class 12 chemicalkinetics.pptx

Class 12 Chemistry- Chemical Kinetics

Notes with Explanation.

1. CHEMICAL KINETICS

2. CHEMICAL KINETICS: definition B ranch of chemistry which deals with the rate of chemical reactions. the factors affecting the rate of reactions and the mechanism of the reaction is called chemical kinetics.

3. Chemical Reactions on the Basis of Rate of Reaction 1. Fast reactions :-Chemical reaction which get completed in 10 - 16 to 10 -14 second ,are known as fast reaction. Example. ionic reactions 2. Slow reactions: - Chemical reactions which completes in a long time from some minutes to some years are called slow reactions. e.g., rusting of iron transformation of diamond etc. 3 . Moderately slow reactions :- Chemical reactions which are intermediate between slow and fast reactions are called moderately slow reactions. Example , Inversion of sugar ,hydrolysis of starch

4. Average rate of reaction The change in the concentration of any of the reactant or product per unit time over a specified interval of time is called average rate of reaction .

5. Similarly,

6. I nstantaneous rate of reaction The change in the concentration of any of the reactant or product at a particular instant of time is called instantaneous rate of the given reaction at that instant of time .

7. Factors Affecting the Rate of Reaction Nature of the reactant :- the rate of the chemical reaction is influenced by the nature of reactant . The ionic reaction are quiet fast whereas the reaction involving the covalent compound are slower . C oncentration on reactant :- The rate of reaction increases on increasing the concentration of the reactant . Temperature :-temp increases reaction rate increases. Surface Area of the Reactants :- Increase in the surface area provide more opportunity for the reaction to come in contact or collide resuting in increased reaction rate

8. Rate law The expression which describe the reaction rate in term of molar concentration of the reactant as determined experimentally is called rate law.

9. The rate constant of the reaction is equal to the rate of reaction when the concentration of each of the reactant is unity .

10. ORDER OF THE REACTION  Sum of the power of concentration. Rate=k [ A] m [B] n • The order of such a reaction is (m + n). • Order of reaction:- positive, negative, zero and fractional. • Example: H 2 + I 2 2 HI reaction order= 1+1= 2 • Reaction order is determined by experiment.

11. For the reaction: aA + bB ⟶ P

12. For example, in above reaction:- if the reaction is first order with respect to both  A  and  B  (a = 1 and b = 1), then overall order is 1+1=2. We call this an overall second order reaction. 2. if the reaction is first order with respect to A  and zero with respect to B  (a = 1 and b = 0), then overall order is 1+0=1. We call this an overall first order reaction. 3. if the reaction is first order with respect to A  and two for B  (a = 1 and b = 2), then overall order is 1+2=3 We call this an overall third order reaction.

13. Molecularity It is defined as the minimum number of molecules, atoms or ions of the reactants(s) required for the reaction to occur. It is equal to the sum of the stoichiometric coefficients of the reactants in the chemical equation of the reaction.  Note:- In general, molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation.

14. Chemical Reaction Molecularity PCl 5   →  PCl 3  + Cl 2     Unimolecular 2HI  →  H 2  + I 2   Bimolecular 2SO 2  + O 2   →  2SO 3 Trimolecular NO + O 3   →  NO 2  + O 2 Bimolecular 2CO + O 2   →  2CO 2 Trimolecular 2FeCl 3  + SnCl 2  → SnCl 2  + 2FeCl 2 Trimolecular

15.  Molecularity Order  It represents the number of the reactant molecules taking part in the elementary reaction.  It represents the sum of the exponents to which the concentration term in the rate law are raised.  It is theoretical property and indicates the number of reactant molecules involved in each act leading to the reaction. It is an experimental property and indicates the dependence of observed reaction rate on the concentration of the reactants.  It is always an integer and can never be fraction or zero. It may be an integer, fraction or zero. DIFFERENCE BETWEEN MOLECULARITY AND ORDER OF THE REACTION

16. ZERO ORDER REACTION  reaction is said to be of zero order if its rate is independent of the concentration of the reactants,

17. INTEGRATED EQUATIONA AND GRAPH

18.  Reaction is said to be first order if its rate is determined by the change of one concentration term only. FIRST ORDER REACTION Consider the reaction, A → products Let [A] O    = Initial Concentration of A, [A] T  = The concentration of A after time t For  the reaction to be of  first order.. The differential equation describing first-order kinetics is given below: Rate=−d[A]/ dt =k[A]……………………………………………….(1)                                                 

