Molecularity of reaction

Molecularity of Reaction

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:02 PM IST

Imagine that you are cooking a complicated recipe in your kitchen. You follow a recipe in which the sequence of adding the ingredients is important. You add A, B and C in steps. When you mix them wrongly or at a wrong time, it won't result in the desired outcome. The case may be compared to that in chemical reactions where the order and manner of contact between the reactant molecules might thoroughly change the results. Something termed "molecularity" defines this interaction in chemistry. In chemistry, it gives the snap-shot of how collision and reaction occur among reacting molecules to form products. It presents the concept of molecularity, encompassing its definition, the types of molecularity, and its point in theory or practical application.

This Story also Contains

Molecularity of Reaction
Significance and Applications
Some Solved Examples

Molecularity of Reaction

Molecularity is referred to as the number of reactant molecules coming together simultaneously to collide with each other and enter into the transition state that finally leads to a chemical reaction. This approach is used to describe theoretically the mechanism of elementary reactions. depending on whether it involves one, two, or three molecules, then it is said to be unimolecular, bimolecular, or termolecular, respectively. For instance, in a unimolecular process, one molecule undergoes a change; this might be just the isomerization of cyclopropane into propene. Knowing the molecularity helps to know what exactly happens at each particular step and which steps the mechanism comprises for a reaction.

The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction which must collide simultaneously in order to bring about a chemical reaction is Molecularity.

Description:

NH4NO2→N2+2H2O(unimolecular) H2+I2→2HI (bimolecular) 2NO+O2→2NO2 (trimolecular)

Molecularity is the theoretical concept. It cannot be zero/non-integer

Type of Molecularity

1. Unimolecular Reactions: This involves one reactant molecule going to the products. A very good example is the decomposition of dinitrogen pentoxide, N₂O₅, to form nitrogen dioxide and oxygen.

2. Bimolecular Reactions: Reactions involving two colliding reactant molecules. The classic example is the reaction of hydrogen, H₂, with iodine, I₂, forming hydrogen iodide, HI.

3. Termolecular Reactions: These are the least frequent due to the extremely low probability of three molecules hitting together at the same time. An example is the formation of ozone from oxygen molecules and atoms in the atmosphere.

Each of these kinds of molecularity details the richness in reaction mechanisms and, therefore, speaks to the complexity of the chemical process.

Significance and Applications

Molecularity of reactions is used in both academic and industrial circles. It helps in the learning of subtleties of the reaction mechanism and kinetics in an academic setup for students and researchers. For example, studies on unimolecular reactions have contributed knowledge with respect to isomerization and stability of species. Knowledge of molecularity becomes very important in applications related to industrial processes, as it is instrumental in the optimization of chemical reactions. For example, the production of ammonia by the method of Haber requires knowledge about the molecularity of the reactions between nitrogen and hydrogen molecules. Tetramolecular processes are very important in atmospheric chemistry, helping to explain the phenomena of formations and destructions of the ozone layer. Knowing molecularity could lead to the design of better catalysts, improvement in yields in reactions, and environmentally more benign chemical processes.

Some Solved Examples

Example 1

Question:
The molecularity of a reaction depends on:
1) Pressure
2) Temperature
3) Both temperature and pressure
4) None of the above

Solution:
The number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction that must collide simultaneously to produce a chemical reaction is Molecularity. The molecularity of a reaction is independent of pressure and temperature.
Hence, the answer is the option (4).

Example 2

Question:
Which one of the following is correct about the molecularity of a reaction?
1) It is the number of reacting species undergoing simultaneous collision in an elementary or simple reaction.
2) It is a theoretical concept and can be calculated by simply adding the molecules of the slowest step.
3) It is always a whole number value only, i.e., 1, 2, 3, etc.
4) All of the above

Solution:
Molecularity is the number of reacting species undergoing simultaneous collision in an elementary or simple reaction. It is always a whole number value only, i.e., 1, 2, 3, etc. It is a theoretical concept and can be calculated by adding the molecules of the slowest step.
So, all statements are correct.
Hence, the answer is the option (4).

Example 3

Question:
A reaction involving two different reactants can never be:
1) Unimolecular reaction
2) First-order reaction
3) Second-order reaction
4) Bimolecular reaction

Solution:
The number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction that must collide simultaneously to bring about a chemical reaction is Molecularity. So, a reaction involving two different reactants can never be an unimolecular reaction.
Hence, the answer is the option (1).

