CHEMICAL KINETICS

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					                               CHEMICAL KINETICS
INTRODUCTION
Chemical thermodynamics predicts stability of a system and the feasibility of a
reaction or a process, while chemical kinetics predicts the speed at which such feasible
process or reaction would occur. Therefore chemical kinetics is the area of chemistry concerned
with the speed or rate at which a chemical reaction occurs. Knowing the speed of a reaction is
important in drug design, in pollution control and food processing.
The rate of a reaction (reaction rate) is expressed as the change in concentration or
product with respect to time. Where A is the reactant and B is the product, such that
in the course of the reaction A is consume therefore its concentration decreases while
B is produce, consequently increasing in concentration.




The negative sign in front of the rate expression in term of concentration of reactant
is signifies decrease in the amount of A as it is consumed.

                                            Similarly


                                                                   A
                                                                   B
                        CONC (M)




                                               t (s)
For example in the reaction of bromine with methanoic acid as given below; the
change in concentration of the Br2 was measured with respect to time at an interval of
50 s.



                  Time (s)                 [Br2] (M)               Rate (M/s)
                     0.0                     0.0120                 4.20 x10-5
                    50.0                     0.0101                 3.52 x 10-5
                   100.0                    0.00846                 2.96 x 10-5
                   150.0                    0.00710                 2.49 x10-5
                   200.0                    0.00596                 2.09 x10-5
                   250.0                    0.00500                 1.75 x 10-5
                   300.0                    0.00420                 1.48 x 10-5
                   350.0                    0.00353                 1.23 x 10-5
                   400.0                    0.00296                 1.04 x 10-5




(Ensure you understand how to solve this simple question in determining the rate at any give time)
Take a look at the concentration of Br2 at t50 and t250 you would observe that the [Br2]
at t50 is double that at t250 and also the rate at t50 is double that at t250. This shows that
there is rate is directly proportional to the concentration.

             Recall O’level maths
where k is called the rate constant; making k the subject of the formula we have


The unit of k in this case is s-1
A plot of rate vs. [Br2] would be a straight line as illustrated in the graph below
confirms the direct proportionality between concentration and rate.

                             [Br2] (M)            Rate (M/s)


                              0.0120              4.20 x10-5              3.50 x 10-3
                              0.0101              3.52 x 10-5             3.49 x 10-3
                             0.00846              2.96 x 10-5             3.50 x 10-3
                             0.00710              2.49 x10-5              3.51 x 10-3
                             0.00596              2.09 x10-5              3.51 x 10-3
                             0.00500              1.75 x 10-5             3.50 x 10-3
                             0.00420              1.48 x 10-5             3.52 x 10-3
                             0.00353              1.23 x 10-5             3.48 x 10-3
                             0.00296              1.04 x 10-5             3.51 x 10-3


               0.00005


               0.00004


               0.00003
  rate (M/s)




               0.00002


               0.00001


                    0
                         0        0.002   0.004      0.006       0.008   0.01    0.012   0.014
                                                     [Br2] (M)
Reaction Rates and Stoichiometry
We have considered a case where 1 mole of react A produced 1 mole of product B




Now we want to consider a case where the ratio is not 1:1 and derive the rate
expression. In the case below 2 moles of A disappear for 1 mole of B to be produced.




This means that the rate of consumption or disappearance of A is twice as fast as the
rate of appearance or production of B.
Therefore in general for any reaction given as

The rate would be expressed as




Working with Volumes:
For example the decomposition of hydrogen peroxide as given below:




We can derive an expression relating rate to change in pressure when that is given.

P = pressure of gas (in this case oxygen), V = volume of gas, n = no of moles of gas,
R = gas constant and T = temperature)




Substituting equation 2 in the rate expression in term of O2 (1), then
Class Work 1.0
   1. Write out the rate expression in terms of each reactant and product
   (a)
   (b)

   2. Consider the reaction

       If oxygen is reacting at a rate of 0.0024 M/s
   (a) At what rate is        being formed?
   (b) At what rate is      reacting?

   3. Determine the rate of production of oxygen and potassium chloride from the
      thermal decomposition of potassium trioxochlorate (V) given that the change
      in pressure is 2 atm and change in time of 2 mins (R= 0.082Latm/Kmol, T =
      25 ).



Solution




                                           2a



                           Given                 = 0.024M/s
b. Similarly




               3a.




               b.

				
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posted:11/17/2012
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