law for this equation,
Rate = k[A][B] has no effect on rate.
ii) When [A] is increase 3x, rate increases 3x, and
increase of [B] 3x causes the rate to increase 9x.
iii) Reduce [A] by half has no effect on the rate, but
reduce [B] by half causing the rate to be half of the
initial rate.
Rate = k[A]
Rate = k[A] [B]2
Rate = k[B]
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
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11.1 REACTION RATE11.1 REACTION RATE
Exercise:The results of the kinetic studies are given below.
exp Initial [ ] (M) Time
interval
(Min)
The change in
concentration
of C (M)[C] [D]
1 0.10 1.0 30 2.5 ×10-3
C + D → E
1 0.10 1.0 30 2.5 ×10-3
2 0.10 2.0 30 1.0 ×10-2
3 0.05 1.0 120 5.0 ×10-3
a) Calculate the rate of reaction for each experiment
a) Determine the order of reaction with respect to C
and D and write the rate law. rate = k [C] [D]2
8.33 ×10-5 ,3.33 × 10-4, 4.17×10-5 M min-1
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY
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11.1 REACTION RATE11.1 REACTION RATE
Exercise:The results of the kinetic studies are given below.
exp Initial [ ] (M) Time
interval
(Min)
The change in
concentration
of C (M)[C] [D]
1 0.10 1.0 30 2.5 ×10-3
C + D → E
c) State the effect on the reaction rate if the
concentration of D is doubled but the concentration
of C remains constant. rate increase by a factor of 4;
1 0.10 1.0 30 2.5 ×10-3
2 0.10 2.0 30 1.0 ×10-2
3 0.05 1.0 120 5.0 ×10-3
S K 0 2 7 C H E M I S T RYS K 0 2 7 C H E M I S T RY