Order of reactions



            The sum of the exponents of the concentration terms in the rate expression of the          chemical reaction.

            Consider a reaction             aA + bB →P

            Rate of reaction =    dx/dt    = k[A]a[B]b

¥          the value of a is the order of reaction with respect to A, and b is the order of reaction       with             respect B.while the sum of a+b is the overall order of reaction.

¥          The order of reaction is not necessarily equal to the number in the co efficient of balance             chemical equation.It is an experimental value.

¥          Order of reaction may be zero, 1st, 2nd or 3rd or may be in fraction.

¥          order of reaction is in fact the number of species ( atoms , molecules, ions) involve in the          rate of chemical reaction.

¥          If a reaction is completed in more than one steps, then order of reaction is determined    from             slow step i.e. rate determining step


      A reaction is zero order with respect to a reactant when changing the concentration of   that             reactant has no effect on the reaction rate.

¥          Metal catalytic reactions like decomposition of NH3 in the presence of Mo or W.

            For such reaction dx/dt = k[NH3]0 is zero order reaction.

¥          photochemical reactions are usually zero order.

2;         Fractional order;    

            in some of the reaction the involvement of some reactants are not in whole number but in             fraction, called fractional order.

¥          para hydrogen is converted into ortho hydrogen at high temperature.

            H2(p) +H2(O/p)→ H2(O) + H2(O/P)

            For this reaction the rate expression is dx/dt = k[H2(P)]1[H2(P/O)]1/2

            Order of reaction is 1+ ½= 1.5

            3;         first order reactions;

            the reaction in which one specie is involved in the rate expression.

            That particle is responsible for the progress of chemical reaction.


            A; Br2   → 2Br                        dx/dt = [Br2]1

            B; H2O2 → H2O 1/2O2          dx/dt = [H2O2]1

            C; Nuclear dissentegration are also first order like

                        88Ra22686Rn222 + 2He4    dx/dt = [Ra]1

4;      second order kinetics

            The reaction in which the rate of reaction is proportional to the product of the       concentration of two reactants each raise to the first power.


            1; Thermal decomposition of NO2

                2NO2 → N2+ 2O2              dx/dt = k[NO2]2

            2; Decomposition of HI in gas phase.

                        2HI → H2 +  I2                        dx/dt = k[HI]2

            3; oxidation of NO with ozone,

               NO + O3 → NO2 + O2                    dx/dt = k [NO][O3]

5;         third order kinetics;

            When three molecules are involved in rate expression


            1; Reduction of NO

              2NO + H2 → N2O +Cl2       dx/dt = k[NO]2[H2]

            2;SnCl2 + 2FeCl3 → SnCl4 + 2 FeCl2                   dx/dt = k[SnCl][FeCl3]2         3rd order

            3; Formation of nitrosyl chloride

                2NO + Cl2  → 2NOCl                   dx/dt = k[NO][Cl2]

            MOLECULARITY OF REACTION          

♣         The total number of molecules taking part in the rate determining step of a chemical       reaction is called "molecularity" of the reaction.

♣         A reaction is unimolecular if only one molecule is involved in its rate-determining step.

♣         A reaction is bimolecular if two molecules take part in its rate-determining step.

             For example: C2H5OH  → C2 H4 + H2O Is a unimolecular reaction because only one      molecule is involved.


♣         REMEMBER:

            ♣         The molecularity of a reaction can not always be predicted by the chemical                                  equation alone, that every reaction must follow the rules of chemical kinetics.

            ♣         Molecularity is always in whole number

        How can we determine order of reaction experimentally?

            In order to determine the order of reaction,

            1; First determine the rate of chemical reaction. From this we will be enable to determine the                involvement of each reactants in chemical reaction;

            Keeping the concentration of one reactant constant, observe the rate of reaction by                                changing the concentration of other reactants.

            Similarly observe the rate of reaction with the change of concentration of all reactants

            From this we will be enable to find out the order of reaction by involvement of the                                                molecules of each reactants in chemical reactio

          Suppose If the rate of reaction become double by double the concentration of reactants,                       It means the the order of reaction is first.( First order reaction)

            If the rate of reaction increase four times by double the concentration, It means the                                 order is second (second order reaction).

            If the rate of reaction increase nine to times by double the concentration, It means the                            order is third (third order reaction).

                        If the rate not effect by increase in concentration, It means the order is zero with respect                         to that reactants