UNIT - 4 CHEMICAL KINETICS 1. CHEMICAL KINETICS : The branch of chemistry dealing with the study of rates of reactions and reaction mechanisms.

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1 UNIT - 4 CHEMICAL KINETICS 1. CHEMICAL KINETICS : The branch of chemistry dealing with the study of rates of reactions and reaction mechanisms. 2. RATE OF CHEMICAL REACTION : The change in concentration of reactants or products in unit time. Its unit is mol l -1 s AVERAGE RATE: The rate of reaction during an appreciable time interval. For a general reaction of the type aa+bb cc+dd, the average rate in terms of the reactants and the products can be given as - = - = = 4. INSTANTANEOUS RATE : The rate of reaction at a particular instant of time. For a general reaction of the type aa+bb cc+dd, the instantaneous rate in terms of the reactants and the products can be given as - = - = = 5. FACTORS AFFECTING RATE OF AREACTION 1. Concentration of reactants 2. Surface area of reactants 3. Temperature 4. Nature of the reactants 5. Presence of catalysts 6. Presence of radiation 6.RATE LAW : Representation of rate of the reaction in terms of concentration of the reactants. For a general reaction of the type aa+bb cc+dd, rate law can be given as Rate = k [A] x [B] y where x and y may or may not be equal to a and b in the balanced equation. 7. RATE CONSTANT ( K ) : It is the rate of the reaction when concentration of the reactants is unity. 8. ORDER OF A REACTION: It is the sum of the powers of all concentration terms in the rate law expression. It can be zero, one, two etc or fractional. Should be found out experimentally. 9. Units of K for reactions with different order ( consider R P) Sl.no. REACTION RATE LAW ORDER UNIT OF K 1 R P Rate=k[R] 0 0 moll -1 s -1 2 R P Rate=k[R] 1 s -1 (time -1 )

2 3 R P Rate=k[R] 2 2 mol -1 ls -1 4 R P Rate=k[R] n n (moll -1 ) 1-n s ELEMENTARY REACTIONS: Reactions which get completed in one step. 11. COMPLEX REACTIONS: Reactions which take place in two or more steps. 12. MOLECULARITY: The number of reacting species which collide simultaneously to bring about a chemical reaction. 13. DIFFERENCE BETWEEN ORDER AND MOLECULARITY Sl.no. ORDER MOLECULARITY 1 Experimentally determined Theoretical concept 2 Can be zero or fractional Cannot be zero or fractional 3 There is overall order for a complex reaction No overall molecularity for a complex reaction 4 Sum of the powers of all concentration terms in the rate law Sum of the coefficient of reactants in a balanced chemical equation 14. INTEGRATED RATE EQUATION A. zero order reaction k= Concentration Vs time plot: B. First order reaction k= log Plot of ln[r] and t Plot of log [R]0/[R] and t

3 15. HALF LIFE PERIOD( t1/2 ): The time during which the concentration of the reactant is reduced to half of its initial concentration. t1/2 α n-1, where n is the order of the reaction. i. For a zero order reaction t1/2 = t1/2 is directly proportional to the initial concentration ii. For a first order reaction t1/2 = t1/2 is independent of the initial concentration. 16. PSEUDO FIRST ORDER REACTION: Reactions of higher order which follow first order kinetics. eg. Acid hydrolysis of ester CH3COOC2H5 + H2O in presence of H + CH3COOH +C2H5OH Rate = k[ch3cooc2h5] 17. EFFECT OF TEMPERATURE i) Reaction rate doubles for every 10 0 rise in temperature. Temperature coefficient = 2 or 3 ii) Quantitative relationship between rate and temperature is given by Arrhenius rate equation k = A where the pre exponential factor A is called the frequency factor and represents the fraction of molecules with energy equal to or greater thanea,the activation energy. iii) Activation energy: The energy required by the reactant molecules to change to products. iv) Plot of k and 1/T v) Maxwell Boltzmann energy distribution

4 i) Most probable kinetic energy: Kinetic energy of maximum fraction of molecules. ii) With increase in temperature the fraction of molecules with higher energy increases. iii) For 10 0 rise in temperature, the fraction of molecules having energy equal to or greater than Ea doubles and hence the reaction rate doubles. iv) If k1 and k2 are the rate constants of a reaction at two different temperatures T 1 and T2 log = [ - ] 18. EFFECT OF CATALYST Intermediate complex theory: i) Catalyst forms an intermediate complex with the reactants and this provides an alternative path with lower activation energy. Lower the Ea faster the process ii) Catalyst will not affect ΔG of the process iii) Catalyst does not change the equilibrium constant as it catalyses the forward and reverse reactions to the same extent. iv) Catalyst helps to attain the equilibrium faster. 14.THEORIES OF REACTION RATES 1)COLLISION THEORY i) Reaction takes place due to effective collision between the reacting molecules. ii) Effective collision :Collsion between properly oriented molecules having sufficient energy leading to breaking of bonds in reactant molecules and formation of new bonds in products. iii) Threshold Energy : The minimum energy the reactant molecules should possess for effective collisions.

