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Chemical kinetics Chemical kinetics: a branch of chemistry which deal about the rate of reaction. Rate of reaction: The change in concentration of reactant.

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Presentation on theme: "Chemical kinetics Chemical kinetics: a branch of chemistry which deal about the rate of reaction. Rate of reaction: The change in concentration of reactant."— Presentation transcript:

1 Chemical kinetics Chemical kinetics: a branch of chemistry which deal about the rate of reaction. Rate of reaction: The change in concentration of reactant or product in a given interval of time is called rate of reaction. Rate of reaction=change in conc. of reactant Time interval

2 A+B C+D Rate=-[d A]\dt = -[dB]\dt =[dC]\dt =[dD]/dt

3 Factor affecting the rate of reaction. Concentration of reactants: The rate of reaction is directly proportional to the conc. Of reactant. The effect of conc. of reactant is also given by law of mass action. 2 Nature of reactants: The nature of reactant is determined by its inner composition thus for ionic reaction. The ratio is very high while if the reactant not strong chemical bond the rate of reaction less

4 Temperature: the rate of chemical reaction increase with increase in temperature generally rate of reaction increase twich as the temperature is increased by 10k such effect of temperature is called temperature coefficient. Catalyst: A catalysts is a substance which do not participate in a reaction but alter the rate of reaction. Law of Mass action: according to this law the rate of chemical reaction is directly preoperational to the product of the molar conc. Of the reactants with each term raised to the power equal to its coefficient aA +bB Product rate [A

5 Characteristic of rate constant For a particular reaction the rate constant is independent of conc. The rate constant only depends upon temperature. The value of K is different for different reaction. Greater the value of K greater will be the rate of reaction A + B C + D K1 C + D A + b K2 K2=1\K1

6 If two chemical reaction are added than the rate constant are multiply If two reaction is subtracted then the rate constant are divided. If a chemical reaction is multiplied by n than rate constant than K2 =K1n Molecularity: Number of molecules which participate in the elementary state in any chemical reaction is called molecularity. On the basis of moleculatity reaction are three type.

7 Unimolecular reaction: O 3 O 2 +[O] N 2 O 5 N 2 O 4 +O 2 \2 Bimolecular reaction: 1HI H2+ I2 NO+ O3 No2+ O2 Trimolecular reaction: 2NO +O 2NO2 2SS2+O2 2So3 In case of complex reactions The molecularity of slowest step is considered 1

8 Order of reaction: The sum of all exponent of concentration of reaction which appear in differential rate equation is know as order of reaction it is denoted by n. Order of reaction is an experimental value its fractional value are possible.it zero value is also possible it provided information about machanism of reaction. aA+bB+cC=product

9 Zero order reaction : in this reactions the rate of reaction does not depend upon conc. Of reaction. 2NH3 N2 +3H2 rate = K[NH3]0 First order reaction: All radioactive changes Dissociation of ammonium nitrate NH4NO3 N2O +2H2O rate = k[NH4NO3]1 Decomposition of hydrogen peroxide H2O2 H2O +O

10 Rate = k H2O2] Second order reaction: Desociation of hydrogen iodide 2HI H2 + I2 Rate =k[HI]2 Desociation of nitrogen peroxide 2NO2 2NO+ O2 RATE = K[NO2] Third order reaction Reaction between nitric oxide and oxygen 2NO +O2 N2O4 RATE =K[NO]2[O2]1

11 Soudo unimolecular reaction or Soudo first order reaction Such reaction in which the molar concentration of a reactant remain partially unchanged and the small change in conc. Does not effect the rate of reaction are called soudo unimolecular reactions. Thus, a reaction which is first order reaction but it look like second order reaction known as soudo unimolecular reaction. Example:- hydrolysis of ester CH 3 COOC 2 H 5 +H 2 O CH 3 COOH +C 2 H 5 OH Rate = K[CH 3 COOC 2 H 5 ] =

12 Hydrolysis of Sucrose:- C 12 H 22 O 11 +H 2 O C 6 H 12 O 6 +C 6 H 12 O 6 Rate = k[C 12 H 22 O 11 ] Units of rate constant for different order of reaction rate = k[concentration] Unit of k= moles/liter.sec For first order reaction rate =k[conc.]1 Unit of k =1\sec For second order reaction rate = k[conc.]2 Unit of k = liter\moles.second

13 For third order reaction Rate = k[conc.]3 Unit of k =liter2\moles2.sec * Rate constant for the first order reaction A product rate =k[A]1 consider initial conc. Of reaction be ‘a’ after ‘t’ time ‘x’ amount of product is formed Thus the conc. Of reactant after ‘t’ time equal to (a-x).thus k = 1\t*log a\a-x

14 k =2.303 log 10 a t a-x Half life period:- The time in which half of the reaction is called half life period. It is denoted by ‘t 1\2 ‘ For the half order reaction k =2.303 log 10 a t a-x if t =t 1\2 a-x = a\2

15 k =2.303 log 10 a log102= k =2.303*0.3010\t 1\2 k =0.693\t 1\2 t 1\2 =0.693\k Expression for rate constant of first order reaction A product Rate =-[dA]\dt rate = k[a] -[dA]\dt = k.[a] t 1\2 a\2

16 -[dA]\dt = k.dt on integrating the above equation -log A =kt +c [integration constant] At t=0, A = A 0 -log e A 0 = c -log e A =kt -loge A 0 log e A 0 -loge A =kt log e A 0 \A =kt k =1\t log e A 0 \A k = 2.303\t.log 10 A 0 \A

17 Effect of temperature on rate constant OR Arrhenius equation Arrhenius gave a relation between rate constant and temperature which is known as Arrhenius equation. According to this equation k = A.e –Eq/RT where, k = rate constant A = frequency factor E A = Activation energy R = Universal gas constant

18 T =Temperature in Kelvin on taking log both the side log e K = loge A.e –Eq/RT log e K = loge A + loge e –Ea/RT log e K = loge A -Ea/RT loge e log e e = 1 log e K =log e A - Ea/RT log 10 K = log 10 A - Ea/RT log1 K = log10 A - Ea/2.303 RT for a reaction, the rate constant at t 1 temperature, while at t 2 temperature it will be K 2 (T 2 > T 1 ) than,

19 log 10 K 1 = log 10 A -Ea/2.303 RT (1) log 10 K 2 = log 10 A - Ea/2.303RT (2) on subtracting (1) from (2) log 10 K 1 - log 10 K 2 =Ea/2.303 RT 2 -Ea/2.303 RT 1 log 10 K 1 /K 2 = Ea/2.303R [1/T 2 - 1/T 1 ] *Theracehold Energy : The minimum amount of energy which is required by molecules of reactant for the fruitful collision is called teracehold energy. *Energy barrio : The amount of energy at which molecules of reactant collides and converted into product is called energy barrier

20 Activation energy : The amount of energy which is required by the molecules of reactant to cross the energy Barrier is called activation energy. Activation energy = Theracehold energy - Energy of reactant


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