Order and Molecularity of Reactions

Molecularity is the number of molecules that react together in an elementary reaction and is equal to the sum of the stoichiometric coefficients of the reactants in that elementary reaction. A chemical reaction that takes place in only one step, i.e. everything that takes place in one step, is called an elementary reaction, while a chemical reaction that takes place in a sequence of two or more steps is called a complex reaction. The sequence of steps in which a complex reaction occurs is called the reaction mechanism. Each step of the mechanism is an elementary step reaction.

PCl5 → PCl3 + Cl2  Unimolecular

2HI → H2 + I2  Bimolecular

2SO2 + O2 → 2SO3 Termolecular

NO + O3 → NO2 + O2  Bimolecular

2CO + O2 → 2CO2  Termolecular

2FeCl3 + SnCI2 → SnCI2 + 2FeCI2  Termolecular

Define Order and Molecularity Reaction

Order :- 

The rate law shows an expression or equation that relates the reaction rate to the concentration of reactants at a constant temperature. The rate of reaction depends on concentration, while the rate constant is affected by temperature and the presence of a catalyst.

The order of a reaction is a power or exponential of a given reactant concentration in the rate law. For example, if the concentration is doubled in the first-order reaction, the rate is doubled. If the concentration is doubled in a secondary reaction, the reaction rate quadruples. Likewise, if the concentration is doubled in a tertiary reaction, the reaction rate increases by the ninth power. 

Consider reaction A+B -> product

Rate = k[A]m [B]n

where k = rate constant, m + n is the order of the reaction

Also for the response:

R1 + R2 + R3 -> product

Rate = k[R1]x[R2]y[R3]z

where k = rate constant; x +y+ z is the order of reaction

E.g

Rate = k[NH4+][NO2-]; where NH4+ = 1st order and NO2- = 1st order. Therefore, general reaction order = 2nd order

The unit of the rate constant depends on the overall reaction order of the rate law.

E.g  2A→2B

        Rate = k[A]2; k= rate/[A]2

Order of reaction is 2

Unit of reaction rate is M-1s-1

Molecularity :-

The molecular nature of the process refers to the number of reacting molecules involved in the elementary reaction. It is the number of molecules present in the fundamental step of the reaction. Any process that takes place in one step is called an elementary process. There are several types of basic steps:

Unimolecular:  Molecules in basic steps, A -> P

Bimolecular:  Two molecules in the primary stage, A+A -> P; A+B -> P

Termolecular:  Three molecules in basic steps, A+B+C -> P; 2A>P

Higher molecular values are rare due to the low statistical probability of four or more molecules colliding simultaneously.

Elementary reactions are considered actual molecular events. This means that the rate of elementary reaction is proportional to the concentration of each reactant molecule. Therefore, we can express the rate law directly from a fundamental reaction. 

Consider the following example illustrating the basic steps for the reaction of NO2 and CO.

NO2(g) + CO(g) → NO(g) + CO2(g)

1st step: NO2 + NO2 →NO3 + NO

2nd step: NO3+ CO→ NO2 + CO2

Total: NO2 + CO→ NO + CO2

Experimental rate law is Rate = k[NO2]2 This means that the above reaction is a secondary reaction. NO2, but zero-order in [CO].

Difference Between Order and Molecularity of Reaction

1. The number of molecules of reactants in an equilibrium reaction is the molecularity, and the sum of the powers of the reactants in the expression of the rate law is the order of the reaction. 

2. Molecularity is always a no. (1,2,3), and reaction order can be an integer, fractional or zero.

3. Molecularity is the theoretical value, and the reaction order is the actual value.

4. We need an equilibrium reaction for molecularity, but it is not necessary in the reaction order.

5. Molecularity determines an equilibrium chemical reaction, but the order of the reactions determines the expression of the rate law.

Conclusion

Molecularity is nothing but the sum of the number of molecules of the reactants involved in the equilibrium chemical equation. Although the molecular nature of the complete reaction does not matter, the overall kinetics of the reaction depends on the rate-determining step. The total number of molecules or atoms whose concentration determines the rate of a reaction is called the order of the reaction.

Reaction order = sum of concentration indices. Conditions of customs law for the reaction 

xA+yB → product

The rate law is Rate= [A]x[B]y

Then comes the general sequence of reactions. n=x+y, where x and y are the order relative to the individual reactants.

Molecules cannot be larger than three, because more than three molecules cannot collide with each other.

Molecularity in the reaction cannot be zero, negative, or fractional. The order of responses can be zero, negative, positive or violation and greater than three. Infinite and falsy values are impossible.

When one of the reactants is present in large excess, the second-order reaction corresponds to the first-order reaction and is called a pseudo-unimolecular reaction.