1. REACTION KINETICS
8.1 REACTION RATE
LEARNING OUTCOMES:
• Define rate law
• Define order of reaction
• Write rate law with respect to the order of reaction

2. The rate law expresses the relationship of the rate of
a reaction to the rate constant and the concentrations
of the reactants raised to some powers.
aA + bB
cC + dD
Rate = k
?
x[B]y
[A]
reaction is x order with respect to A
reaction is y order with respect to B
• x and y are called the orders for each reactant
• The rate constant, k, is a proportionality constant in the
relationship between rate and concentrations.

3. Reaction Order
with respect to
-the order of a reaction w.r.t
a reactant, is the exponent of
its concentration term in the
rate expression,
Rate
k[A] [B]
m
n
(m is the order w.r.t A) (n is the order w.r.t B)
can be 0, 1, 2 or fractions,-ve
order must be determined by experiment!!!

4. Reaction Order
-the total reaction order is the
sum of all exponents on all
concentration terms;
Rate = k [A]m[B]n[C]p
Total order = m + n + p

5. Rate Law Example1:
Overall Reaction Order
•Sum of the order of each component in the
rate law.
•rate = k[H2SeO3][H+]2[I ]3
•The overall reaction order is 1 + 2 + 3 = 6.

6. Rate Law Example 2:
Reaction
CH3CN
Rate Law
Rate = k[CH3CN]
CH3NC
CH3CHO
Rate = k[CH3CHO]3/2
CH4 + CO
2 N2O5
4 NO2 + O2
Rate = k[N2O5]
H2 + I2
2 HI
Rate = k[H2][I2]
+3
Tl +
Hg2+2
+1
Tl + 2 Hg
+2
Rate = k[Tl+3][Hg2+2][Hg+2]-1
2 NO(g) + O2(g) 2 NO2(g)
Rate = k[NO]2[O2]
The reaction is second order with respect to [NO],
first order with respect to [O2],
and third order overall

7. Rate Law Example 3:
• Reaction Order
– Consider the reaction of nitric oxide with
hydrogen according to the following equation.
2NO(g ) 2H 2 (g )
N 2 ( g ) 2 H 2 O( g )
– The experimentally determined rate law is
2
Rate k[NO] [H 2 ]
– Thus, the reaction is second order in NO, first
order in H2, and third order overall.

8. TIPS 1:
• The orders are determined experimentally except for one case: An
elementary reaction.
• Elementary reactions are one step reactions which are the individual
steps in a mechanism. (For an elementary reaction only: the
balancing coefficients determine the order.) - Important
Example 1 for a multistep reaction: 1 CH2Br2 + 2 KI ---) 1 CH2I2 + 2 KBr
If experimentation found that m & n were both first order; then:
rate = k [CH2Br2 ]1 [KI]1
Example 2 for an elementary reaction: 2 O3 ---) 3 O2 (told it is elementary)
No need for experimentation; order comes from balancing coefficients:
rate = k[O3]2

9. TIPS 2:
A rate law is an equation that relates the rate
of a reaction to the concentration of reactants
(and catalyst) raised to various powers.
• As a more general example, consider the
reaction of substances A and B to give D and
E.
aA bB
C
dD eE
C catalyst
– You could write the rate law in the form
m
n
p
Rate k[A] [B] [C]
– The exponents m, n, and p are frequently, but not
always, integers. They must be determined
experimentally and cannot be obtained by simply
looking at the balanced equation.

10. Rate Law Question 1:
aA
cC
Explain the differences between differential rate equation and rate law
using the reaction above
SOLUTION:
Differential rate equation
Rate Law
relates the rate of disappearance relates the rate of reaction to
of reactants and the rate of
the rate constant and the
formation of products
concentrations of the reactants
raised to some power
1 d [ A]
1 d [C ]
rate = =
a dt
c dt
rate k[ A]
x

11. How the rate is affected by the reactant concentration
Rate Law Question 2:
aA
cC
If order with respect to A is zero and the rate constant is
5.1 x 10-4 molL-1s-1, calculate the rate of reaction
SOLUTION
rate k[ A]x
rate k[ A]0
rate k
4
1
rate 5.1x10 molL s
1

12. How the rate is affected by the reactant concentration
Rate Law Question 3:
The rate of a reaction increases eight-fold when the concentration
of one of the reactants is doubled. Determine the order of the
reaction with respect to that reactant.
ANSWER: 3

13. How the rate is affected by the reactant concentration
Rate Law Challenging Question 1:
If the order with respect to NO2- is zero and first order
respect to NH4+ , calculate the value of x when initial
concentration of NH4+ is 2 times higher than Exp 1.
Exp
Initial
concentration,
NH4+ (M)
Initial
concentration,
NO2- (M)
Initial rate,
( Ms-1)
1
2
0.0100
0.200
0.200
5.4 x 10-7
x
ANSWER: 10.8 x 10-7Ms-1

14. Rate Law Challenging Question 2:
Given the initial data measured for the reduction of nitric oxide with hydrogen,
answer the question:
What is the rate law for the reaction:
2NO(g) + 2H2(g)
N2(g) + 2H2O(g)
a) Use the data to determine the order of
the reaction with respect to:
b) Fill in the blanks to write a rate equation
for the reaction:

15. Rate Law Challenging Question 3:
a) Use the data to
determine the order of
the reaction with respect
to:
(a) thiosulphate
(b)acid
b) Write a rate equation for the reaction:

16. Rate Law Challenging Question 4:
For the reaction below, the rate equation is found to be
P2 (g)
+
2X2(g)
C(g)
At a temperature T, the rate of reaction is W. How does
the reaction rate change if
(a) only the partial pressure of P2 is doubled?
(b) the partial pressure of P2 and X2 is doubled?
(c) the partial pressure of P2 is halved and X2 is tripled?
Rate Law Outcomes Based Learning Question 1:
( must use the prior knowledge of Boyle’s Law)
(d) the volume of the vessel is doubled?
(e) the volume of the vessel is halved?
ANSWER:(a) 4W, (b) 8W, (c) 3/4W, (d) 1/8W, (e) 8W

17. Rate Law Outcomes Based Learning Question 2:
( must use the prior knowledge of differential rate equation)
The decomposition of N2O5 at 45oC is a first-order reaction.
2N2O5 (g)
4NO2 (g) + O2 (g)
Calculate the rate constant if the initial concentration of N2O5 is
1.5 x 10-3 M at a rate of decomposition of 8.6 x 10-7 Ms-1.
ANSWER: k = 2.87 x 10-4 s-1