Rate equation and order of reaction by xiuliliaofz

VIEWS: 7 PAGES: 14

									   Rate equation and order of
   reaction
   7S Chan Po Hang ( 3 )
      Ng Lai Man ( 24 )




2001/1                      1
Rate equation
For a general reaction:
         2A + B + 3C  D
The rate law for the above reaction can be expressed as


Rate of reaction = k Ax By Cz

where k = rate constant for the reaction at temperature
       [A], [B] and [C] = concentrations
       x, y and z = order of reaction


                                                     2
Rate constant

  The rate constant k, is temperature
  dependent.
  At a fixed temperature,it is a constant
  for a reaction.
                         k

  The unit of k depends on the overall
  order of the reaction.
           rate of reaction
E.g.    k
           A B C
                x    y   z



    unit of k = M   1-(x+y+z) s-1


                                            3
Order of Reaction

  The overall order of reaction is the sum of
  powers of the concentration of individual
  reactants.
  The order of reaction can only be determined
  experimentally.

  (a) For a zeroth order reaction,
  the rates of reaction are independent of the
  concentration of reaction.


                                                 4
(b) For a first order reaction,
the rate is directly proportional to the
concentration of the reactant.

(c) For a second order reaction,
the rate of the reaction is directly
proportional to the square of the
concentration of the reagent.



                                           5
For a simple reaction: A  products


              Order of reaction      Rate law        Unit of k

                                     d
   (a)   Zeroth order                   A         Mol dm-3 s-1
                                     dt
                                  d
   (b)   First order                 A  k A         s-1
                                  dt
                                  d
   (c)   Second order                A  k A2   Mol-1 dm3 s-1
                                  dt
                                  d
   (d)   N th order                  A  k An   Mol-2 dm6 s-1
                                  dt

                                                                 6
Simple rate equations determined
from experimental results

 (1) Initial rate method
 Example:
 In the kinetic study of the reaction,
 CO(g) + NO2(g)  CO2(g) + NO(g)

 Experiment   Initial concentration Initial concentration Initial
              of CO                 of NO2                rate M/s
 1            0.1 M                0.1 M                 0.015
 2            0.2 M                0.1 M                 0.030
 3            0.1 M                0.2 M                 0.030
 4            0.4 M                0.1 M                 0.060
                                                                     7
  Establish the rate equation for the reaction.

Solution
0.015 = k (0.1)m (0.1)n ----------(1)
0.030 = k (0.2)m (0.1)n ----------(2)
0.030 = k (0.1)m (0.2)n ----------(3)
    : 0.5  0.5 
(1)                     (1)
                            : 0.5  (0.5) n
                  m

(2)                     (2)
m=1                n=1
0.060=k(0.4)(0.1)  k=1.5
        Rate =k[CO][NO2]

                                                  8
(2) Rate-concentration graph
Example:
Tabulated below are the initial rate of decomposition of
  hydrogen peroxide at different concentrations. Plot a
  graph of initial rate against concentration.




                                                       9
Solution:




            10
(3) Log(rate)-log(concentration)
graph
log(rate)= n log[A]+log k
Slope = order




                                   11
12
(4) Integrated Rate equation

      Order          Equation

      Zeroth       [A]=-k0 + [A]0

                       A0    k1t
       First        In
                        A
     Second         K2t + 1/[A]0

                                      13
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