Elementary Reactions

A chemical reaction in which one or more chemicals react directly to form products is known an elementary reaction. In all chemical reactions, reactants get converted into products. A chemical reaction occurs when electrons in the reactants move, and thus forming and breaking chemical bonds. Law of conservation of Mass states that matter can be neither created nor destroyed .It can be converted from one form to other and this is happening in all elementary reactions. The process is comprised of a single reaction step and single transition state. Elementary reactions are generally divided into three types based on the number of reactants (atoms, ions or molecules involved).

• Unimolecular Reactions
• Bimolecular Reactions
• Trimolecular reactions

Unimolecular reactions

In Unimolecular reactions are chemical reactions in which a single molecule undergoes isomerisation (a chemical process by which a compound is changed into its isomeric forms) to form products.    (A → Products).An elementary step in which an ion or molecule decomposes to form products is known as Unimolecular step or Unimolecular process. All Unimolecular reactions are first order reactions (Reactions which depends on the concentration of only one reactant).

Rate of Unimolecular reactions at constant temperature is proportional to the concentration of reactants. Rate of reactions are based on Collision Theory (The theory states that the rate of product formation is equal to the number of reactant molecules collided)

d(A)/dt  =  -k(A)

For example:   O₃  → O₂  +  O, where the rate of reaction is k(O₃)

Bimolecular Reactions

Two atoms, molecules or ions react together in bimolecular reactions to form products.

A   +    B  →  Products.  The Rate of bimolecular reactions can be expressed as follows:

d(A)/dt  =   d(B)/dt   =  -k(A)(B).

Bimolecular reactions are 2^nd order reactions in which the concentration of both reactants are considered.

Cl + CH₄ = HCl + CH₃ where the rate of reaction is k(Cl)(CH₄)

Trimolecular Reactions

When three atoms, molecules or ions collide to form products, the reaction is known as Trimolecular reactions.

A +  B + C  →   Products.

Rate of these reactions can be expressed as:

d(A)/dt  =  d(B)/dt  = d(C)/dt  =  k(A)(B)(C)

An example for trimolecular reaction:

O +  NO + N₂  →  NO ₂ + N₂

In short an elementary process expresses how the molecules or ions in a reaction react each other to form products. Elementary reaction is also known as elementary step or elementary process.

Questions:

• What is meant by elementary Reactions?
• How can you express the rate of a Unimolecular reaction?