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Home  / GENERAL CHEMISTRY Textbook / Chapter 5. How Chemical reactions proceed

Chapter 5. How Chemical reactions proceed

As a result of the studies of various chemical reactions, it has been confirmed that during such reactions the existing chemical bonds break and new ones form. Thus, for example, in the reaction of hydrogen (H2) and chlorine (Cl2) we get hydrogen-chlorine. The formula of this reaction can be illustrated as follows:  

H2 + Cl2 = 2HCl

During this reaction, the old bonds are broken between the hydrogen atoms in the hydrogen molecule (H2) and between the chlorine atoms in the chlorine molecule (Cl2) while new bonds are formed between the hydrogen and chlorine atoms in the hydrogen-chlorine molecule (HCl).

It was originally supposed that the initial hydrogen molecules (H2) and chlorine molecules (Cl2) collide with each other resulting in the breaking of the old bonds and the formation of free hydrogen and chlorine atoms which are united into molecules of hydrogen-chlorine. That is, it was supposed that the old bonds break during the reactions.  

However, this explanation contradicts the experiments. According to experimental data, in order to break a bond between chlorine atoms in the chlorine molecule (Cl2), the molecular chlorine should be heated to a temperature more than 700°C, while many reactions containing chlorine proceed at temperatures of 100°C to 200°C and even at normal room temperature.

Numerous studies of mixtures taken from reactors have shown that besides the initial and final products, these mixtures contain atoms (H, Cl) formed of initial molecules in very small quantities, and that the interaction of these atoms with the initial molecules proceed at a great rate, thereby forming the final product and new atoms.

That is, it turned out that the interaction of hydrogen molecules with chlorine proceeds not via the presupposed scheme:

H2 + Cl2 → 2HCl,                   

 but in another scheme:  

Cl2 → Cl + Cl                    (1)

H2 + Cl →HCl + H            (2)

Cl2 + H → HCl + Cl.          (3)

      This interaction mechanism is known as a chain mechanism.

Stage 1 is known as the stage of initiation. The active species - chlorine atom - is originated at this stage. Stages 2 and 3 are the chain's propagation. However, the given scheme contains contradictions.  

At Stage 2, the chlorine atom interacts with a hydrogen molecule to form the final product (HCl) and a hydrogen atom. During the interaction of a chlorine atom (Cl) with hydrogen (H2), the bond in the hydrogen molecule breaks at a temperature of 30°C, while the breaking of the bond in a hydrogen molecule requires a temperature of more than 3,000°C. 

Analogously, at Stage 3, a hydrogen atom reacts at a high rate of speed with the consecutive initial chlorine molecule (Cl2) and forms the final product (HCl) with a new chlorine atom. This reaction takes place at a great speed at room temperature. In the course of this reaction, the bond in molecule Cl2 breaks. It has been experimentally identified that a temperature of 700°C is required to break such a bond.

The answers to the following questions will heid we get our information about atom and molecule structure and what is this information based upon? What is the physical nature of chemical bonding? What other bonds, besides covalent ones, can form molecules?  

In order to answer these questions one should know the kinetic and potential energies of the electrons in the atoms, and correspondingly, the rates and transition trajectories of their electrons.  

Molecules and atoms cannot be seen even through a microscope. The kinetic energy of electrons cannot be measured in any way. That is, from the pure experimental viewpoint, the argument concerning the amount of energy in the electrons and their transitional speeds, and, therefore, the conclusions about the structure of molecules and about the electronic energy in these molecules, are actually unfounded. 

In spite of all this, scientists have managed to define the size of the molecule, its structure, its speed rate, and even the kinetic energy of the electrons transiting inside the molecule.  

Exactly how scientists discovered such seemingly impossible phenomena will be studied later, when you increase your knowledge with the forthcoming material.

Chapter 5. How Chemical reactions proceed  
Brief History >>
Helium-Like Atoms >> 
Conclusions >>   
Chapter 5 Textbook Questions >>