19. For a first-order reaction, the rate constant can be mathematically expressed as follows: k=2.303tlog[A]0/[A] From the definition of reaction half-life, at t = t 1/2 , [A] = [A] /2. Substituting these values in the expression for the first-order rate constant, the following equation is obtained: k=2.303t1/2log[A0]/([A]0/2) Rearranging the expression to find the value of t 1/2 : t1/2=2.303klog(2)=0.693k Thus, the half-life of a first-order reaction is given by 0.693/k. HALF LIFE TIME OF FIRST ORDER REACTION

20. Examples: a. Hydrolysis of an ester : CH3COOC2H5 + H2O(Large excess) → CH3COOH + C2H5OH The rate law for the reaction can be written as Rate =K[CH3COOC2H5][H2O] --Since water is present in large excess, its concentration remains practically constant during the course of the reaction. Thus above rate law can be written as Rate = K’[CH3COOC2H5] . The reaction is actually second order but in practice it follows first- order kinetics. Thus, it is a pseudo-first order reaction .

21. TEMPERATURE AND RATE OF REACTION The rate constant is nearly  doubled  for a chemical reaction with a rise in temperature by 10°. Until 1889, there was no fixed way to physically measure the  temperature dependence  of the  rate  of a chemical  reaction . In 1889, Svante Arrhenius give an equation called Arrhenius equation.

22. ARRHENIUS EQUATION It was first proposed by Dutch chemist, J.H. van’t Hoff but Swedish chemist, Arrhenius provided its physical justification and interpretation. Where, A = Arrhenius factor or the frequency factor. pre-exponential factor. It is a constant specific to a particular reaction. R is gas constant Ea is activation energy

23. EXPLANATION OF ARREHENIUS EQUATION According to the Arrhenius equation, a reaction can only take place when a molecule of one substance collides with the molecule of another to form an unstable intermediate. This intermediate exists for a very short time and then breaks up to form two molecule of the product. The energy required to form this intermediate is known as activation energy (E a ). Threshold energy (ET)  The minimum amount of energy which the reactant must possess in order to convert into products is known as threshold energy. Activation energy (E a ) The additional amount of energy, required by the reactant so that their energy becomes equal to the threshold value is known as activation energy.

24. ( i ) If k 2  and k 1  are rate constant at temperature T 2  and T 1 ; then Important points about Arrhenius equation (ii)E a  is constant for a particular reaction. (iii) E a  doesn't depend on temperature, volume, pressure, etc., but gets affected by catalyst. QUES:-Why does the rate of a reaction increase when the temperature increases? Solution : When the temperature increases, the fraction of molecules that have kinetic energies more than the activation energy of the reaction increases. Therefore, the total activation energy of the reaction decreases. Thus, the rate of the reaction increases.

25. EFFECT OF CATALYST ON ACTIVATION ENERGY A catalyst provides an alternative route for the reaction with a lower activation energy. The action of the catalyst can be explained by intermediate complex theory. According to this theory, a catalyst participates in a chemical reaction by forming temporary bonds with the reactants resulting in an intermediate complex. This has a transitory existence and decomposes to yield products and the catalyst.

26. COLLISION THEORY According to this theory, the reactant molecules are assumed to be hard spheres and the reaction is postulated to occur, when molecules collide with each other. The number of collisions between the reacting molecules taking place per second per unit volume is known as collision frequency (Z AB )· But only those collisions in which the colliding species are associated with certain minimum amount of energy and collide in proper orientation result in the product formation, such collisions are called  effective collision. It is given by:- Rate = Z AB  e  -E a /RT    Where, Z AB  represents the collision frequency of reactants A and B and e  –E a /RT   represents the fraction of molecules with energies equal to or greater than the activation energy of the reaction. 

27. OVERALL VIEW

28. ASSIGNMENT WORK Q.1 what is the difference between order and molecularity. Q.2 What is the unit of rate constant of second order reaction. Q.3 what is Arrehenious equation. Q.4 Define:- collision theory and arrehenious factor. Q.5 write the integrated equation of first order reaction. Q.6 how is temperature depends upon rate of reaction.

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