CHEMICAL KINETICS CLASS- XII

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CHEMICAL KINETICSCLASS- XII

• VINAY KUMAR
• PGT CHEMISTRY
• KV NTPC KAHALGAON
• PATNA REGION

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• It is the branch of physical chemistry which
  deals with the study of the rate of a chemical
  reaction and the mechanism by the reaction occur.
• RATE OF THE CHEMICAL REACTION OR AVERAAGE RATE OF
  REACTION - it is the change in the
  concentration of reactant or product with time in
  which a chemical reaction proceed.
• Rate of reaction Decrease in the
  concentrationof R
• time taken
• Or Increase in
  the concentrationof P
• time taken
• Unit of rate is Mol L-1 S-1 or atm S-1 (For
  gaseous reaction)

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• Or Rate of reaction -??R ??P
  
• ?t ?t
• INSTANTANIOUS RATE OF REACTION- it is the rate
  of the reaction at the particular moment of time
  and measured as a very small concentration change
  over a very small time interval. ?t? 0 for a
  reaction R? P

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• Instantaneous rate -d?R d?P
• dt dt
• r inst. -d?R - slope of R
• dt
• r inst. d?P slope of P
• dt
• FACTORS AFFECTING THE RATE OF A CHEMICAL
  REACTION-
• Nature of reactant
• Concentration of reactant
• Temperature
• Surface area of reactant
• Radiation

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• GENERAL EXPRESSION FOR RATE OF REACTION-
• For a general chemical reaction
• aA bB ? cC dD
• Rav. -1 ??A -1 ??B 1 ??C
  1 ??D
• a ?t b ?t c
  ?t d ?t
• Rinst. -1 d?A -1 d?B 1 d?C
  1 d?D
• a dt b dt c dt
  d dt

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• RATE LAW- It is experimentally determined
  expression which relates the rate of reaction
  with the concentration of reactants.
• For a hypothetical reaction
• A B ? Products
• Rate ? ?Am ?Bn
• Rate k ?Am ?Bn
• Where k is the rate constant .
• If ?A ?B 1 Mol L-1 than Rate k
• Thus rate constant is the rate of reaction when
  concentration of each reactant in the reaction
  is unity.

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• ORDER OF REACTION- It may be defined as the sum
  of the power of the concentration of reactants in
  the rate law expression. Order of chemical
  reaction can be 1,2 or 3 and even may be
  fractional.
• MOLECULARITY OF REACTION- The total number of
  reacting species( molecules, atoms or ions)
  taking part in an elementary chemical reaction.
  The molecularity of a reaction may not be
  fractional.

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• INTEGRATED RATE LAW FOR ZERO ORDER CHEMICAL
  REACTION-
• Consider a general zero order reaction
• R ? P

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2
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• Comparing eq-2 with strait line equation
  y m x c , if we plot R against t we get a
  strait line with slope -k and intercept equal to
  R0.
• Further simplify equation 2 we can get the rate
  constant k

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• Half life for zero order reaction-

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• INTEGRATED RATE LAW FOR FIRST ORDER CHEMICAL
  REACTION-

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• Half life for the first order of reaction-

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• PSEUDO FIRST ORDER REACTION-

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• DETERMINATION OF ORDER OF REACTION-
• 1. Graphical Method-
• This method is applicable to those
  reactions wherein only one reactant is
  
  involved.
• 2. Initial rate Method-
• This method is used for those reactions where
  more than one reactant is involved.
• In this method we carried out some series of
  experiments.

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• We change the one reactants concentration and
  determine the rate of reactions by keeping the
  constant concentration of each other reactants
  and compare the rate from initial concentration
  rate.
• Similarly, we repeat the experiments for all
  other reactants and compare the rate from initial
  concentration rate and finally determine the
  overall rate of reaction.

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• 3. Integrated rate law Method-
• In this method we put the data of the reaction
  under investigation in all the integrated rate
  equation one by one .
• The expression which gives a constant value of
  rate constant decide the order of reaction.