Summary

The molecularity of a reaction is the central theme in chemistry and acts more or less as a window to knowledge regarding the mechanism/fittingness of a set of chemical reactions. It classifies varied reactions into classes based on the number of molecules taking part as reactants in any one elementary reaction. These categories are very useful both for theoretical insights and practical applications right from academic studies to industry production lines and environmental science. Thus, knowing the molecularity allows easier control and prediction of the course of chemical reactions; hence, the field has opened avenues for several advances within the domain of chemistry.

Frequently Asked Questions (FAQs)

1. What is molecularity in a chemical reaction?

Molecularity is the number of reactant molecules or atoms that must collide simultaneously to produce the reaction as written in the balanced equation. It represents the number of species involved in the rate-determining step of an elementary reaction.

2. Can a reaction have zero molecularity?

No, a reaction cannot have zero molecularity. The minimum molecularity is 1, which occurs in unimolecular reactions. Molecularity represents the number of reacting species, so at least one molecule must be involved for a reaction to occur.

3. What is a unimolecular reaction?

A unimolecular reaction is a reaction with a molecularity of 1, meaning only one reactant molecule is involved in the rate-determining step. Examples include isomerization reactions or the decomposition of a single molecule.

4. What is the maximum molecularity typically observed in reactions?

The maximum molecularity typically observed in reactions is 3 (termolecular reactions). Reactions with higher molecularities are extremely rare due to the low probability of more than three molecules colliding simultaneously with the correct orientation and energy.

5. Can molecularity change during a reaction?

No, molecularity does not change during a reaction. It is a fixed property of an elementary reaction step and is determined by the number of reactant molecules in the balanced equation for that step.

6. How does molecularity differ from reaction order?

Molecularity refers to the number of molecules participating in an elementary reaction step, while reaction order is determined experimentally and relates to how the rate of reaction depends on reactant concentrations. Molecularity is always a whole number, but reaction order can be fractional.

7. How does molecularity relate to the rate-determining step?

Molecularity specifically refers to the number of reactant species involved in the rate-determining step of an elementary reaction. It does not consider species involved in faster steps that occur before or after the rate-determining step.

8. How does temperature affect molecularity?

Temperature does not affect molecularity. Molecularity is a fundamental property of the reaction mechanism and remains constant regardless of temperature changes. However, temperature can affect reaction rate and the likelihood of successful collisions.

9. What is the relationship between molecularity and stoichiometry?

Molecularity and stoichiometry are related but distinct concepts. Stoichiometry deals with the quantitative relationships between reactants and products in a balanced equation, while molecularity focuses on the number of molecules involved in the rate-determining step of an elementary reaction.

10. How does molecularity affect reaction kinetics?

Molecularity directly affects reaction kinetics by influencing the rate law of elementary reactions. Higher molecularity generally leads to more complex rate laws and lower reaction probabilities due to the need for more molecules to collide simultaneously.

11. Can isotope effects change the molecularity of a reaction?

Isotope effects do not change the molecularity of a reaction, as molecularity is determined by the number of reactant molecules involved, not their isotopic composition. However, isotope effects can influence reaction rates and potentially favor different reaction mechanisms with different molecularities.

12. What is a bimolecular reaction?

A bimolecular reaction is a reaction with a molecularity of 2, meaning two reactant molecules or atoms must collide simultaneously for the reaction to occur. This is the most common type of reaction in solution chemistry.

13. Can a reaction with overall molecularity of 2 have a unimolecular step?

Yes, a reaction with an overall molecularity of 2 can have a unimolecular step if it occurs through multiple elementary steps. The overall molecularity is determined by the rate-determining step, which could be bimolecular, while other steps might be unimolecular.

14. What is a termolecular reaction?

A termolecular reaction is a reaction with a molecularity of 3, meaning three reactant molecules or atoms must collide simultaneously for the reaction to occur. These reactions are rare due to the low probability of three-body collisions.

15. Can molecularity be fractional?

No, molecularity cannot be fractional. It always represents a whole number of discrete molecules or atoms participating in an elementary reaction step. Fractional values are possible for reaction orders but not for molecularity.

16. What is the difference between elementary and overall reactions in terms of molecularity?

Elementary reactions have a molecularity that matches their reaction order and represents the actual number of molecules colliding. Overall reactions may have multiple steps with different molecularities, and their overall order may not match the molecularity of any single step.

