When a small piece of platinum is added to a mixture of ethene and hydrogen, the following reaction occurs:

Doubling the concentration of hydrogen has no effect on the reaction rate. Doubling the concentration of ethene also has no effect.

(a) What is the kinetic order of this reaction with respect to ethene? With respect to hydrogen? What is the overall order?

(b) Write the unusual rate equation for this reaction.

(c)Explain this strange rate equation, and suggest what one might do to accelerate the reaction.

It may be defined as the relationship between the concentrations of the reactants and the observed reaction rate. Each reaction has its own rate equation which may be determined experimentally by altering the concentration of the reactants and then measuring the change in rate.

Consider a general reaction of the type:

aA + bB → products

The reaction rate is usually proportional to the concentrations of the reactants [A] and [B] which are raised to powers a and b respectively. The rate expression can, then be written as:

rate = k_r[A]^a[B]^b , where k_r= rate constant.

The order of a reaction may be defined as the sum of the exponents to which the concentration terms in the rate law are raised to express the observed rate of reaction.

From general equation of the type: aA + bB → products , the rate expression can be written as, rate = k_r[A]^a[B]^b .

a and b are called the orders of the reaction with respect to A and B. Depending on whether (a+b) is equal to 0,1,2, or 3, the reactions are said to be of zero order, first order, second order, and third order respectively.

(a)The rate of the reaction is independent of the concentration of ethene. So, order with respect to ethene is zero order.

Also, the rate of the reaction is independent of the concentration of hydrogen. So, order with respect to hydrogen is zero order.

Overall order for the reaction is zero-order.

(b) The rate of the reaction is only dependent to rate constant (k_r) . Hence, the unusual rate law equation can be written as rate = k_r .

(c) As mentioned, the rate law does not depend on the concentration of the reactants. So, it must depend on the platinum catalyst.

The surface area of the platinum catalyst must be increased or by adding more amount of the catalyst, the reaction rate can be increased.