CHEMICAL KINETICS CLASS XII CHEMISTRY STUDY MATERIAL

CHAPTER 4. CHEMICAL KINETICS

Chemical kinetics- The branch of physical chemistry which deals with thestudy of rate of reaction and their mechanism is called chemical kinetics.

Rate of chemical reaction- The change in concentration of any reactant or product per unit time is called rate of reaction. TYPES OF RATE OF REACTION- Average rate of reaction- The rate of reaction measured over the longtime interval is called average rate of reaction.                    Average rate-∆[R]/∆t∆x/∆t= =  +∆[p]/∆t Instantaneous rate of reaction- The rate of reaction measured at aparticular instant of time is called instantaneous rate of reaction. Instantaneous rate dx/dt= -d[R]/dt=+d[P]/dt

FACTORS AFFECTING RATE OF REACTION- Concentration of reactant :Rate of reaction increases with increase in concentration of raactants. Surface area: The smaller is the particle size, greater is the surface area, faster is the rate of reaction. Temperature:  Rate of reaction increases with increase in temperature Nature of reactant: The reaction rate is also affected by nature of reactant. Presence of catalyst: A catalyst speeds up the rate of reaction and help to attain the equilibrium quickly. But it doesnot affect rate constant. Radiation: It affects the rate of photochemical reactions.

RATE CONSTANT (k)-It is equal to the rate of reaction when molecularconcentration of reactants is at unity.

RATE LAW- The expression which describes the reaction rate in terms of molar concentrations of the reactants as determined experimentally is called rate law. For a reaction,       aA+bB→ cC+dD Rate law = k[A]p[B]q

MOLECULARITY –The total number of reactants taking part in an elementary chemicalreaction is called molecularity.

ORDER OF REACTION- The sum of powers to which the molar concentrations in the rate law equations are raised is called order of reactions. HALF-LIFE PERIOD- The time during which the concentration of the reactant is reduced to half of its initial concentration is called half-life period.

ACTIVATION ENERGY- The minimum extra amount of energy absorbed by reactant molecules so that their energy becomes equal to the threshold energy is called activation energy. Activation energy = Threshold energy –kinetic energy

Collision Theory of Chemical Reactions: According to the collision theory, a)      A chemical reaction takes place due to collision among reactant molecules b)      All collision do not lead to product formation.Those collisions which form products are called effective collisions. c)      For product formation, the colliding molecules should have a minimum amount of energy called threshold energy. d)     Energy alone does not determine the effectiveness of collision.The reacting molecules must also collide in proper orientation to bring about formation of products.

TEMPERATURE COEFFICIENT- The ratio of rate constant at twotemperatures having difference of 100C is called temperature coefficient. Temperature coefficient = (Rate constant at T+100C) / Rate constant at T0C Arrhenius Equation- K= Ae-Ea/RT K-rate constant A-Arrhenius factor/frequency factor Ea-Activation energy R- gas constant T-Temperature

Pseudo first order reaction:  A pseudo first order reaction is a reaction that is truly second order but can be approximated to be first order under special circumstances. For Example 1.      Hydrolysis of an ester CH3COOC2H5 + H2O  →  CH3COOH + C2H5OH 2.      Inversion of cane sugar C12H22O11+ H2O  à C6H12O6  + C6H12O6

Write the difference between order and molecularity of reaction. ORDER MOLECULARITY It is the sum of the powers of concentration terms in the rate law expression. It is the number of reacting species undergoing simultaneously Collision in a reaction. It is determined experimentally it is a theoretical concept Order mayor may not be a whole number Molecularity is always a natural number. Order of reaction can be zero. Molecularity can not be zero

ConConceptual  Question: The rate law for a reaction is Rate = K [A] [B] 3/2 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.

7.      Conceptual  Question 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   = K= mol-2L2s-1

Conceptual  QuestionA first order reaction takes 69.3 min for 50% completion. Determine the timeneeded for 80% completion. Ans. K=0.693 T1/2 =  0.693/69.3min =  10-2min-1 T= 2.303 log   [R0] K             [R] T= 2.303/10-2log5 T= 160.9min

Conceptual  QuestionThe 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 9[A]2

Conceptual  QuestionThe rate law for a reaction isRate = K [A] [B] 3/2 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.

Conceptual  Question Show that in a first order reaction, time required for completion of 99.9% is 10 times of half-life (t1/2) of the reaction.

