Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

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Chemistry : Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

The document Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev is a part of the Chemistry Course Physical Chemistry.
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CHEMICAL KINETICS

“Chemical kinetics involves the study of the rates and mechanism of chemical reactions.” The rates of reactions:

(a) The definition of rate: Consider a reaction of the form
A + 2B  → 3C + D.............(1)
in which the molar concentration of participants are [A], [B], [C] & [D].
The rate of consumption or decomposition of the one of the reactants at a given time is Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev where R is A or B. The rate of formation of one of the products is  Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRevwhere P is C or D.
The rate of reaction can be expressed with respect to any species in equation (1).
 

Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

Thus, the rate of reaction can be defined with respect to both reactants and products.
For example:
4NO2 (g) + O2 (g)  → 2N 2O2 (g)
find the expression for rate of reaction.
 Sol. 
4NO2 (g) + O2 (g)  → 2N 2O2 (g)

Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

(b)  Rate laws and rate constant: The rate of a react ion will generally depends on temperature pressure and concentration of species involving in the reaction.
The rate of reaction is proportional to the molar concentration of reacting species.
i.e. A + B + C + D + ……. → Product
then, rate of reaction = k[A][B]b [C]c [D]d ………..

where [A] is the concentration of reactant A, [B] is the concentration of reactant B and so on. The constant a is known as the reaction order with respect to species A, b the reaction order with respect to species B and so on.
The overall reaction order is equal to the sum of the individual reaction orders (a + b + c + d + ……..). Finally the constant k is rate constant for the reaction.
The rate constant dependent on concentration but also on temperature & pressure.

This relationship is known as a rate law. 

(c) Order of the reaction:

A + B + C + ………  → Product
The rate law = v = k[A]a [B]b [C]c ……..
The order of reaction = a + b + c + ……
For example: if rate law = v = k[A]1/2 [B]
Then, it is half order in A, first order in B and three half Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRevorder overall.

Molecularity of a Reaction : The number of reacting species (atoms, ions or molecule) taking parting an elementary reaction, must collide simultaneously, in order bring about a chemical reaction is called molecularity of a reaction.
Relationship between Rate law, order and the rate constant:

Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

Then, rate of react ion =  Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

The unit of rate or reaction is mol liter–1 sec–1 i.e. mol L–1 s–1.
where M represent mol L–1 or mo les per liter & n is order of reaction.
The unit of rate constant (k)
Rate of reaction = k[A]n
unit of rate of reaction = unit of k × [unit of concentration]
MS–1 = unit of k × [M]n

 ⇒  unit of k = Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

i.e., unit of k     = M1 - n S-1 = mol1 - n Ln - 1 S-1

 Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

Problem. Find the order of the reaction if unit of rate constant or the reaction is (dm3)3/2 mol–3/2 s–1.
 Sol. 
Unit of rate constant = (dm3)3/2 mol–3/2 s–1 (given)
We know that,
Unit of rate constant = M1 – n s–1
For nth order
i.e. M1 - n s -1 = (dm3 )3 / 2 (mol)-3 / 2 s -1

 Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

∵      1 L = 1 dm3
&  Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

 Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev
Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

i.e. it is 5/2 order reaction.
 

Determine Reaction order: 

Using the following data for the reaction, we determine the order of the reaction with respect to A and B, over all order and rate constant for the reaction

 Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

Sol. A + B  → Product
rate of react ion = k[A]a [B]b
5.25 × 10–4 = k[2.30 × 10–4]a [3.10 × 10–5]b ...(1)
4.20 × 10–3 = k[4.60 × 10–4]a [6.20 × 10–5]...(2)
1.70 × 10–2 = k[9.20 × 10–4]a [6.20 × 10–5]b ...(3)

Divide equation (2) by equation (3), we get

 Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev

2.47 × 10–1 = (0.5)a
(0.247) = (0.5)a
(0.5 × 0.5) ≈ (0.5)a
(0.5)a ≈ (0.5)a
or taking log we can find the value of a.
a = 2
Divide equation (1) by equation (2) we get

 Introduction to Chemical Kinetics - Chemical Kinetics Chemistry Notes | EduRev
1.25 × 10–1 = [0.5]a [0.5]b = [0.5]2 [0.5]b
= 0.25 [0.5]b
5 × 10–1 = [0.5]b
⇒ 0.5 = [0.5]b
⇒ b = 1 

Therefore, the reaction is second order in A and first order in B and third order overall.

rate = k[A]2 [B]
5.2 × 10–4 Ms–1 = k(2.3 × 10–4 M)2 (3.1 × 10–5]M
⇒ k = 3.17 × 108 M–2 s–1
i.e. the over all rate law is 
rate = (3.17 × 108 M–2 s–1) [A][B]

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