Molecularity of Reaction JEE

Order of Reaction and its Differences

Introduction


Chemical reaction involves one or more than one molecule. A single-step reaction which takes place in one single step is called as an elementary reaction. While the chemical reaction that involves series of two or more step is called as a complex or complicated reaction. In the complicated reaction, the sequence of steps through which it takes place is called as the mechanism of chemical reaction. Each sequential step in the mechanism is an elementary step reaction. During the course of chemical reaction, these molecules constantly bump into each other. Based on that, the chemical reaction takes place.

Definition


The molecularity of reaction can be defined as the number of molecules reacting with each other in the reaction. In other words, it can be defined as the number of molecules which undergo collision that ultimately resulting in to a chemical reaction. Hence, molecularity of reaction indicates the number of molecules that takes part in the chemical reaction.
The minimum number of reactants (atoms, ions, or molecules) required for the reaction to occur is called as the molecularity of an elementary reaction. The molecularity of the chemical reaction is equal to the sum of the stochiometric coefficients of the reactants in the chemical equation of the reaction. It is also defined as the number of reactant molecules taking part in a single step of the reaction. 

Examples:


  • 1. The Simple Case of Unimolecular Reaction

  • In case, when there is only one molecule in the reaction, the reaction is known as unimolecular reaction and its molecularity is one. This is a case of single-step chemical reaction. In this reaction, only one molecule transforms in to products. For example, degradation of dinitrogen tetroxide.

    N2 O4 (g)→ 2 NO2 (g)

    This reaction is of first order reaction as the reaction rate is directly proportional to the concentration of dinitrogen tetroxide raised to the first power.

    Another example of unimolecular reaction is:

    P Cl5 → P Cl3 + Cl2   
  • 2. Bimolecular reaction

  • In this case, there are two molecules of reactant that take part in the chemical reaction.

    2 HI → H2 + I2

    There are 2 molecules of hydrogen iodide take part in this reaction, and hence, it is an example of bimolecular reaction.

    Another example of bimolecular reaction is:

    NO + O3→ NO2 + O2
  • 3. Trimolecular Reaction

  • In this case, there are three molecules of reactant that take part in the chemical reaction.

    2 SO2 + O2→ 2 SO3

    There are 2 molecules of sulfur dioxide and one molecule of oxygen that collide and take part in the chemical reaction. Hence, it is called trimolecular reaction.

    Other examples of this type are mentioned as follow:

    2 CO + O2→ 2 CO2

    2 FeCl3 + SnCl2→ SnCl4 + 2 FeCl2


    “The molecularity of chemical reaction can also be defined as the minimum number of reacting particles (either molecules, atoms, or ions) that come closer and collide in a rate determining step to form one product or more than one product.”

    Complex example of molecularity of chemical reaction


    The decomposition of hydrogen peroxide (H2O2) is a complex reaction and it involves more than one step. This reaction takes place in two different steps.

    The overall decomposition reaction can be represented as:

    H2 O2→ H2 O + ½ O2


    This produces a molecule of water. This reaction can be divided in to following two steps.

    Step 1 involves: H2 O2→ H2 O + [O]

    Step 2 involves: [O] + [O] → O2


    From these two steps, first step is slow whereas the second step is fast. Now, as we know, the slowest step of complex reaction is a rate-determining step. In this case, the first step is rate-determining step. This step involves only one molecule of hydrogen peroxide. According to the above-mentioned definition of molecularity, this reaction is an example of unimolecular reaction. 

    Facts about Molecularity:


  • • The concept of molecularity of the chemical reaction is theoretical.

  • • The value of molecularity cannot be zero, negative, fractional, infinite, and imaginary. So, it can only be positive integer.

  • • The value of molecularity cannot be greater than 3 as more than three molecules may not mutually collide or come closer during the course of the chemical reaction.

