IB syllabus > kinetics (hl) > 16.2 

16.2 - Reaction mechanism


16.2.1: Explain that reactions can occur in more than one step and that the slowest step determines the rate of the reactions (rate determining step).


Reaction mechanisms

Chemical reactions are processes in which new substances are formed. They can be represented by chemical equations in which the reactants are written on the left hand side and the products on the right, the two sides being joined by an arrow that indicates that change has taken place.

We are now familiar with the idea that all substances are made of particles, each compound having its own unique arrangement of atoms or ions within the compound. Logically, if new substances are formed in the course of a chemical reaction then a rearrangement of the fundamental atoms must have taken place.

Using the balanced equation, the situation seems fairly simple, but on studying the kinetics of a reaction it becomes apparent that even the most simple reaction actually involves several steps, moving from reactants to products. We call these different steps the mechanism of the reaction. They are the different stages in which particles collide to make intermediate or transition states, which then in turn either break down or collide further to give the final products.

The distinct stages that a reaction must pass through are called the 'mechanism' of the reaction.

Example

Nucleophilic substitution of 1-chloroethane

STEP 1: CH3CH2-Cl + OH- --> [CH3CH2(OH)-Cl]-*

STEP 2: [CH3CH2(OH)-Cl]-* --> CH3CH2-OH + Cl-

The species [CH3CH2(OH)-Cl]-* is an activated complex or transition state with a lifetime of only fractios of a second, but it is necessarily a stage that the reaction must go through.

This reaction has a two step mechanism.

The study of chemical kinetics aims to get information about the actual processes occurring during the course of the reaction, the mechanism. The intermediates, activated complexes and transition states can almost never be detected in the course of the reaction and so this branch of chemistry relies on analysis of the data obtained from experimentation, to provide evidence for the existence of a mechanism.

Kinetics is a purely experimental science - there are no theories that will allow a mechanism to be elucidated by consideration of the species taking part in the reaction.


Rate determining step

If a chemical reaction proceeds via several steps and one step is much slower than the others, then this slow step will have the effect of slowing everything else down. It' s like a traffic jam in road works during a journey. The stage of the journey through the road works has a much greater influence on the journey time than the other (faster) stages.

When we study the rate of a reaction we are really studying the rate of the rate determining step of the reaction. It is the slowest step that we are examining, because the fast steps happen too quickly and have little or no overall effect on the rate.

Hence, any data we obtain by experiment must be related to the rate determining (slowest) step.

We call the slowest step of a mechanism the 'rate determining' step, as it has the greatest effect on the overall rate of the reaction.

We can only measure the rate of the reaction under study in terms of the actual reactants and products and so any mechanistic steps must be found by indirect means. This is done by analysing the data obtained from the rate experiments.


Molecularity

The data obtained from a series of kinetics experiments allows us to solve the rate expression for the orders of reaction with respect to the reactants A and B

Rate = k [A]x[B]y

These orders, X and Y, give evidence about the number of particles involved in the rate determining step of the reaction. We call the total number of particles involved in this step the 'molecularity'.

Example: In the reaction between nitrogen monoxide and oxygen (one of the steps in the formation of photochemical smog):

NO + O2 --> NO2 + O

There is one particle of nitrogen monoxide and one particle of oxygen colliding in this step and so the molecularity is 2


16.2.2: Describe the relationship between reaction mechanism, order of reaction and rate-determining step. Only examples to one- or two-step reactions where the mechanism is given will be assessed. TOK: Agreement between rate equations and a suggested mechanism only provides evidence to support a reaction mechanism. Disagreement disproves the mechanism.


Analysing experimental data

The aim of any series of kinetics experiments is to determine the values of the components of the rate expression:

Rate = k [A]x[B]y

and to use this information to work out the mechanism of the reaction. Further analysis at different temperatures can also lead to information regarding the activation energy of the process.

Experiment Study the rate of disappearance of a reactant or appearance of a product (or something that can be related to either of their concentrations) Vary the concentrations of the different reactants and measure the rate of the reaction with the different concentrations.
       
Use the orders in the rate expression to determine the rate constant 'k' - don't forget to be careful with the units. Use the effect of concentration on the reaction rate to determine the orders with respect to the individual reactants
     
Repeat the experiment at different temperatures   The order with respect to an individual reactant gives the molecularity of that species in the rate determining step (slowest step) of the mechanism.  
     
Get the activation energy of the reaction using the Arrhenius equation: k = Ae-Ea/RT Suggest a possible mechanism for the reaction.  

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