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• Temperature dependence of a rate of a reaction-
• Most of the chemical reactions are accelerated
  by increase in temperature. For example, in
  decomposition of N2O5, the time taken for half of
  the original amount of material to decompose is
  12 min at 50oC, 5 h at25oC and 10 days at 0oC. We
  also know that in a mixture of potassiumpermangana
  te (KMnO4) and oxalic acid (H2C2O4), potassium
  permanganate gets decolourised faster at a higher
  temperature than that at a lower temperature.

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• It has been found that for a chemical reaction
  with rise in temperature by 10, the rate
  constant is nearly doubled.
• The temperature dependence of the rate of a
  chemical reaction can be accurately explained by
  Arrhenius equation.
• It was first proposed by Dutch chemist, J.H.
  vant Hoff but Swedish chemist, Arrhenius
  provided its physical justification and
  interpretation.
• k A e -Ea /RT 1

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• where A is the Arrhenius factor or the frequency
  factor. It is also called pre-exponential factor.
  It is a constant specific to a particular
  reaction. R is gas constant and Ea is activation
  energy measured in joules/mole (J mol 1).
• It can be understood clearly using the following
  simple reaction
• 2H2(g) I2(g)? 2HI(g)
• According to Arrhenius, this reaction can take
  place only when a molecule of hydrogen and a
  molecule of iodine collide to form an unstable
  intermediate.

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• It exists for a very short time and then breaks
  up to form two molecules of hydrogen iodide.
  According to Arrhenius, this reaction can take
  place only when a molecule of hydrogen and a
  molecule of iodine collide to form an unstable
  intermediate.
• It exists for a very short time and then breaks
  up to form two molecules of hydrogen iodide.

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• The energy required to form this intermediate,
  called activated complex (C), is known as
  activation energy (Ea). Reaction coordinate
  represents the profile of energy change when
  reactants change into products. Some energy is
  released when the complex decomposes to form
  products. So, the final enthalpy of the reaction
  depends upon the nature of reactants and products.

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• Ludwig Boltzmann and James Clark Maxwell used
  statistics to predict the behaviour of large
  number of molecules. According to them, the
  distribution of kinetic energy may be described
  by plotting the fraction of molecules (NE/NT)
  with a given kinetic energy (E) vs kinetic
  energy. Here, NE is the number of molecules with
  energy E and NT is total number of molecules.
• The peak of the curve corresponds to the most
  probable kinetic energy, i.e., kinetic energy of
  maximum fraction of molecules.

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• There aredecreasing number of molecules with
  energies higher or lower than this value. When
  the temperature is raised, the maximum of the
  curve moves to the higher energy value and the
  curve broadens out, i.e., spreads to the right
  such that there is a greater proportion of
  molecules with much higher energies.
• The plot of ln k vs 1/T gives a straight line.
  Thus, it has been found from Arrhenius equation
  that increasing the temperature or decreasing the
  activation energy will result in an increase in
  the rate of the reaction and an
  exponentialincrease in the rate constant.

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• slope Ea/ R and intercept ln A. So we can
  calculate Ea and A using these values. At
  temperature T1, equation (1) is
• ln k1 Ea/RT1 ln A (2)

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• At temperature T2 eq.(1) is
• (3)
• A is the constant for this particular reaction.
• K1 and k2 are the rate constant for the
  temperatures T1 and T2 respectively.
• Substracting eq(2) from eq(3)

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• (4)
• Effect of Catalyst on the rate of a chemical
  reaction-
• A catalyst is a substance which alters the rate
  of a reaction without itself undergoing any
  chemical change at the end of the chemical
  reaction. For example MnO2 increases the rate of
  decomposition of potassium chlorate to a great
  extent.

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• According to intermediate complex theory a
  catalyst participate in a chemical reaction by
  forming temporary bonds with the reactants
  resulting in a intermediate complex.
• This has a transitary existence and decompose to
  yield products and the catalyst.

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• Collision Theory of a Chemical Reaction-
• According to this theory the molecules of
  reactants are having sufficient kinetic energy so
  they may collide with each other and make product
  molecules.
• The number of collisions per second per unit
  volume of the reaction mixture is known as
  collision frequency (Z).
• A B ?Products
• rate of reaction can be expressed as
• Rate P ZAB e Ea/RT

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• Where ZAB the collision frequency of reactants
  A B.
• P Probability or steric factor.
• e Ea/RT fractions of molecules with energies
  equal to greater than Ea.