Sometimes Miss Chemistry.....a lot!!!

Rate Constants and the Orders

Usually, the rate of a reaction is a function of the concentrations of reactants. For example, the rate of the reaction

2 NO + O2 = 2 NO2

has the form:

Rate = k [O2] [NO]2

The rate is proportional to the concentration of O2, usually written as [O2] and is proportional to the square of [NO], or [NO]2. The orders of 1 and 2 for [O2] and [NO] respectively has been determined by experiment, NOT from the chemical equation. The total order of this reaction is 3 (=2+1).

Note the rates and order in the following example reactions:

H2 + I2 = 2 HI,
Rate = k [H2] [I2],
Total order 2.

H2 + Br2 = 2 HBr,
Rate = k [H2] [Br2]1/2,
Total order 1.5.

In particular, note that orders are NOT determined from the spectrophotometry of the reaction equation.

Rates as Functions of Reactant Concentrations

The order with respect to (wrt) a reactant are determined experimentally by keeping the concentration of other reactants constant, but vary the concentration of one of the reactant, say A in a general reaction

a A + b B + c C = products

If concentrations of B and C are kept constant, you can measure the reaction rate of A at various concentrations. You can then plot the rate as a function of [A]. For a zeroth order reaction, you will get a horizontal line.


For a first order reaction, the plot is a straight line (linear), as shown above, because

rate = k [A] (a straight line)

Note that rate = k when [A] = 1.

For a second order reaction, the plot is a branch of a parabola, because
rate = k [A]2

For a reaction with an infinite order, the plot is a step function. The rate is small, almost zero, when [A] less than 1. When [A] is greater than or equal to 1, then the reaction rate is very large. This model applies to nuclear explosion, except that [A] = 1 is actually the critical mass of the fission material.
rate = k [A]00



Well, we all know that one of the key conditions in an atomic bombs is to have a critical mass of the fission material, 235U or 239Pu. When such a mass is put together, the reaction rate increases dramatically, leading to an explosion. Thus, this model seems to apply, however, the mechanism for the fission reaction is not discribed by the order of the fission material.