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Chemical Kinetics Class 12 Notes Chemistry Chapter 3

Rate of Reaction:

  • Rate of change of extent of reaction is the rate of reaction.
  • Rate of reaction is positive for product and negative for reactant.
  • For reaction aA →bB
    Rate =1/b(Δ[B]/ Δ t)  = -1/a (Δ [A]/ Δt)
  • It goes on decreasing as the reaction progress due to decrease in the concentration(s) of the reactant(s).

    Chemical Kinetics Class 12 Notes Chemistry Chapter 3
    Chemical Kinetics Class 12 Notes Chemistry Chapter 3

  • Unit of rate of reaction: mol L-1 s-1
  • The rate measured over a long time interval is called average rate and the rate measured for an infinitesimally small time interval is called instantaneous rate.
  • In a chemical change, reactants and products are involved. As the chemical reaction proceeds, the concentration of the reactants decreases, i.e., products are produced.
  • The rate of reaction (average rate) is defined as the change of concentration of any one of its reactants (or products) per unit time.

Order of Reaction
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Where m and n may or may not be equal to a & b.
m is order of reaction with respect to A and n is the order of reaction with respect to B.
m + n +… is the overall order of the reaction.

Elementary Reaction:

  • It is the reaction which completes in a single step.
  • A reaction may involve more than one elementary reactions or steps also.
  • Overall rate of reaction depends on the slowest elementary step and thus it is known as rate determining step.

Molecularity of Reaction:

  • Number of molecules taking part in an elementary step is known as its molecularity.
  • Order of an elementary reaction is always equal to its molecularity.
  • Elementary reactions with molecularity greater than three are not known because collisions in which more than three particles come together simultaneously are rare.
Chemical Reaction
Molecularity
PCl → PCl3 + Cl2  
Unimolecular
2HI  → H2 + I2 
Bimolecular
2SO2 + O → 2SO3
Trimolecular
NO + O → NO2 + O2
Bimolecular
2CO + O2  →  2CO2
Trimolecular
2FeCl3 + SnCl2 → SnCl2 + 2FeCl2
Trimolecular
 


Differential and Integrated Rate Laws:
Zero Order Reactions:
For Reaction: A → Product
[A]0-[A] = k0t
Where,
[A]0 = Initial concentration of A
[A]t = Concentration of A at time t.  
k0  =  Rate constant for zero order reaction.
Chemical Kinetics Class 12 Notes Chemistry Chapter 3

Half Life:
t1/2 = [A]0/2k
Unit of rate constant = mol dm-3s-1

Examples: 

  •  Enzyme catalyzed reactions are zero order with respect to substrate concentration.
  •  Decomposition of gases on the surface of metallic catalysts like decomposition of HI on gold surface.

First Order Reactions:
A → Product
(Δ [A] /A) = -k1Δt
or k1=( 2.303/ t)log ([A]0 / [A]t

Half Life:
t1/2 = 0.693/k1
Half life is independent of the initial concentration of the reactant for a first order
reaction.
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Units of k=  s-1
Examples:
N2O5 →  2NO2 + 1/2O2
Br2 → 2Br
2HNO3 → 2NO + H2O
H2O2→ H2O + 1/2O2 

Pseudo First Order Reactions:
These are the reactions in which more than one species is involved in the rate determining step but still the order of reaction is one.

Examples:

  • Acid hydrolysis of ester: CH3COOEt + H3O→CH3COOH + EtOH 
  • Inversion of cane sugar:

  Chemical Kinetics Class 12 Notes Chemistry Chapter 3


  • Decomposition of benzenediazonium halides C6H5N=NCl +H2O → C6H5OH +N2 +HCl

Half – Life of a nth Order Reaction:
kt1/2 =  (2n-1-1)/(n-1)[A0]n-1
Where, n = order of reaction ≠1

Parallel  Reactions:
The reactions in which a substance reacts or decomposes in more than one way are called parallel or side reactions.
If we assume that both of them are first order, we get.
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
k1 = fractional yield of B × kav
k2 = fractional yield of C × kav
If k1 >  k2 then
A → B main and
A → C is side reaction
Let after a definite interval x mol/litre of B and y mol/litre of C are formed.
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
i.e
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
This means that irrespective of how much time is elapsed, the ratio of concentration of B to that of C from the start (assuming no B  and C in the beginning ) is a constant equal to k1/k2.

