CHAPTER --4:- --.CHEMICAL KINETICS (1 MARK QUESTIONS) 1. What do you understand by the rate determining step of a reaction? 2. Find the molecularity of following reaction. RCOOR + H 2O -------- H+ RCOOH + R OH 3. The rate constant of a reaction is 5.0 X 10-5 L mol -1 min -1. What is the order of the reaction? 4. Why rate of the reaction does not remain constant throughout? 5. What is the order of reaction whose rate constant has the same units as the rate of reaction? 6. Write Arrhenius equation. 7. Define rate constant or specific reaction rate. 8. The reaction A + 3B ---- 2C obeys the rate equation. Rate = k [A] 1/2 [B] 3/2. What is the order of this reaction?
109 9. What are the units of rate constant for a first order reaction? 10. Give one example of a reaction where order and molecularity are equal. 11. What do you understand by half-life period of a reaction? 12. Hydrolysis of ethyl acetate with NaOH is a reaction of second order while with HCl, it is of first order. Why? 13. What is the effect of adding catalyst on the free energy ( G) change of a reaction? 14. The rate of a reaction whose rate law is Rate = k [B] n becomes double on doubling the concentration of B. Find the value n. 15. A reaction is 50% complete in 2 hrs. And 75% complete in 4 hrs. What is the order of reaction? 16. Give one example of a first order reaction. 17. Define threshold energy. 18. The reaction A+B -- C has zero order. What is the rate equation? 19. Which reactions proceed with constant rate?. 20. How does catalyst affect rate of reaction? 21.. For a chemical reaction represented by R P the rate of reaction is denoted by d [R]/dt or +d[p]/dt. State the significance of plus and minus sign. 22. Express the rate of reaction in terms of disappearance of hydrogen andappearance of ammonia in the given reaction. N2(g) + 3 H2 (g) 2NH3 (g) 23. Why rate of reaction does not remain constant throughout? 24. Write the unit of first order rate constant of a gaseous reaction if the partial pressure of gaseous reactant is given in bar. 25. What will be the order of reaction, if the rate of reaction does not depend on the concentration of any of the reactant? 26. For the elementary step of a chemical reaction :H2 + I2 2HI rate of reaction [H2] [I2]What is the (i) molecularity and (ii) order of the reaction. 27. For a chemical reaction A B. The rate of the reaction is given as Rate =k [A] n the rate of the above reaction quadruples when the concentration of A is doubled. What is the value of n? 28. Mention one example of zero order reaction. 29. What is the value of the order of reaction of radioactive decay? 30. Express the relation between the half life period of a reactant and initial concentration for a reaction of nth order. 31. A reaction is 50% complete in 2 hours and 75% complete in 4 hours. What is the order of reaction?
110 32. Suggest an appropriate reason for the observation : On increasing temperature of the reacting system by 10 degrees, the rate of reaction almost doubles or even sometimes becomes five folds. 33. For a chemical reaction, activation energy is zero and at 300K rate constant is 5.9 10-5 s 1, what will be the rate constant at 400K? 34. Two reactions occuring at the same temperature have identical values of Ea. Does this ensure that also they will have the same rate constant? Explain. 35. The rate constant of a reaction is given by the expression k = Ae Ea/RT. Which factor in this expression should register a decrease so that the reaction proceeds rapidly? 36. For a chemical reaction rate constant k = 5.3 10-4 mol L 1 s 1, what will be the order of reaction? 37. Write the rate law and order for the following reaction : AB2 + C2 AB2C + C (slow) AB2 + C AB2C (Fast)] 38. The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to 3 times how will it affect the rate of formation of Y. 39. When rate of reaction becomes equal to specific reaction rate. 40. 87.5% of the substance disintegrated in 45 minutes (first order reaction).what is its Half life. ANSWERS TO 1 MARK QUESTION 1. The slowest step in a reaction is known as rate determining step. 2. Molecularity of reaction = 2 (there are two reactant molecules in balanced equation) 3. The order of reaction is 2. 4. Rate of reaction depends upon concentration of reactants which keep on decreasing with time. Hence, rate of reaction does not remain constant throughout. 5. Zero order. 6. Ae-Ea/RT Where,k = Rate constant Ea = Activation energy A = Arrhenius factor 7. When the molar concentration of each reactant is unity, the rate of reaction is called specific reaction rate. 8. Order =(1/2) + (3/2) = 2 9. s-1 10. For elementary reaction, order and molecularity are same. These reactions are carried out
