Chapter - 1 Chemical Kinetics - PowerPoint Presentation

1. Chapter - 1Chemical KineticsChemical kinetics is concerned with the rate and mechanism of chemical change. Homogeneous reaction:- A reaction which occurs entirely in one phase.Heterogeneous reaction:- A reaction which occurs two or more phases.Homogeneous reactions divided in gas phase reaction and in liquid(solution).Here we are study the kinetics of Homogeneous reaction.

2. Rate of Chemical Reaction:-The rate of chemical reaction the speed or the velocity of the reaction.It is defined as the change in concentration of reactants or products.In a chemical reaction concentration of reactant decreases while that of products increases with time.

3. Consider a general reaction A  BAs this reaction proceeds concentration of the reactant A decreases while that of products B increases Rate of reaction = rate of disappearance of A A = - d[A] / dt OR Rate of reaction = rate of appearance of B B = d[B] / dt Where [A] and [B] are the concentrations of reactant A and product B

4. Rate of a reactionThe rate of a reaction is defined as the decrease in concentration of a reactant in unit time or increase in concentration of a product in unit timeThe unit of rate are expressed in moles per litre per second ( mol /lit/s) for reaction in solution and moles per cubic centimeter per second ( mol / cm3/s) for a gaseous reaction.

5. Law of Mass action and Rate Law:-The dependence of rate of the reaction on reactant concentration at a given temperature, is given by rate law of mass action.Statement:- “The rate of a chemical reaction at any instant is proportional to the molar concentration of the reacting substance at that instant”.

6. Consider a simple reaction A  ProductIf [A] is the concentration of the reactant at time t, then according to law of mass action we can write.Rate of the reaction at time t α [A] ----1Rate = - d[A] / dt α [A] ---- 2 Equation which shows how the reaction rate is related to concentration is called rate law or rate equation.Eq. 2 is a differential form of the rate equation

7. If two or more reactants are involved in the reaction, the rate equation takes a complicated form.For example consider the following reaction A + B  product A + 2B  products The rate equation for above reaction are Rate = - d[A] / dt α [A] x [B] ------3 Rate = - d[A] / dt α [A] x [B]2 ------4

8. Let us consider a general reaction aA + bB  product The rate r at time t is experimentally found to be related to the concentration of species present at that time by an equation r = k[A]a x [B]b ------- 5 where the exponent a, b are usually integers or half integers ( ½, 3/2 ----). It is taken that the power to which a concentration should appear in this predicted rate law is the same as the number of molecules of that substance involved in the chemical equation.

9. The following experimental results show that the law of mass action does not always hold.1) 2HI  H2 + I2 r = k[HI]22) NaOH +CH3Br  CH3OH + NaBr r = k[NaOH] [CH3Br]3) CH3CHO  CH4 + CO r = k[CH3CHO ]3/2 4) 2N2 O5  2N2 O4 + O2 r = k[N2 O5 ]5) 2KClO  KClO3 r = k[KClO]

10. Rate law for reactions 1 and 2 are as predicted form law of mass action. Where as for reaction 3,4 and 5 are not as predicted. This means that the exponents in the rate law can differ the coefficients in the balanced chemical equation. Rate law must be determined from measurements of reaction rates and cannot be deducted from the reaction stoichiometry.

11. Rate Constant and its Significance:-The proportionality constant k in rate equation is called as specific velocity constant or rate constant or specific reaction rate. The rate constant is characteristic of the reaction and is different for different reaction. It depends on temperature and it usually increases with the rise in temperature.

12. To define the rate constant consider equation - d[A] / dt α [A]The equation can be written as - d[A] / dt = k[A] ------- 1k in Eq. 1 is the rate constant. when the concentration [A] of the reactant is unity, the rate of the reaction is equal to the rate constant.Thus the rate constant is numerically equal to the rate of the reaction when all the reactants are at unit concentration.

13. Order of Reaction:- It as an experimentally determined quantity. It is obtained from the rate equation applicable to the reaction.Consider a reaction that involve several reactants A, B, C. Let the rate equation applicable to the reaction be Rate = k[A]n1 x [B]n2 x [C]n3 -------1 The rate of the reaction in Eq. 1 indicates is proportional to n1th power of the concentration of reactant A, that is [A], to the n2th and n3th powers of the reactants B and C, that is, [B] and [C] respectively.