5 iv) Activation Energy: The energy required by the reactant molecules to have threshold energy. Activation Energy = Threshold Energy Energy of reacting species v) Rate= PZAB,, ZAB is collision frequency, P is steric factor) vi) Collision frequency: The number of collisions per second per unit volume of the reaction mixture. 15. Slowest step is the rate- determining step: Some of the elementary reactions in a reaction path are relatively slow. the slowest reaction step determines the maximum rate, because a reaction cannot proceed faster than its slowest step. 16. ACTIVATED COMPLEX THEORY i) Reactants combine to form an activated complex. ii) Energy required by the reactant molecules to form the activated complex is called activation energy. iii) Activated complex is unstable ( a high energy species) and hence decomposes to form Products. iv) Activated complex is in equilibrium with the reactants. 1 MARKS QUESTION Q 1 What is the order of reaction whose rate constant has the same units as the rate of reaction? Ans.. Zero order Q 2.Give one example of pseudo first order reaction. Ans. Hydrolysis of an ester CH 3 COOC 2 H 5 + H 2 O CH 3 COOH + C 2 H 5 OH Q 3. Ans- Why boiling of an egg or cooking of rice in an open vessel take more time at a hill station? At a hill station, due to higher altitude atmospheric pressure is low. Hence, water boils at a Lower temperature. At a lower temperature, the rate of reaction decreases. Hence it takes Longer time. Q 4. What is the order of reaction whose rate constant has the same units as the rate of reaction? Ans- Zero order. Q5.. Why are reactions of higher order less in number? Ans- A reaction takes place because the molecules collide.the chances for a large number of.. molecules or ions to collides simultaneously are less. Hence, the reactions of higher order.. are less

6 Q6. Give units of specific rate constant for a zero order reaction. Ans mol L -1 s -1. Q7. Why the Rate of reaction does not remain constant throughout the course of reaction? Ans- Rate of reaction depands on concentration of reactants and since concentration becomes Less and less as the reaction progresses, the rate also goes on decreasing. Q8. Among the elementary processes in the proposed mechanism of reaction which is rate Determining step? Ans- The slowest step. Q9. State any one condition under which a biomolecular reaction may be kinetically of first Order? Ans- A biomolecular reaction may become kinetically of first order if one of the reactants Is present in excess. Q10. How is rate of reaction related to concentration of the reactants? Ans- Rate of reaction increases with increase in concentration except in zero order reaction. As the concentration increases total number of collisions will increase. Therefore probability Of effective collisions will increase. 2 MARKS QUESTION Q 1 Can the reaction be an elementary process? Explain. Ans. No, an elementary process would have a rate law with orders equal to its molecularities and therefore must be in integral form Q 2 The conversion of molecules X to Y follows the second order of kinetics. If concentration of X is increased 3 times, how will it affect the rate of formation of Y Ans. Rate = k [A] 2 = k [3A] 2 = k [9a] 2 The rate of formation will become nine times. Q3 Higher molecularity reactions (viz. molecularity, 4 and above) are very rare. Why? Ans: Molecules of reactants collide simultaneously and go to products.. The probability of colliding 4 or 5 species simultaneously is very rare Q 4. Decomposition of NH3 (g) on surface of catalyst 2NH >N2 (g) + 3H2(g), Q.5.Under low pressure follows first order kinetics while at high pressure it is zero order reaction. Why? Ans heterogenous catalysis molecules of NH 3 are absorbed on surface. Under lower conc. the surface of catalyst is not completely occupied. When pressure is high the surface is completely occupied and further increase in pressure (conc.) does not affect the rate. Q 5) Write the difference between order and molecularity of reaction.

