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JEE Main Mock Test Series 2020 & Previous Year Papers

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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).

     Notes | EduRev
     Notes | EduRev

  • 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
 Notes | EduRev
 Notes | EduRev
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.
 Notes | EduRev

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.
 Notes | EduRev
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:

   Notes | EduRev


  • 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.
 Notes | EduRev
 Notes | EduRev
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.
 Notes | EduRev
i.e
 Notes | EduRev
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.

 Notes | EduRev Notes | EduRev

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
 Notes | EduRev

 Notes | EduRev…....(i)

 Notes | EduRev…......(ii)

 Notes | EduRev….......(iii)
Integrating equation (i), we get
 Notes | EduRev
 Notes | EduRev   
 Notes | EduRev   
 Notes | EduRev 

Arrhenius Equation:
k = A exp(-Ea/RT)
Where, k = Rate constant
A = pre-exponential factor
Ea = Activation energy
 Notes | EduRev 
 Notes | EduRev

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
 Notes | EduRev
 Notes | EduRev
 Notes | EduRev
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
     Notes | EduRev

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.
 Notes | EduRev
Units: dps or Becquerrel

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