Q26 of 26 Page 1

(i) Graphically explain the effect of temperature on the rate constant of reaction? How can this temperature effect on rate constant be represented quantitatively?

ii) The decomposition of a hydrocarbon follows the equation


K = ( 4.5 X 1011 S -1) e-28000/KT


Calculate Ea.


OR


i) In the reaction


Q + R → Products


The time taken for 99% reaction of Q is twice the time taken for 90% reaction of Q.


The concentration of R varies with time as shown in the figure below:



What is the overall order of the reaction? Give the units of the rate constant for the same. Write the rate expression for the above reaction.


ii) Rate constant for a first order reaction has been found to be 2.54 x 10-3s-1. Calculate its three- fourth life.


(i)


• Rate constant of a reaction depends on temperature and it is said that rate constant increases almost doubly for evey increase of 10 ◦ of temperatureand this dependency can be expressed quantitatively by Arrhenius equation :


k = A e – Ea/RT


Where , K= rate constant, A= frequency factor or Arrhenius factor R= universal gas constant and T= temperature (Kelvin) , Ea = energy of activation for the reaction.


• To represent this relationship graphically we have to reform the equation as :


By taking ln on the both sides we get,


lnK = - Ea/RT + lnA


and by plotting ln k values of a certain reaction against the (1/T) values we get the following straight line graph with a negative slope = - Ea/R and an intercept of ln A


.


(ii)


• From Arrhenius equation k = A e – Ea/RT .


• R= N0K , where R = ideal gas constant , k= boltzman constant and N0 = avogadro’s number.


Hence K =R/ N0 .


From the given equation , e – Ea/kT = e-28000N0/RT = e – Ea/RT


Hence ,


=


Or, Ea = 28000 X 6.023 X 1023


=1.744 X 1028 joule/molecule .


OR


(i)


• From the given plot the order of the reaction should be of zero order.


Hence, Q + R → Products is a zero order reaction .


• Because, the concentration of R ,


[R] varies with time which is shown by the plot. And that means the rate of the reaction is actually proportional to the concentration of reactant R (zero power on [R] ).


i.e. rate = d[R]/dt =k R0 = K (R0 = 1] which upon integration results in,


[R] = -kt + [R]0( where , [R] is the final concentration and [R]0 is the initial concentration


or, k = [R] – [R]0


t


• The dimension of the rate constant for the reaction is concentration/time hence, unit = M/s.


• The rate expression for the given zero order reaction is ;


rate = d[R]/dt =kR0 = K


hence, rate=k


(ii)


• For the given 1st order reaction K=2.54 x 10-3 s-1


• For a 1st order reaction t= 2.303 log a


k a-x


where, a = initial amount of the reactant and a-x denotes final amount until the reaction is taken into consideration.


• Now , for 3/4 th life a – x =a – (3/4) a= a/4


• t3/4 = 2.303 log a


2.54x10-3 a/4


= 2.303 log 4


2.54x10-3


=2.303 X 0.6020 [ log 4 =0.6020]


2.54x10-3


= 545.82 second.


More from this chapter

All 26 →
22

(i) Which of the following biomolecule is insoluble in water? Justify.


Insulin, Haemoglobin, Keratin.


(ii) Draw the Haworth structure for α-D-Glucopyranose.


(iii) Write chemical reaction to show that glucose contains aldehyde as carbonyl group.


 23

John had gone with his mother to the doctor as he was down with fever. He then went to the chemist shop with his mother to purchase medicines prescribed by the doctor. There he observed a young man pleading with the chemist to give him medicines as he had nasal congestion. The chemist gave him cimetidine. John advised and also explained to the young man that he should only take the medicines prescribed by the doctor.

Answer the following questions:


(a) Did the chemist give an appropriate medicine? Justify your answer.


(b) John’s action was appreciated by his mother. List any two reasons.


 24

(a) Write the mechanism of hydration of ethene to form ethanol.

(b) How are the following conversions carried out?


(i) Propanol to propan-2-ol.


(ii) Propanol to 1-propoxypropane


(c) Give the structure and the IUPAC name of the major product obtained in the following reaction:



OR


(a)Write the mechanism of the reaction of HI with methoxymethane.


b) Identify A and B in the following reactions:


(i)



(ii)



(c) Give the structure and the IUPAC name of the major product obtained in the following reaction:



conc. H2SO4


 25

(a) A blackish brown coloured solid (A) which is an oxide of manganese, when fused with alkali metal hydroxide and an oxidizing agent like KNO3, produces a dark green coloured compound (B). Compound (B) on disproportionation in neutral and acidic solution gives a purple coloured compound (C). Identify A, B and C and write the reaction involved when compound (C) is heated to 513 K.

(b)


(i) E0 M3+ / M2+values for the first series of transition elements are given below.


Answer the question that follows:



Identify the two strongest oxidizing agents in the aqueous solution from the above data.


ii) Copper (I) ion is not known in aqueous solution


iii) The highest oxidation state of a metal is exhibited in its oxide.


OR


(a) Write balanced equations to represent what happens when


(i)Cu2+ is treated with KI.


(ii) Acidified potassium dichromate solution is reacted with iron (II) solution.


(ionic equation)


(b)


i) The figure given below illustrates the first ionization enthalpies of first, second and third series of transition elements. Answer the question that follows



Which series amongst the first, second and third series of transition elements have


the highest first ionization enthalpy and why?


ii) Separation of lanthanide elements is difficult. Explain.


iii) Sm2+, Eu2+ and Yb2+ ions in solutions are good reducing agents but an


aqueous solution of Ce4+ is a good oxidizing agent. Why?