Molecularity Of A Chemical Reaction

As we have discussed earlier what is a chemical reaction, how the reaction is classified and how the rate law of the reaction is written. Here we will discuss about the property of reaction called 'Molecularity', which helps in understanding the mechanism of the reaction.

Before going to define molecularity of a reaction in detail, let's have a look some basic terms related to a chemical reaction(we know that a chemical reaction occurs in a single step or more than one step);
(a) If a reaction is completed in a single step then it is called an "Elementary Step" and the rate of reaction depends on this elementary step.
(b)When a reaction is completed in more than one elementary step, then the rate of reaction depends on the slowest step at which the overall reaction proceeds ,out of all the other steps. And this slow step is called as "Rate Determining step". 


Let us now understand about the Molecularity of the reaction. What is the molecularity of a chemical reaction and how is it defined? How are chemical reactions classified on the basis of molecularity?, etc........
MOLECULARITY
Chemical reactions occur when atoms, ions and molecules collide with each other to bring about a chemical change and form a new bonded molecule called a product. Therefore, it is important to know the number of reactive species that bring about a chemical change. This brings us to the concept of molecularity of a reaction.
What is a molecularity??
"The molecularity of a reaction is equal to the sum of the number of reactant molecules participating in a balanced chemical reaction (or an elementary step or rate determining step) that must collide simultaneously in order to bring about a chemical change". In other terms, the molecularity of a reaction indicates the number of molecules participating in a chemical reaction. The molecularity of any reaction is always a whole number.
Thus,
........."The Molecularity Of The Chemical Reaction Is Equal To The Sum Of The Stoichiometric Coefficients Of The Reactants In The Balance Chemical Equation Of The Reaction".......

The concept of molecularity of a chemical reaction is theoretical. The value of atomicity cannot be zero, negative, fractional, infinite and imaginary. It can only be a positive integer. 
The value of molecularity can never exceed 3. Because more than three molecules can not collide or come close to each other during a chemical reaction.

Consider a chemical reaction;
                  aA + bB+ cC ➝ products 
The molecularity of the above reaction is the sum of stoichiometric coefficients "a+b+c".
For Example
(a) NO(g)+O₃(g) ➝ NO₂(g)+O₂(g)  
           Molecularity =1+1 =2
(b) 2 NO(g) +O₂(g) ➝ 2 NO₂(g) 
           Molecularity =2+1 =3

For elementary reactions, the molecularity can be obtained directly from the stoichiometry (or stoichiometric coefficient) of the reaction. Whereas for non-elementary reactions, the molecularity cannot be obtained directly from the stoichiometry of the reaction. This rate can be obtained from the stoichiometry of the determining step or the slowest step of that reaction.
For Example 
Elementary Reactions;
NH₄NO₂ ➝ N₂(g) + 2H₂O(l) ➛ Uni-molecular
N₂O₅ ➝ N₂O₄ + ½O₂(g) ➛ Uni-molecular

Non-elementary Reactions; 
The decomposition of hydrogen peroxide(H₂O₂) is a nonelementry reaction and involves more that one elementary step.
Step-1;  Involves, H₂O₂ ➝ H₂O + O (Slow)
Step-2;  Involves, OO+ OO  ➝ O₂      (Fast)
The overall decomposition reaction is the rate determining step and can be represented as:
H₂O₂(g) ➝ H₂O(l) + ½ O₂(g)  
This step involves only one molecule of hydrogen peroxide. According to the above-mentioned definition of molecularity, this reaction is an example of a unimolecular reaction. 
[ H₂O₂ ➝ H₂O + ½ O₂ ➛ Uni-molecular ]


