Elementary and Complex Reactions

Introduction

Imagine a steak sizzling on the grill or vinegar that foams, bub­bling with baking soda in a homemade volcano. These quite ordinary events illustrate chemical reactions at work. From the food items we cook to the fuel we burn, the occurrence of chemical reactions is absolute in daily life. Grasping how these processes work takes much mystery out of the world around us and brings into very sharp focus just how important chemistry is in nature and industry.

The paper fosters a very interesting theme: chemical reactions in general, with primary views on elementary and complex reactions. We will try to explain the meaning and differences between these two kinds of reactions, illustrate some different aspects of examples, and show their importance for real-life applications and academic studies. By the end of this paper, the mechanisms of such reactions will be overviewed, together with their broader implications.

Chemical Reactions

Defining Elementary and Complex Reactions

Thus, chemical reactions can be broadly classified into two categories under study: elementary and complex reactions. An elementary reaction is a one-step process where reactants directly go to form products. The nature of such a reaction is pretty simple and contains one transition state. However, complex reactions have a number of steps and intermediates. They usually progress in a series of elementary steps to finally produce the products.

Elementary Reactions

The stoichiometry of an elementary reaction allows one to get the rate law directly. Assuming there is a reaction that involves, the rate of the reaction will directly be proportional to the concentration of the reaction. Such a type of reaction is a foundation for the understanding of mechanisms of reaction and kinetics.

Types and Examples of Reactions

Elementary Reactions

An elementary reaction may be unimolecular, bimolecular, or termolecular, based on how many reactant molecules become involved in it. The reactant molecule count for unimolecular reactions is a single molecule. One example would be the isomerization of cyclopropane to propene. In the bimolecular reactions, two molecules collide; one example is the reaction between hydrogen and iodine, forming hydrogen iodide. Termolecular reactions are hardly found because it is highly improbable that three molecules will collide simultaneously, but they do occur, like in the reaction between nitrogen dioxide and carbon monoxide to form nitrogen monoxide and carbon dioxide.

Complex Reactions

On the other hand, complex reactions could be further divided by the type of mechanism: sequential reactions, parallel reactions, and chain reactions. An example of a sequential reaction is the stepwise oxidation of glucose in cellular respiration. Examples of parallel reactions include the multiple pathways to different products of benzene nitration. The cases of chain reactions, on their part, go through stages of initiation, propagation, and termination, such as the polymerization of ethylene to form polyethylene.

Complex reactions are those reactions when a sequence of elementary reactions or single-step reactions gives us the products. Since the complex reactions occur in multiple steps thus, molecularity for such reactions cannot be determined. It can only be determined for elementary reactions.
For example:

${\mathrm{NO}}_2+\mathrm{CO}\to \mathrm{NO}+{\mathrm{CO}}_2$

The rate of the reaction is given experimentally as follows:
$\text{rate}=\mathrm{k}{\left[{\mathrm{NO}}_2\right]}^2$

Now, clearly, the order of this reaction is 2 but since it is a complex reaction, thus the molecularity of this reaction cannot be determined.
NOTE: For any complex or elementary reaction, it has been found that:
Molecularity $\le 3$

The rate of the reaction is given experimentally as follows:

Because the probability of simultaneous and effective collision of three molecules is very low. Thus, tetramolecular or higher molecularity reaction is rarely observed.

Difference between Order of Reaction and Molecularity

Relevance and Applications

Applications of Chemical Reactions in Real Life

Chemical reactions take a prominent place in areas related to the production of energy up to pharmaceuticals. Industry makes use of complex reactions to perform chemical synthesis, which is involved in the production of fuels and materials. An example of such a complex reaction involves the Haber-Bosch process. This process synthesizes ammonia from nitrogen and hydrogen; in terms of fertilizers, these are very critical elements. Knowledge of the complex biochemical reactions, from the health point of view, leads to the invention of drugs and control of diseases.

Scholarly Value

The study of elementary and complex reactions has immense applications in academia for students to understand some of the major principles of chemistry and biochemistry. It could be that the basis for many a laboratory experiment or research project undertaken for understanding mechanisms of reactions, kinetics, and thermodynamics is rooted in such reactions. Thus, knowledge of major reactions has applications in the fields of chemical engineering, environmental science, and biotechnology.

Some Solved Examples

For a better understanding of the topic and to learn more about Elementary and Complex Reactions with video lesson we provide the link to the

YouTube video:

Example 1

Question:

For a complex reaction, CH₃COOH CH₄ + CO.
experimental rate law, r = k [CH₃CHO]^3/2, the order and molecularity respectively?

1)3/2 , 3/2

2)Not defined , 3/2

3) 3/2 , Not defined

4)none of above

Solution:

The order of the reaction is the sum of the power of reactant concentration.

So, Order = 3/2
Molecularity is not defined for Complex (multi-step) reactions.

Example 2

Question:
The overall rate of a reversible reaction may decrease with the increase in temperature. When the activation energy of the forward reaction is less than that of the backward reaction, the increase in the rate of the backward reaction is more than that of the forward reaction on increasing the temperature. Which of the following is correct?

1) Statement-1 is true, Statement-2 is true, and Statement-2 is not the correct explanation for Statement-1.

2) Statement-1 is true, Statement-2 is true, and Statement-2 is the correct explanation for Statement-1.

3) Statement-1 is true, Statement-2 is false.

4) Statement-1 is false, Statement-2 is true.

Solution:
The overall rate of a reversible reaction may decrease with the increase in temperature. When the activation energy of the forward reaction is less than that of the backward reaction, the increase in the rate of the backward reaction is more than that of the forward reaction on increasing the temperature. So, Statement-1 is true, Statement-2 is true, and Statement-2 is the correct explanation for Statement-1.

Hence, the answer is option (2).

Summary

The other way around, elementary and complex chemical reactions lie at the heart of a great many natural and industrial processes. Elementaries are one-step processes; hence, their investigation affords immediate insight into mechanisms and kinetics. Complexes involve many steps and intermediates of importance in synthesis, synthesis of a broad range of products, and behavior vis-à-vis vital biological processes. Understanding these provides insight into the chemical world and its applications in philosophy, technology, industry, and medicine.