14. The reaction is said to be of order n1 with respect to the reactant A. similarly it is of the order n2 and n3 with respect to reactant B and C respectively. The overall order of the reaction ‘n’ is given by the Eq. n = n1 + n2 + n3 ------2Definition:- The order of the reaction with respect to a particular reactant is equal to the power to which the concentration of that reactant is raised in the experimentally determined rate equation. Or Order is define as sum of the powers of the concentrations of various reactants involved in the rate equation.

15. The experimentally determined rate law for the reaction. 2N O + O2  2N O2Which takes place in the gas phase at the room temperature and atmospheric pressure is -d[O2] / dt = k[NO]2 [O2]The reaction is third order from the rate equation. 3KClO  KClO3 + 2KCl d[KClO3] / dt = k[KClO]3The reaction is third order from the rate equation

16. Molecularity of a Reaction:-Molecularity of a Reaction:- It is a theoretical concept and is based on the mechanism of the reaction. Each step in the mechanism of the reaction is called an elementary reaction.“The number reactant molecules that are involved in elementary step represents the molecularity of an elementary reaction”. CH3CHO  CH4 + CO molacularity = 12HI  H2 + I2 molacularity = 2NaOH +CH3Br  CH3OH + NaBr molacularity = 23KClO  KClO3 + 2KCl molacularity = 3

17. On the basis of molecularity the elementary reactions are classified as unimolecular (involving one molecule), bimolecular (involving two molecules)or termolecular (involving three molecules).The order of the reaction may be zero or even fractional.The molecularity of the reaction can never be zero and fractional.

18. Difference between order and molecularity:- Sr. No Order of a reaction Molecularity of a reaction 1It is sum of powers of concentrations of reactants involved in the rate equationIt is the number of reactants molecules involved in the elementary step reaction  2It is an experimentally determined propertyIt is an theoretical property 3It can be fractional and can be zeroIt cannot be fractional and cannot be zero 4It can be changed with experimental conditions like concentration, pressure.It is (fixed for a reaction) independent of experimental conditions of the reaction 

19. Integrated Rate law Expression Integrated Rate law Expression: Integrated rate law equation gives the concentration of reactants at any time t, during the course of reaction.Mathematical expression for Zero-order reaction:Zero order reaction is uncommon, but some heterogeneous reaction has order of zero. The rate of zero order reaction remains constant and it does not change with time, as it is independent of the concentration of the reactant.Rate = constant = ko ------ 1ko is the rate constant of zero order reactionIf CA is the concentration of a reactant A in a zero order reaction, at time t, we can write Eq. 1 Rate = - = ko ------ 2 

20. Integrated Rate law Expression: Integrated rate law equation gives the concentration of reactants at any time t, during the course of reaction.Mathematical expression for Zero-order reaction:Zero order reaction is uncommon, but some heterogeneous reaction has order of zero. The rate of zero order reaction remains constant and it does not change with time, as it is independent of the concentration of the reactant.Rate = constant = ko ------ 1ko is the rate constant of zero order reactionIf CA is the concentration of a reactant A in a zero order reaction, at time t, we can write Eq. 1 Rate = - = ko  

21. If ‘a’ mole is the initial concentration of A that is the concentration at the beginning of the reaction and if ‘x’ mole react in time t will be (a-x) mole. This is CA. substituting the value of CA in Eq. 2 we get, - = - (a-x) = - = ko ------ 3 Eq. 3 can be integrated between the limits t = 0 to t = t for time and x = 0 to x = x for ‘x’.Rearranging of Eq. 3 gives,dx = ko dt ------- 4Integrating Eq. 4 between the above limits, we get, = = ko ------ 5ko is constant at a constant temperature, is taken out of the integral sign.[x]ox = ko [t]ot x = ko t ------- 6Eq. 6 is the integrated form of the rate constant for a zero order reaction. The equation can be used to calculate ko.  

22. Integrating Eq. 4 between the above limits, we get,==ko ------ 5 ko is constant at a constant temperature, is taken out of the integral sign. [x]ox = ko [t]ot x=kot -------- 6 Eq. 6 is the integrated form of the rate constant for a zero order reaction. The equation can be used to calculate ko.