  • There are several examples of chemical reaction whose molecularity seems to be more than three from the stochiometric equations. Examples of these reactions are:
  • 1. Reaction between hydrogen bromide and oxygen

  • This reaction can be written as:

    4 HBr → 2 H2O + 2 Br2(in presence of oxygen)

    At the first attempt, the molecularity of this reaction seems to be five. However, its molecularity is two. This type of reaction involves two or more elementary steps and the molecularity of this each elementary step is not more than three.
    The three elementary steps of above-mentioned reaction are as follow:

  • i. HBr + O2→HOOBr

  • ii. HOOBr + HBr→ 2 HOBr

  • iii. [HOBr + HBr→ H2O + Br2]

  • The molecularity of all above elementary steps is 2. Hence, molecularity of the overall reaction is also two, not five.

  • 2. The reaction between nitrogen dioxide and fluoride molecule.

  • The overall reaction can be shown as:

    2 NO2 + F2→ 2 NO2F

    According to stochiometric addition, the reaction seems to be trimolecular reaction. However, it is bimolecular reaction.
    The elementary steps of this reaction are as follow:

  • i. NO2 + F2→ NO2 + F

  • ii. NO2 + F → NO2F

  • The first step of the reaction is slow while the second step is fast. Hence, the first step is rate-determining step of the chemical reaction.

    Now, according to the definition, molecularity of the reaction is the number of colliding molecules in the rate-determining step. Hence, this chemical reaction is an example of bimolecular reaction. 

    The condition in which the molecularity of the chemical reaction is greater than 3 is rare. As we know, the chemical reaction takes place due to the collision among the reactant molecules present in the reaction system. It is a fact that as a number of reactant molecules increases or say as the molecularity of chemical reaction increase, the chance of their coming closer and colliding simultaneously decreases. Hence, when there are more than three molecules in reaction, the probability of all molecules coming closer and colliding simultaneously is low, and that is why the value of molecularity greater than 3 is a rare condition. Generally, this type of reaction in which more than 3 molecules react, involves more than one elementary step. From theses steps, one step is slow that determines the rate of the reaction as well as the molecularity.

    Order of Reaction


    Rate-expression or rate-law of reaction can be defined as the mathematical expression representing the dependence of rate of reaction on the concentration of reactants. The summation of the powers of the concentration terms of every reactant mentioned in the rate-expression is called as order of reaction. 

    For example, consider following reaction. 
    aA + bB→Products
    Rate-expression for this reaction is:
    Rate  [A]m [B]n

    In this case, the order of reaction with respect to the reactant A is m and the order of reaction with respect to the reactant B is n. However, according to the definition, the overall order of this reaction is m + n. 

    As we know, the molecularity of this reaction is a + b. It may or may not equal to the order of reaction i.e. m + n. (Note- value of ‘a’ may or may not equal to ‘m’. Similarly, the value of ‘b’ may or may not similar to ‘n’.)

    Main Difference Between Molecularity and the Order of Reaction


    Several reactions have the same molecularity and order of reaction. However, this is not true in every case. They might differ and it depends upon the reaction. Sometimes, these two terms might seem to be a bit confusing. Anyhow, there are several major differences between these two concepts, which are enlisted as follow.

  • 1. Molecularity of the reaction can be defined as the total number of reacting species in a rate-determining step. On the other hand, the order of reaction is the summation of powers of concentration of the reactant molecules in the rate equation of the reaction.

  • 2. As discussed earlier, the molecularity of a chemical reaction is always a whole number, or say, positive integer. On the other hand, the value of order of reaction may be a whole number, fractional, or zero.

  • 3. The molecularity is a theoretical concept, while order of reaction is an experimentally determined value.

  • 4. As discussed, the molecularity of complex chemical reaction might not the stochiometric sum. Hence, it is meaningful only in the case of simple reactions or individual steps. It is not meaningful for an overall complex reaction.

  • Whereas the order of reaction is meaningful for the overall reaction. It is not meant for the each and every individual steps of the complex reaction.
  • 5. The molecularity of reaction is based on the number of reactant molecules that take part in the rate-determining step of reaction. While, the order of reaction depends upon the molar concentration of reactant molecules.