111 only in one step. 2HI -- H 2 + I 2. 11. The time taken for half of the reaction to complete is known as half life period of that reaction. 12. Rate of hydrolysis of ethyl acetate with NaOH depends upon concentration of both CH3COOC2H5 and NaOH while with HCl it depends only upon the concentration of ethyl acetate. 13. Free energy of the reactants and products remains the same. Hence, there is no change in G on adding catalyst. 14. n = 1 15. First order 17. Threshold energy is minimum energy which the colliding molecules must possess so that the collision between them may be effective. Threshold energy = Activation energy + energy possessed by reactant molecules. 18. Rate =k [A] o [B] o 19. Zero order reactions. 20. A catalyst can increase rate of reaction by lowering down the activation energy. It gives alternate path to the reaction. 21. ( ) sign represents decrease in concentration with time while (+) sign represetns increase in concentration. 22. Rate= -1/3d[H2]/dt rate=+1/2d[nh3]/dt 23. It is because concentration of reactants goes on decreasing with time. 24. s -1 25. zero order 26. i)2 ii) 1 27. n=2 28. 2NH3 (g) N2 (g) +3H2(g) (at1130k and Pt as catalyst) 29. First order 30. t1/2 α 1/[R]0 n-1 where n is order of reaction. 31. First order 32. Increasing the temperature of the substance increases the fraction of molecules which collide with energy greater than Ea. 33. 5.9 10 5 s 1 34. No, because the Rate depends on the nature and concentrations of reactants and also pre-exponential factor. 35. Ea should. Decrease. : Rate = k [AB2] [C2]; Order = 1 + 1 = 2] 36. zero order reaction 37. Rate = k [AB2] [C2]; Order = 1 + 1 = 2]
112 38. The rate will increase 9 times. 39. When the concentration of reactant is Unity. 40. 15 Minutes. (2 MARKS QUESTIONS) 1. What is (i) Rate law expression? (ii) Rate determining step? 2. (i) This reaction is of first order and rate constant of reaction is 5.7 X 10-3 S -1. Find the value of t 1/2. 3. Define (i) Activation energy (ii) Collision freq 4. For a reaction A--- B, the rate of reaction doubles when concentration of A is increased by 4. What is the order of reaction? 5. Time required to decompose SO 2Cl 2 to half of its initial amount is 1 hour. If the decomposition is a first order reaction, calculate the rate constant of the reaction. 6. The conversion of molecules from A to B follows second order kinetics. If the concentration of A is increased to five times, how will it affect the rate of formation of B? 7. The rate of decomposition of N 2O 5 is 2.4 x 10-4 mol L -1 s -1 when [N 2O 5] is 0.36 M. What is the rate law and value of k for this first order reaction? 8. In a first order reaction, 75% of reactants disappeared in 1.386 hrs. Calculate the rate constant of the reaction. 9. Rate of formation of product for second order reaction is 9.5 x 10-5 mol L -1 s -1. The initial concentration of reactant was found to be 0.01 mol/l. a) Write rate law for second order reaction. b) Calculate the rate constant for the given second order reaction. 10. What do you understand by effective collision? What are the factors affecting the Effective collision? 1. (i) Define half life period. (THREE MARKS QUESTIONS) (ii)the half-life period of reaction is 10 minutes. How long it will take for concentration of reactant to be reduced to 10% of original? 2. Explain the term Activation energy and threshold energy? How temperature affects the rate of reaction?
113 3. What do you understand by integrated rate equation? Deduce the integrated rate equation for first order reaction? 4. A chemical reaction is of second order w.r.t. a reactant. How will the rate of reaction be affected if the concentration of this reactants : (a) Doubled; (b) Reduced to 1/8th. 5. From the following data for a chemical reaction between A and B at 300 K [A] mol/l [B] mol/l Initial rate (mol L 1 sec 1 ) 2.5 10 4 3 10 5 5 10 4 2.5 10 4 6 10 5 4 10 3 1 10 3 6 10 5 1.6 10 2 Calculate (i) the order of reaction with respect to A and with respect to B.(ii) the rate constant 300K 6. (i) Distinguish between elementary and complex reaction? (ii) What are the factors affecting the rate of chemical reaction? 7. The decomposition of phosphine4ph 3(g)---- P 4(g) + 6H 2(g) has rate law; Rate = k [PH 3]. The rate constant is 6.0 10 4 s 1 at 300K and activation energy is 3.05 10 5 J mol 1. Calculate the value of the rate constant at 310K. (R = 8.314 J k 1 mol 1 ). 8. The decomposition of hydrocarbon follows the equation k = (4.5 10 11 s 1 )e 28000 K/T. Calculate Ea. 9. Show that for a first order reaction, time required for 99% completion is twice for the time required for the completion of 90% of reaction. 10. The rate of reaction triples when the temperature changes from 20 C to50 C. Calculate the energy of activation. [R = 8.314 J k 1 mol 1, log 3 =0.48] 5 MARKS QUESTIONS 1. (a) For a reaction A + B Products, the rate law is given by r = k [A] 1/2 [B] 2 What is the order of reaction? (b) The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y? ( c) Write two points of difference between order and molecularity of a reaction. 2. a)define (i) Activation energy (ii) Collision frequency
114 b) The rate constants of a reaction at 500k& 700k are 0.02s -1 & 0.07s -1 respectively. Calculate the value of E a& A. 3. (i) A reaction is first order in A & second order in B. a) Write differential rate equation. b) How is the rate affected on increasing the concentration of B three times? c) How is the rate affected when conc of both A&B is doubled? ( ii) Name the factors which affect the rate of reaction. 4. a) Deduce the expression of half-life for zero order reaction. b) Show that time required for 99.9% completion of the first order reaction is 10 times of t 1/2 for first order chemical reaction. 5. a) For a reaction A + B Products, the rate law is given by r = k [A] 1/2 [B] 2. What is the order of reaction? (b) T he conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y?