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Chemical kinetics

Information about the mechanism of a reaction is often provided by the use of chemical kinetics to determine the rate equation and the reaction order in each reactant.

Consider the following reaction for example:

In this case, experiments have determined that this reaction takes place according to the rate law r = k 2 {\displaystyle r=k^{2}}. This form suggests that the rate-determining step is a reaction between two molecules of NO. A possible mechanism for the overall reaction that explains the rate law is:

Each step is called an elementary step, and each has its own rate law and molecularity. The elementary steps should add up to the original reaction. (Meaning, if we were to cancel out all the molecules that appear on both sides of the reaction, we would be left with the original reaction.)

When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step (in the above reaction) is the slowest step, it is the rate-determining step. Because it involves the collision of two NO molecules, it is a bimolecular reaction with a rate r {\displaystyle r} which obeys the rate law r = k 2 {\displaystyle r=k^{2}}.

Other reactions may have mechanisms of several consecutive steps. In organic chemistry, the reaction mechanism for the benzoin condensation, put forward in 1903 by A. J. Lapworth, was one of the first proposed reaction mechanisms.

A chain reaction is an example of a complex mechanism, in which the propagation steps form a closed cycle.In a chain reaction, the intermediate produced in one step generates an intermediate in another step.Intermediates are called chain carriers. Sometimes, the chain carriers are radicals, they can be ions as well. In nuclear fission they are neutrons.

Chain reactions have several steps, which may include:

Even though all these steps can appear in one chain reaction, the minimum necessary ones are Initiation, propagation, and termination.

An example of a simple chain reaction is the thermal decomposition of acetaldehyde (CH CHO) to methane (CH) and carbon monoxide (CO). The experimental reaction order is 3/2, which can be explained by a Rice-Herzfeld mechanism.

This reaction mechanism for acetaldehyde has 4 steps with rate equations for each step :

For the overall reaction, the rates of change of the concentration of the intermediates CH and CH CO are zero, according to the steady-state approximation, which is used to account for the rate laws of chain reactions.

d/dt = k – k + k - 2k = 0

and d/dt = k – k = 0

The sum of these two equations is k – 2 k = 0. This may be solved to find the steady-state concentration of CH radicals as = (k / 2k).

It follows that the rate of formation of CH is d/dt = k = k (k / 2k)

Thus the mechanism explains the observed rate expression, for the principal products CH and CO. The exact rate law may be even more complicated, there are also minor products such as acetone (CH COCH) and propanal (CH CH CHO).