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 -1 3. 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-1s-1

2 R P Rate=k[R] 1 s-1 (time-1)

3 R P Rate=k[R]2 2 mol-1ls-1

4 R P Rate=k[R]n n (moll-1)1-n s-1

10. 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

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 100 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

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 100 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.

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

CH3COOC2H5 + H2O → CH3COOH + C2H5OH

Q 3. Why boiling of an egg or cooking of rice in an open vessel take more time at a hill station?

Ans- 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

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

2NH3 ------------>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 NH3 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.

Ans. ORDER MOLECULARITY

It is the sum of the powers of It is the number of reacting speci-

Concentration terms in the rate -es undergoing simultaneously

law expression. Collision in a reaction.

It is determined experimentally it is a theoretical concept

Order of reaction need not be a whole It is whole no. only

number

Order of reaction can be zero. 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

t1/2

= 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 = 2.303 log [R0]

k [R]

t= 2.303 log 1

60 1/10

t = 2.303 log10

60

t = 2.303 = 3.38X 10-2

s-1

60

3 MARKS QUESTION

Q.1

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

Q2. The rate of a particular reaction quadruples when the temperature changes from 293K to

313K. Calculate activation energy.

Ans. K2/K1 = 4,

T1= 293K T2 = 313K

log [k2/k1] = Ea(1/T1-1/T2)/ 19.15

Thus on calculating and substituting values we get…..

Ea = 52.86 KJ mol-1

Q3. If the decomposition of nitrogen oxide as

2N2O5────> 4NO2 + O2

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 [N2O5]

k= Rate

[N2O5]

K=1.5 x 10-6

0.05

K= 3.0 x 10-5

ii) What concentration of N2O5 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

[N2O5] = 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.

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

L2s

-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 NH3 on platinum surface is a zero order reaction. What are the rate of

production of N2 and H2.

If k= 2.5 x 10-4

Ans. 2NH3 →N2 + 3H2

Rate= d[N2] = 1 d[H2]

dt 3 dt

rate = k x [NH3]0 =k =2.5 X 10

-4

d [N2] =2.5 X 10-4

dt

d[H2] =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 Ea were negative ,the exponential factor can . be

written as ex where x=Ea/RT .If x<<1,we have

Ex =1+x+x

2/2!+x

3/3!+…..=1+x

=1+Ea/RT

Therefore ,k=A(1+Ea/RT),i.e, k>>A which is impossible.

Hence, Ea cannot be negative.

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 .

(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 N2 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 SO2 present in the atmosphere to SO3 which is absorbed

by the water vapours of the atmosphere. To slow down its formation ,amount of SO2 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.

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.