Zero Order Reactions

A reaction is said to be of zero order if its rate is independent of the concentration of the reactants, i.e., the rate is proportional to the zeroth power of the concentration of the reactants.

For the reaction

       A → products

to be of zero order,

      -dx/dt = k[A]0 = k

Some photochemical reaction and a few heterogeneous reactions are zero-order reactants. Such reactions are not common.

 

Example:

1.   Photochemical reaction between hydrogen and chlorine:

                       hv

      H2(g) Cl2(g)  →  2HCl(g)

This photochemical reaction is zero-order reaction. The reaction is studied by placing H2 and Cl2 gases over water. the rate of reaction is studied by nothing the rate at which water rises in the vessel due to dissociation of HCl formed. the rate of rise of water is the same as the rate of disappearance of H2 and Cl2, i.e., the concentration of the gases phase will not change with time, although the quantities will change.

 

2.   Decomposition of N2O on hot platinum surface:

       N2O →  N2  +  1/2 O2

       Rate  [N2O]0 =  k[N2O]0 = k

       d[N2 O]/dt = k

 

3.   Decomposition of NH3 in presence of molybdenum or tungsten is a zero-order reaction.

             [Mo]

       2NH3   →   N2 + 3H2

The surface of the catalyst is almost completely covered by NH3 molecules. The adsorption of gas on the surface cannot change by increasing the pressure or concentration of NH3. Thus, the concentration of gas phase remains constant although the product is formed. Therefore, this reaction zero order kinetics.

     Other examples of zero order are:

4.   Decomposition of NI on the gold surface.

5.   Iodations of acetone in presence of H+ ions.

                         H+

     CH3COCH + I2  →  ICH2COCH3 + HI

The rate equation of this reaction does not include [I2] factor, i.e.,

     -dx/dt = k[CH3 COCH3 ][H+]

Characteristics of zero order reaction

(a)  The concentration of reactant decreases linearly with time.

        [A]t  =  [A]0 - kt

(b)  The time required for the reaction to be complete, i.e., time at which [A] is zero.

      tcompletion = [A]0/k = (Initial concentration)/(Rate constant)

(c)  The units of k are mol L-1 time-1.

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