Class- XII-CBSE-Chemistry                      Chemical Kinetics 
                      CBSE NCERT Solutions for Class 12 Chemistry Chapter 4 
                      Back of Chapter Questions 
                      1.       For the reaction R → P, the concentration of a reactant changes from 0.03 M to
                               0.02 M in 25 minutes. Calculate the average rate of reaction using units of time
                               both in minutes and seconds.
                               Solution:
                                       [ ]            [ ]
                               Given  R 1 = 0,03;  R 2 = 0.02; Average rate of reaction =
                               change in concentration     Δ[R]
                                                        =
                                    time intervel           Δt
                                  [ ]     [ ]
                                   R 2 − R 1
                               =
                                    t2 −t1
                               Average rate of reaction in minutes = −(0.02−0.03)M min−1
                                                                                  25
                               Negative sign indicate the decrease the concentration 
                                    (        )
                                      −0.01
                                                       −1            −4         −1
                               =−              M min      =4×10 M min
                                        25
                                                                           4×10−4
                               Average rate of reaction in seconds =               M s−1
                                                                              60
                                              −6     −1
                               =6.67×10 M s
                               Concept Insight: 
                               Average rate depends upon the change in concentration of reactants or products 
                               and time taken for that change to occur. 
                               Rate of disappearance of R = Decrease in concentration of R/Time taken 
                               Rate of appearance of P = Increase in concentration of P/Time taken 
                                                                                                                         −1
                      2.       In a reaction, 2A → Products, the concentration of A decreases from 0.5 mol L
                                             −1
                               to 0.4 mol L      in 10 minutes. Calculate the rate during this interval?
                               Solution:
                                                    [ ]
                               Average rate = 1Δ A
                                                 2 Δt
                                      [ ]      [ ]
                                    1 A 2 − A 1
                               =−
                                    2 t2−t1
                                    1(0.4−0.5)
                               =−
                                     2      10
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                    Class- XII-CBSE-Chemistry                 Chemical Kinetics 
                                   (     )
                            =−1 −0.1  
                                 2 10
                                           −1     −1
                            =0.005 mol L min  
                            =5×10−3M min−1 
                            Concept Insight: 
                            For expression the rate of reaction where stoichiometric coefficients of reactants 
                            or products are not equal to one, rate of disappearance of any of the reactants or 
                            rate  of  appearance  of  products  is  divided  by  their  respective  stoichiometric 
                            coefficients. 
                                                                                                 1
                                                                                                 2    2
                                                                                              [ ] [ ]
                    3.      For a reaction, A + B → Product; the rate law is given by, r = k A     B . What is 
                            the order of the reaction? 
                            Solution: 
                            Sum of power of the concentration of the reactants in the rate law expression is 
                            called the order of that chemical reaction. 
                                            1
                                            2   2
                                        [ ] [ ]
                            Given r = k A     B . 
                            Hence the order of the reaction = 1 + 2 = 21 = 2.5 
                                                               2          2
                            Concept Insight: Sum of power of the concentration of the actants in the rate law 
                            expression is called the order of the chemical reaction. 
                    4.      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? 
                            Solution: 
                            The reaction X → Y follows second order kinetics. 
                            Therefore, the rate equation for this reaction will be: 
                                        2
                                     [ ]
                            Rate = k X  
                                [ ]           −1
                            Let  X = a mol L , 
                            then equation (1) can be written as: 
                            Rate = k.(a)2 
                                 1
                            =ka2 
                                                                                       [ ]             −1
                            If the concentration of X is increased to three times, then  X = 3a mol L     
                            Now, the rate equation will be: 
                                                                    (   )2     (  2)
                                                        Rate2 = k 3a      =9 ka  
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                    Class- XII-CBSE-Chemistry                 Chemical Kinetics 
                            Rate =9 Rate  
                                 2          1
                            Hence, the rate of formation will increase by nine times. 
                    5.      A first-order reaction has a rate constant 1.15 10−3s−1. How long will 5 g of this 
                            reactant take to reduce to 3 g? 
                            Solution: 
                            Given: 
                                            [ ]
                            Initial amount   R 0 = 5 g 
                            Final concentration [R] = 3 g 
                            Rate constant (k) = 1.15 × 10−3s−1 
                            Formula: 
                                          [ ]
                            t = 2.303log R 0 
                                           [ ]
                                  k         R
                                              [ ] [ ]
                            Put the values of  R 0, R , Rate constant (k) in above formula 
                            t =   2.303  log5  second 
                                       −3
                                1.15×10      3
                            = 2.303     ×0.2219 seconds 
                                     −3
                              1.15×10
                            =444.196 second 
                            =444 seconds (approx) 
                    6.      Time required to decompose SO Cl  to half of its initial amount is 60 minutes. If 
                                                             2   2
                            the  decomposition  is  a  first  order  reaction,  calculate  the  rate  constant  of  the 
                            reaction. 
                            Solution: 
                            Given:  Time required to decompose SO Cl  to half of its initial amount (t ⁄ ) =
                            60 minutes                               2   2                               1 2
                                                  st
                            Formula for t ⁄  of 1  order reaction, 
                                          1 2
                                   0.693
                            t ⁄ =          
                             1 2
                                      k
                            ∴k=0.693 
                                    t ⁄
                                     1 2
                                                                               0.693
                            By putting the value t ⁄  in the equation the k =       
                                                  1 2                           60
                            k = 0.01155 min−1 
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                    Class- XII-CBSE-Chemistry                 Chemical Kinetics 
                                               k = 0.01155/60 sec−1 = 1.925×10−4s−1 
                    7.      What will be the effect of temperature on rate constant? 
                            Solution: 
                            The rate constant of reaction is  doubled or triple with an 10 oC or K rise in 
                            temperature. 
                            However, the exact d rate of a chemical reaction on temperature is given by 
                            Arrhenius equation, 
                                     Ea
                                   −
                            k = Ae RT 
                            Where, 
                            k = rate constant 
                            A= Arrhenius factor or the frequency factor 
                            T= temperature 
                            R= gas constant 
                            Ea = activation energy 
                            Concept Insight: The effect of temperature on rate constant is given by Arrhenius 
                            equation. 
                    8.      The rate of the chemical reaction doubles for an increase of 10 K in absolute 
                            temperature from 298 K. Calculate Ea. 
                            Solution: 
                            Given that 
                            initial temperature (T ) = 298 K 
                                                  1
                                              (   )   (          )
                            final temperature  T   = 298+10 K=308 K 
                                                2
                            We know that the rate of the reaction doubles when temperature is increased by 
                               o
                            10 . 
                            Therefore, let assume value of k1 = k and that of k2 = 2k 
                                                 −1    −1
                            Also, R = 8.314 J K    mol  
                                        k     E T −T
                            Formula: ln 2 =     [ 2  1] 
                                        k     R T T
                                         1        1 2
                               k         E     T −T
                            log 2 =           [ 2     1] 
                               k     2.303R     T T
                                 1                1 2
                            log2k =      E    [308−298] 
                                k     2.303R 298×308 
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