Reaction Based On Molecularity
The molecularity of a process tells how many molecules are involved in the process. And depending on the number of reactive molecules which means how many species (atoms, molecules or ions) are reacting with each other to form the product, chemical reactions are of different types-
  1. Uni-molecular Reaction (A reaction whose molecularity is equal to one is called a molecular reaction. As the name implies, it involves one molecule.)
  2. Bi-molecular Reaction (A reaction whose molecularity is equal to two is called Bimolecular reaction. It involves simultaneous collision between two reacting species, with appropriate orientation and sufficient energy. Pseudo first order reaction is also termed as Bi-molecular reactions.)
  3. Tri-molecular Reaction (A reaction whose molecularity is equal to one is called trimolecular reaction. It involves simultaneous collision between three reacting species, with appropriate orientation and sufficient energy. These are rare as it is difficult to have effective collisions with three molecules.)
For Example
NH₄NO₂ ➝ N₂(g) + 2 H₂O(l)  ➛ Uni-molecular
N₂O₅ ➝ N₂O₄ + ½ O₂(g)  ➛ Uni-molecular
H₂O₂(g) ➝ H₂O(l) + ½ O₂(g)  ➛ Uni-molecular
NO(g)+O₃(g) ➝ NO₂(g)+O₂(g)  ➛ Bi-molecular
C₁₂H₂₂O₁₁ + H₂O ➝ 2 C₆H₁₂O₆  ➛ Bi-molecular
NaOH + HCl➝ Na-Cl+ H₂o  ➛ Bi-molecular
2 NO(g) +O₂(g) ➝ 2 NO₂(g) ➛ Tri-molecular
2 FeCl₃+ SnCl₄➝ SnCl₂+ 2 FeCl₂  ➛ Tri-molecular
2 SO₂(g) + O₂(g) ➝ 2 SO₃  ➛ Tri-molecular
2 CO(g) + O₂(g)➝ 2 CO₂(g)  ➛ Tri-molecular



Characteristics Of Molecularity
  1. Molecularity of a reaction is always an integer value because it refers to the mechanism of the reaction and hence, it can not be zero or any fractional value. 
  2. Molecularity is a theoretical concept which means that it does not change with Changes in experimental conditions (like pressure and temperatures).
  3. There are no any reaction encountered in nature which have molecularity four or greater than four. This is because more than three molecules may not mutually collide with each other.
  4. Hence, 1 ≤ Molecularity ≥ 3 
  5. Hence, maximum value of molecularity is equal to 1 and minimum value of molecularity is equal to 3.
  6. Molecularity of a reaction is assigned for each elementary steps of the reaction mechanism. 
  7. Molecularity can be obtained from a balanced chemical equation. Therefore, the chemical equation must be balanced before calculating the molecularity of a reaction. 
  8. Note that the molecularity refers only to an elementary reaction.


Hope you have found this article helpful!!
Do you have suggestions? Please write in comment box!!!
Feel free to comment if you have any queries!!

Comments

Post a Comment

Most Viewed Posts

Zeroth Law Of Thermodynamic: The Thermal Equilibrium Law

Image
In thermodynamics, we will see two types of walls. One is the Diathermic wall and the other is the adiabatic wall. Let us now go through them one by one.  • So the first is the Diathermic wall- " A Diathermic wall is a wall that allows the easy exchange of heat between two bodies (or between two systems or between system and surrounding) but does not allow the exchange of mass between them ". This means that it is a good conductor of heat. It is sometimes called as metallic wall because it is made of metal. • While the other is the adiabatic wall- " An Adiabatic wall is that wall which neither allows exchange of heat nor mass between two systems (or between system and surrounding) ". Since there is no exchange of heat and mass between the system and its surroundings, the system remains insulated from its surroundings. ➛ That's why whenever we separate the two systems by an adiabatic wall, then there is no concept of equilibrium between them. But whenever we sep...
Read more

Characteristics Of Entropy

Image
Entropy is an intrinsic property of a substance and is not affected by the external position of a system or its motion relative to other systems. It is a measure of the randomness of the system. Thus, the higher the randomness of the system, the higher the entropy of the system and the lower the randomness of the system, the lower the entropy of that system. Entropy   " Entropy is defined as a measure of the system's thermal energy that is incapable of doing work ". It is a state function.  For Example Heat is a form of energy that is incapable of doing work. Therefore, the more heat added to the system (or tranfer from the system), the higher (or the lower) will be its entropy.  The formula for change in entropy is given by the equation;                      ➩ ∆S = ∆Q/T For a reversible process, the change in entropy is given as; The Total Change In Entropy Entropy change is zero at equilibrium condition or whenev...
Read more

Reversed Carnot Cycle

Image
We have already discussed the concept of “ Carnot Cycle ”   & “ Its Efficiency ” and we have also seen the concept of “ Carnot Theorem And Its Explanation ” in our previous post. As we have already discussed that the Carnot Cycle is a reversible cycle and every process in the Carnot cycle is a reversible process. Therefore, every process of the Carnot cycle can be reversed. And the cycle can be operated in the opposite direction as well. In this article, we're interested in learning more about the reversed Carnot cycle, also known as the reversed Carnot heat engine cycle. Reversed Carnot Heat Engine It is important to note that if a reversible process is reversed, all related energy types—such as heat energy or work energy—will have the same magnitude but the direction of their interaction with matter will be changed. Let us first look at the Reverse Carnot Heat Engine  cycle here; The reversed Carnot heat engine cycle is depicted in the following figure. According to the...
Read more