Chemical Kinetics Class 12 Notes Chemistry Chapter 3Chemical Kinetics Class 12 Notes Chemistry Chapter 3

Sequential Reactions:
This reaction is defined as that reaction which proceeds from reactants to final products through one or more intermediate stages. The overall reaction is a result of several successive or consecutive steps.
A → B → C and so on
Chemical Kinetics Class 12 Notes Chemistry Chapter 3

Chemical Kinetics Class 12 Notes Chemistry Chapter 3…....(i)

Chemical Kinetics Class 12 Notes Chemistry Chapter 3…......(ii)

Chemical Kinetics Class 12 Notes Chemistry Chapter 3….......(iii)
Integrating equation (i), we get
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Chemical Kinetics Class 12 Notes Chemistry Chapter 3   
Chemical Kinetics Class 12 Notes Chemistry Chapter 3   
Chemical Kinetics Class 12 Notes Chemistry Chapter 3 

Arrhenius Equation:
k = A exp(-Ea/RT)
Where, k = Rate constant
A = pre-exponential factor
Ea = Activation energy
Chemical Kinetics Class 12 Notes Chemistry Chapter 3 
Chemical Kinetics Class 12 Notes Chemistry Chapter 3

Temperature Coefficient: 
The temperature coefficient of a chemical reaction is defined as the ratio of the specific reaction rates of a reaction at two temperature differing by 10oC.
μ = Temperature coefficient= k(r+10)/kt
Let temperature coefficient of a reaction be ' μ ' when temperature is raised from Tto T2; then the ratio of rate constants or rate may be calculated as
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Its value lies generally between 2 and 3.

Collision Theory of Reaction Rate

  • A chemical reaction takes place due to collision among reactant molecules.
  • The number of collisions taking place per second per unit volume of the reaction mixture is known as collision frequency (Z).
  • The value of collision frequency is very high, of the order of 1025 to 1028 in case of binary collisions.
  • Every collision does not bring a chemical change.
  • The collisions that actually produce the products are effective collisions.
  • The effective collisions which bring chemical change are few in comparison to the form a product are ineffective elastic collisions, i.e., molecules just collide and
  • disperse in different directions with different velocities.
  • For a collision to be effective, the following two barriers are to be cleared.
  1. Energy Barrier
  2. Orientation Barrier
    Chemical Kinetics Class 12 Notes Chemistry Chapter 3

Radioactivity:
All radioactive decay follow 1st order kinetics
For radioactive decay A →B
-(dNA/dt) =l NA
Where, l =  decay constant of reaction
NA  = number of nuclei of the radioactive substance at the time when rate is calculated.
Arrhenius equation is not valid for radioactive decay.
Integrated Rate Law: N= Noe-lt
Half Life:  t1/2= 0.693/λ
Average life time: Life time of a single isolated nucleus, tav= 1/λ
Activity: Rate of decay
A = dNA/dt, Also, At = Aoe-lt
Specific Activity: activity per unit mass of the sample.
Chemical Kinetics Class 12 Notes Chemistry Chapter 3
Units: dps or Becquerrel

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FAQs on Chemical Kinetics Class 12 Notes Chemistry Chapter 3

1. What is chemical kinetics?
Ans. Chemical kinetics is the branch of chemistry that studies the rate at which chemical reactions occur and the factors that influence these rates. It involves studying the speed of reactions, the mechanisms by which they occur, and how different factors such as temperature, concentration, and catalysts affect the reaction rate.
2. How is reaction rate determined in chemical kinetics?
Ans. The reaction rate in chemical kinetics is determined by measuring the change in concentration of reactants or products as a function of time. This can be done by monitoring the absorbance of light, measuring the volume of gas produced, or analyzing the concentration using techniques such as spectroscopy or chromatography.
3. What are the factors that affect the rate of a chemical reaction?
Ans. Several factors can influence the rate of a chemical reaction, including temperature, concentration of reactants, surface area of reactants, presence of a catalyst, and the nature of the reactants. Increasing the temperature, concentration, and surface area, as well as the addition of a catalyst, generally increases the reaction rate.
4. How does temperature affect the rate of a chemical reaction?
Ans. Temperature has a significant effect on the rate of a chemical reaction. As temperature increases, the kinetic energy of the particles involved in the reaction also increases. This leads to more frequent and energetic collisions between the reactant molecules, resulting in a higher reaction rate. In general, the rate of a reaction doubles for every 10-degree Celsius increase in temperature.
5. What is the role of a catalyst in chemical kinetics?
Ans. A catalyst is a substance that speeds up a chemical reaction by providing an alternative reaction pathway with lower activation energy. It does not get consumed in the reaction and does not appear in the overall balanced chemical equation. Catalysts increase the reaction rate by lowering the energy barrier for the reaction to occur. They are highly specific to the reaction they catalyze and can significantly increase the efficiency of industrial processes.
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