7 Ans. ORDER MOLECULARITY It is the sum of the powers of Concentration terms in the rate law expression. It is determined experimentally Order of reaction need not be a whole number Order of reaction can be zero. It is the number of reacting speci- -es undergoing simultaneously Collision in a reaction. it is a theoretical concept It is whole no. only It can t be zero or fractional Q.6 A first order reaction takes 69.3 min for 50% completion. Set up on equation for determining the time needed for 80% completion. Ans. k=0.693 t 1/2 = 0.693/69.3min = 10-2 min -1 Q7. The rate constant for first order reaction is 60/s. How much time will it take to reduce the concentration of the reaction to 1/10 of its initial value. Ans:- t = log [R 0 ] k [R] t= log /10 t = log10 60 t = = 3.38X 10-2 s Q.1 3 MARKS QUESTION Ans 1 The sum of powers of the concentration of the reactants in the rate law expression is called order of reaction. For a general reaction: aa+ bb --- Products

8 Q2. The rate of a particular reaction quadruples when the temperature changes from 293K to 313K. Calculate activation energy. Ans. K 2 /K 1 = 4, T1= 293K T2 = 313K log [k2/k1] = Ea(1/T1-1/T2)/ Thus on calculating and substituting values we get.. Ea = KJ mol -1 Q3. If the decomposition of nitrogen oxide as 2N 2 O 5 > 4NO 2 + O 2 follows a first order kinetics. (i) Calculate the rate constant for a 0.05 M solution if the instantaneous rate is 1.5 x 10-6 mol/l/s? Ans. Rate = k [N 2 O 5 ] k= Rate [N 2 O 5 ] K=1.5 x K= 3.0 x 10-5 ii) What concentration of N 2 O 5 would give a rate of 2.45 x 10-5 mol L -1 s -1 Rate = 2.45 x 10-5 mol L -1 s -1 [N 2 O 5 ] = Rate = 2.45 x 10-5 K 3.0 x 10-5 = 0.82 M 4(a). Draw a schematic graph showing how the rate of a first order reaction changes in concentration of reactants.

9 Variation of rate of first order reaction with concentration. (b). rate of reaction is given by the equation Rate = k [A] 2[B] What are the units of rate constant for this reaction? Ans. Rate = k [A] 2[B] K = mol L -1 s -1 (mol L -1 ) 2 (mol -1 ) K= mol -2 L 2 s -1 Q 5 Define Threshold energy and activation energy. How they are related? Ans. Threshold Energy: It is the minimum amount of energy which the reactant molecules must possess for the effective collision in forming the products. Activation Energy: It is the excess energy required by the reactants to undergo chemical reaction. Activation energy = Threshold energy Average kinetic energy of molecule Q.6 The decomposition of NH 3 on platinum surface is a zero order reaction. What are the rate of production of N 2 and H 2. If k= 2.5 x 10-4 Ans. 2NH 3 N 2 + 3H 2 Rate= d[n 2 ] = 1 d[h 2 ] dt 3 dt rate = k x [NH 3 ] 0 =k =2.5 X 10-4 d [N 2 ] =2.5 X 10-4 dt d[h 2 ] =3 x rate dt =3x 2.5 X 10-4 moll -1 sec -1 Q 7.Can a reaction have negative activation energy?explain it, Ans. Acc. To Arrhenius equation K =A.e -Ea/RT.If E a were negative,the exponential factor can. be written as e x where x=e a /RT.If x<<1,we have E x =1+x+x 2 /2!+x 3 /3!+..=1+x =1+E a /RT Therefore,k=A(1+E a /RT),i.e, k>>a which is impossible. Hence, E a cannot be negative.

10 Value based question Q1. A number of chemical reactions or physical processes are taking place around us some of which are quite slow whereas some other are quite fast. In a number of cases we wish to speed up the slow reactions whereas in some other cases we wish to slow down the fast reactions. Now answer the following questions: (a) Hydrogen peroxide is very commonly used to bleach hair and also as antiseptic under the name perhydrol. How should it be preserve and why? (b) Why is it suggested that onions should be refrigerated before cutting them? (c) In winter, curding of milk becomes difficult.what is the simplest way to follow so that curding can occur and why? (d) Why does powdered sugar dissolve faster than big crystals sugar? (e) Do you think that diamond is for ever? Why or why not? Ans- (a) hydrogen peroxide is an unstable liquid and decomposes easily in presence of light or heat or long standing into water and oxygen. To slow down its decomposition, it should be stored in coloured bottles and kept well stoppered. Further a few drops of glycerol can be added which act as negative catalyst and slow down the decomposition. (b) Chemical vapours coming on cutting onions at room temperature bring tears in the eyes if this process is carried out at lower temperature the evolution of vapors is slowed down. (c) curding of milk takes place faster at a temperature which is neither too high nor too low(called optimum temperature). In winter, temperature is low and reaction envolved in curding of milk is so slow that sometimes it does not occur at all time. The simplest way is to keep the pot containing the milk for curding half buried n drum containing wheat flour which provides the required optimum temperature. (d) Powdered sugar has large surface area than crystalline sugar of the same mass. Greater the surface area, faster is the process of dissolution. (e)diamond is not forever because conversion of diamond to graphite is feasible or spontaneous (thermodynamically ). However, the process is so slow that the change is not perceptible at all. Q2. Food items get spoil more quickly in summer because the reactions occurring are faster at higher temperature than at lower temperature. therefore, use of refrigerator has become very common in almost every house. Manufactures also follow certain methods to prolong the life of food product from rancidity. Now answer the following questions : (a)why should milk and dairy products such as cream, yogurt etc. not be placed in the door of the refrigerator even if it has built in space for these and why? (b)for non-vegetarian people where should they store meat? Which process is speeded up or slowed down? (c)preservation of oily food items by vacuum is becoming more popular by manufacturers. How does it work? Ans.(a)The door of refrigerator has higher temperature than the shelves.hence reactions involving the spoilage of the food take place faster in the door. b)meat should be stored in the coldest part of the refrigerator. It slows down the growth of microorganism which are responsible for the spoilage.