S. No. IMPORTANT QUESTIONS (CHAPTER-4) CHEMICAL KINETICS QUESTION WITH ANSWER/ VALUE POINTS 1 Define the term ‘order of reaction’ for chemical reactions. The sum of powers of the concentrations of the reactants in the rate law expression is called theorder of reaction. 2 Identify the order of reaction for which the rate constant is expressed in units of L–1mol s–1. Second order. 3 What is Activation Energy? Activation energy of a reaction: The minimum extra energy absorbed by the reactant molecular so that their energy becomesequal to threshold energy is called activation energy. Activation energy = Threshold energy – Energy possessed by reactant molecules 4 List the factors on which the rate of a chemical reaction depends. Rate of reaction depends on(i) Concentration (ii) Temperature(iii) Nature of reactant (iv) Pressure of the gaseous reactant(v) Surface area (vi) Catalyst 5 Distinguish between ‘rate expression’ and ‘rate constant’ of a reaction. Rate expression is an experimentally determined expression which relates the rate of reaction with the concentration of the reactants whereas rate constant is the rate of reaction when concentration of each reactant in the reaction is unity. Consider a general reaction aA+ bBc C+ dD The rate expression for this reaction is Rate = k [A]m [B]n Where the proportionality constant k is called rate constant. 6 What is meant by a pseudo first order reaction? Give an example of a pseudo first order reaction. A reaction which is of higher order but follows the kinetics of first order under special conditions is called a pseudo first order reaction. Example, Acid hydrolysis of ethyl acetate. 7 A reaction is of second order with respect to a reactant. How will the rate of reaction be affected if the concentration of this reactant is (i) doubled (ii) reduced to half? (i) If [A] is doubled than rate rate becomes 4 times. (ii) If [A] is reduced to half then rate, rate becomes1/4 times 8 A reaction is second order in A and first order in B. (i) Write the differential rate equation. (ii) How is the rate affected on increasing the concentration of A three times? (iii) How is the rate affected when the concentrations of both A and B are doubled? (i) Rate = - = d R/dt =k [A]2[B]  (ii) When concentration of A is tripled Rate = k [3A]2[B] = 9k [A]2[B] i.e., Rate of reaction will become 9 times. (ii) When concentration of both A and B is doubled Rate = k [2A]2[2B] = 8k [A]2[B] i.e., Rate of reaction will become 8 times. 9 The rate constant for a first order reaction is 60 s-1. How much time will it take to reduce the initial concentration of reactant to 1/10 th of its value? 10 Hydrogen peroxide, H2O2 (aq) decomposes to H2O(l) and O2(g) in a reaction that is of first order in H2O2 and has a rate constant, k = 1.06 × 10–3 min–1. (i) How long will it take 15% of a sample of H2O2 to decompose? (ii) How long will it take 85% of a sample of H2O2 to decompose? 11 A first order decomposition reaction takes 40 minutes for 30% decomposition. Calculate its t1/2value.(Given log 1.428 = 0.1548) 12 For a decomposition reaction the values of rate constant k at two different temperatures are given below: k1 = 2.15 × 10–8 L mol–1s–1 at 650 K k2 = 2.39 × 10–7 L mol–1s–1 at 700 K Calculate the value of activation energy for this reaction. (R = 8.314 J K–1mol–1) 13 The rate constant for a reaction of zero order in A is 0.0030 mol L–1 s–1. How long will it take for the initial concentration of A to fall from 0.10 M to 0.075M? 14 What do you understand by the rate law and rate constant of a reaction? Identify the order of a reaction if the units of its rate constant are: (i) L–1mol s–1 (ii) L mol–1s–1 An experimentally determined expression which relates the rate of reaction with the concentration of reactants is called rate law while the rate of reaction when concentration of each reactant is unity in a rate law expression is called rate constant. (i) zero order reaction (ii)  second order reaction 15 A reaction is of first order in reactant A and of second order in reactant B. How is the rate of this reaction affected when (i) the concentration of B alone is increased to three times (ii) the concentrations of A as well as B are doubled? According to the given reaction: Rate = k [A] [B]2 (i) When the concentration of reactant ‘B’ is increased three times the rate of reaction becomes 9 times. i.e., Rate = k [A] [3B]2 = 9k[A] [B]2 (ii) When the concentration of reactants A and B are doubled, then rate becomes 8 times. i.e., Rate = k [2A]1 [2B]2 = 8k[A] [B]2 16 The rate of a reaction increases four times when the temperature changes from 300 K to 320 K. Calculate the energy of activation of reaction, assuming that it does not change with temperature. (R = 8.314 J K–1mol–1) 17 The rate constant for a zero order