Chemical kinetics class xii
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CHEMICAL KINETICS CLASS- XII. VINAY KUMAR PGT CHEMISTRY KV NTPC KAHALGAON PATNA REGION. 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.

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Chemical kinetics class xii

CHEMICAL KINETICSCLASS- XII

VINAY KUMAR

PGT CHEMISTRY

KV NTPC KAHALGAON

PATNA REGION


  • 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)


Or Rate of reaction = study of the rate of a chemical reaction and the mechanism by the reaction occur.-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


Instantaneous rate = study of the rate of a chemical reaction and the mechanism by the reaction occur.-dR] = +dP]

dtdt

  • 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


  • GENERAL EXPRESSION FOR RATE OF REACTION:- study of the rate of a chemical reaction and the mechanism by the reaction occur.

    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


  • RATE LAW:- study of the rate of a chemical reaction and the mechanism by the reaction occur.It is experimentally determined expression which relates the rate of reaction with the concentration of reactants.

    For a hypothetical reaction

    A + B  Products

    Rate  A]m B]n

    Rate = k A]m B]n

    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.


  • ORDER OF REACTION:- study of the rate of a chemical reaction and the mechanism by the reaction occur.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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1

2


2 study of the rate of a chemical reaction and the mechanism by the reaction occur.

  • 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 [R]0.

  • Further simplify equation 2 we can get the rate constant k



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1 study of the rate of a chemical reaction and the mechanism by the reaction occur.

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7 study of the rate of a chemical reaction and the mechanism by the reaction occur.

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t

3




  • DETERMINATION OF ORDER OF REACTION:- study of the rate of a chemical reaction and the mechanism by the reaction occur.

    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.


  • We change the one reactant’s 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.


3. Integrated rate law Method:- the rate of reactions by keeping the constant concentration of each other reactants and compare the rate from initial concentration rate.

  • 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.


  • Temperature dependence of a rate of a reaction:- the rate of reactions by keeping the constant concentration of each other reactants and compare the rate from initial concentration rate.

    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 potassiumpermanganate (KMnO4) and oxalic acid (H2C2O4), potassium permanganate gets decolourised faster at a higher temperature than that at a lower temperature.


  • 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. van’t Hoff but Swedish chemist, Arrhenius provided its physical justification and interpretation.

    k = A e -Ea /RT 1


  • where temperature by 10°, the rate constant is nearly doubled.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.


  • 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.


  • 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.


  • 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.


  • 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.


  • slope = – 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.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)


  • At temperature T2 eq.(1) is 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.

    (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)


(4) 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.

  • 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.



  • Collision Theory of a Chemical Reaction:- participate in a chemical reaction by forming temporary bonds with the reactants resulting in a intermediate complex.

  • 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


  • Where Z participate in a chemical reaction by forming temporary bonds with the reactants resulting in a intermediate complex.AB = 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.


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