11 (c)rancidity of oily food items is due to oxidation of oils and fats by the oxygen of air. By vacuum,the amount of oxygen coming in contact with the food is minimized.as a result, the oxidation reaction is considerably slowed down and the life of the food item is prolonged. Q3.A number of harmful reactions are occurring in the troposphere(where we live)as well as in the stratosphere also called ozonosphere ).These reactions are affecting our environment adversely. The environmentalists are feeling highly concerned to slow down the reactions involved so that we can stay healthy.now answer the following questions: (a)use of chlorofluorocarbons(cfcs) in refrigerators or aerosol spray has been banned?which reactions are slowed down by not using them? (b) Exhaust pipes of vehicles are fitted with catalytic converters. What type of reactions are speeded up by these catalyst? (c) In INDIA, sometimes, smog appears in winter what steps should to reduce its formations? Ans- (a) CFCs undergo photochemical decomposition to produce chlorine which depletes the ozone layer which protects us from harsh UV rays. By not using them, the amount of chlorine present in the stratosphere will we considerably reduced and hence, the reaction involving depletion of ozone layer will be slow down (b) Exhaust pipes of vehicles are fitted with such catalytic converters which speed up the reduction of pollutants of oxides of nitrogen produced due to combustion of petroleum products to free N 2 i.e, they accelerate the purifine reactions. (c) Smog in winters is due to condensation of sulphuric acid of particles on particulates of the atmosphere.sulphuric acid particles are present due to oxidation of SO 2 present in the atmosphere to SO 3 which is absorbed by the water vapours of the atmosphere. To slow down its formation,amount of SO 2 present in atmosphere should be reduce. This can be done using low sulphur or sulphur free fuels. Q4. Chemical kinetics has an important role to play in medical field. Some drugs and medicines speed up or slow down certain metabolic process. These drugs and medicines have beneficial effect on one hand but have harmful effects on the other hand. Now answer the following questions: (I) In USA teenagers and young adults smoke or ingest marijuana for recreation use. It is most illegal substance in USA. What does it slow down and what does it speed up causing adverse effect on health? (II) Oxytocin injections are given to mammals? What do they speed up? What are their harmful effects? (III) What do enzymes do in our body? What whould have happened if they were not present in our body? Ans- (i) This drugs slow down the flow of brain. As a result a person is unable to concentrate and remember things. Secondly, it speed up heart rate to dangerous level. (ii) Oxytocin speed up milk production in mammals. Its harmful effect include lowering of blood pressure and change of heart beat. (iii) Enzymes are biological catalyst which speed up the reactions occurring in our body. These reactions are so complex that if they were not present,reaction whould not have been so slow that life whould have been very low and sluggish. Q5. Chemical kinetics has an important role in agriculture. Synthetic fertilizers are used to speed up the growth of fruits and vegetables whereas pesticides are used to slow down the speed of growth of insects,fungi and weeds.

12 Now, answer the following questions: (i) What are the harmful effect of using synthetic fertilizers. What solutions do you suggest to the problem? (ii) What are the harmful effects of using pesticides? What solutions do you suggest to the problem? Ans-(i) During rainful, nitrogenous fertilizers wash away into rivers, lakes, Ponds and oceans. It helps the algae to bloom. When short live algae die, decomposing bacteria consume dissolved oxygen. The solution of the problem is that natural fertilizers like cow-dung etc. should be encouraged, i.e., production of organic food should be increased. (iii) Pesticides have neurological effect on humans such as headache and hand tremors. A number of them are responsible for cancer. The solution to the problem is that their used should be minimized and such chemicals should be produced which can kill pests but have no effects on human.

UNIT-4 CHEMICAL KINETICS CONCEPT

UNIT-4 CHEMICAL KINETICS CONCEPT Thermodynamics helps us to predict the feasibility of chemical reaction by using G as parameter but it cannot tell everything about the rate of reaction. Rate of chemical