reaction is 0.0030 mol L–1 s–1. How long will it take for the initial concentration of the reactant to fall from 0.10 M to 0.075 M? 18 The thermal decomposition of HCO2H is a first order reaction with a rate constant of 2.4 × 10–3 s–1 at a certain temperature. Calculate how long will it take for three-fourths of initial quantity of HCO2H to decompose. (log 0.25 = – 0.6021) 19 A reaction is of second order with respect to a reactant. How is the rate of reaction affected if the concentration of the reactant is reduced to half? What is the unit of rate constant for such a reaction? As the reaction is of second order  Rate= k [R]2 If the concentration of the reactant reduced to half, then rate of reaction becomes 1/4th  of the initial rate. The unit of rate constant is mol–1 L s–1. 20 For a first order reaction, show that time required for 99% completion is twice the time requiredfor the completion of 90% of reaction. 21 Rate constant ‘k’ of a reaction varies with temperature ‘T’ according to the equation: Where Eais the activation energy. When a graph is plotted for log k vs.1/T, a straight line with a slope of – 4250 K is obtained. Calculate ‘Ea’ for the reaction. (R = 8.314 JK–1mol–1) 22 A first order reaction is found to have a rate constant k = 5.5 × 10–14 s–1. Find the half life of the reaction. 23 Define Half life of a reaction. Give equation for the half life of zero order and first order reaction. The half life (t½ ) of a reaction is the time in which the concentration of reactant is reduced to one half of its initial concentration [R]o. For a first order reaction t½ = 0.693/k    (i.e., independent of [R]o.) For a zero order reaction t½ = [R]o/2k i.e., t½ α [R]o . 24 Consider the following graphs and answer the following questions: (A)                                                                   (B) i)                    Predict the order of the reaction for graph (A) and (B) ii)                  What is the slope of the curve in graph (A) and (B) iii)                What is the unit of rate constant in each case? Answer:  i) Graph (A) – Zero order and Graph (B)- First Order                  ii) Slope of Graph (A)= - k & Slpoe of Graph (B)= - k                  iii) Unit of rate constant for Graph (A) = molL-1s-1 and Graph (B) = s-1 25 Answer: VALUE BASED QUESTIONS Q.1 Chemical kinematics has an important role in the field of agriculture. Synthetic fertilizers are used to speed up the growth of fruit and vegetable where as pesticides are used to slow down the speed of growth of insects , fungi and weeds and now answer the following question : A) What are the harmful effects of using synthetic fertilizers? What solution do you suggest to the problem? b) What are the harmful effects of pesticides?  What solution do you suggest to the problem? Ans: During the rainfall nitrogenous fertilizers washed away in to the rivers, lakes and oceans. It helps the algae to bloom when short lived algae dies, decomposing bacteria consume dissolve oxygen as a result aquatic life begin to perish. Secondly artificial fertilizers deplete the soil of organic matter. As a result,it losses ability to hold water and more subject to erosion. The solution to the problem is that natural fertilizer like cow dung etc should be encouraged. b) Pesticides have neurological effects on humans such as headache and hand tremors.  A number of them responsible for cancer. The solution of the problem is that there use should be minimized such chemical should be produced which can kill pests but have no effect on human. Q2. Food item get spoiled more quickly in summer because the reaction occurring are faster at high temperature then at low temperature. Therefore use of refrigerator has become very common in almost every house. Manufactures also follow certain methods to prolong the life of food products from the rancidity now answer the following: a) Why should d milk and other dairy products not be placed in door of the refrigerator even if it has built in space for these and why? b) For non-vegetarian people where should they store e meat? Which process /reaction is speeded up or slowed down? c) Preservation of oily food items by vacuum is becoming more popular by manufactures, How does it work? Ans : a) The door of refrigerator has higher temperature than the shelves. Hence reactions involving spoilage of the food take place faster in the door. B) Meat should be stored in the coldest art of the refrigerator .It slows down the growth f microorganism which are the responsible of the spoilage c)Rancidity of the oily food item is due to oxidation of oils and fats by oxygen f the air by vacuum the amount t of oxygen coming in contact with the food is minimized. As a result oxidation reaction is considerably slowdown life of the food item is prolonged.