17. How does molecularity affect the units of the rate constant?

The units of the rate constant depend on the molecularity of the reaction. For a first-order (unimolecular) reaction, the units are s⁻¹. For a second-order (bimolecular) reaction, the units are M⁻¹s⁻¹. The units change to ensure dimensional consistency in the rate law.

18. Can a reaction with molecularity 2 have a reaction order of 1?

Yes, a reaction with molecularity 2 can have a reaction order of 1 if it's not an elementary reaction. This situation can occur in complex reaction mechanisms where the rate-determining step is different from the overall stoichiometry of the reaction.

19. What is pseudo-first-order reaction and how does it relate to molecularity?

A pseudo-first-order reaction is a bimolecular reaction that behaves like a unimolecular reaction because one reactant is in large excess. The molecularity remains 2, but the reaction appears to have first-order kinetics due to the constant concentration of one reactant.

20. How does the concept of molecularity apply to catalyzed reactions?

In catalyzed reactions, the catalyst participates in the reaction but is regenerated, so it doesn't appear in the overall equation. However, the catalyst can affect molecularity by providing an alternative reaction pathway with different elementary steps and potentially different molecularities.

21. What is the significance of molecularity in understanding reaction mechanisms?

Molecularity is crucial for understanding reaction mechanisms as it provides insight into the number of species involved in each elementary step. This information helps in proposing and validating reaction mechanisms and predicting reaction rates and kinetics.

22. How does molecularity relate to the collision theory of reactions?

Molecularity directly relates to collision theory as it represents the number of molecules that must collide simultaneously with the correct orientation and energy for a reaction to occur. Higher molecularity reactions are less probable due to the decreased likelihood of simultaneous multi-body collisions.

23. Can a reaction change its molecularity if conditions (like pressure or concentration) change?

No, the molecularity of a reaction does not change with conditions like pressure or concentration. These factors can affect reaction rate, but molecularity is a fundamental property of the reaction mechanism and remains constant under varying conditions.

24. What is the molecularity of a decomposition reaction?

The molecularity of a decomposition reaction is typically 1 (unimolecular) because it involves the breakdown of a single molecule. However, some decomposition reactions might have higher molecularities if they involve catalysts or proceed through complex mechanisms.

25. How does molecularity affect the probability of a reaction occurring?

Higher molecularity generally decreases the probability of a reaction occurring because it requires more molecules to collide simultaneously with the correct orientation and energy. Unimolecular reactions are more probable than bimolecular, which are more probable than termolecular reactions.

26. What is the relationship between molecularity and the rate-determining step?

The molecularity of a reaction is determined by the rate-determining step, which is the slowest step in a multi-step reaction mechanism. The molecularity represents the number of reactant molecules involved in this slowest, rate-determining step.

27. Can a reaction have different molecularities for forward and reverse reactions?

Yes, a reaction can have different molecularities for forward and reverse reactions, especially in complex reaction mechanisms. For example, a decomposition reaction might be unimolecular in the forward direction but bimolecular in the reverse direction.

28. How does solvent affect molecularity?

The solvent does not directly affect molecularity, as molecularity is a property of the reaction mechanism. However, solvents can influence reaction rates and mechanisms, potentially favoring pathways with different molecularities, especially in solution-phase reactions.

29. What is the molecularity of a chain reaction?

Chain reactions typically involve multiple steps with different molecularities. While individual propagation steps might be unimolecular or bimolecular, the overall molecularity of a chain reaction is not a meaningful concept. Each step in the chain must be considered separately.

30. How does molecularity relate to the concept of active complexes or transition states?

Molecularity relates to the number of reactant molecules that come together to form the active complex or transition state in an elementary reaction step. The molecularity determines the number of species involved in forming this high-energy intermediate state.

31. Can enzymes change the molecularity of a reaction?

Enzymes cannot change the fundamental molecularity of a reaction, but they can provide alternative reaction pathways with different molecularities in individual steps. The overall effect may appear to change the reaction order, but the molecularity of each elementary step remains constant.

32. What is the difference between molecularity and multiplicity in chemical reactions?

Molecularity refers to the number of reactant molecules in an elementary reaction step, while multiplicity in chemistry typically refers to the number of unpaired electrons in a molecule or atom, which is related to its spin state. These concepts are unrelated in reaction kinetics.

33. How does molecularity affect the units of reaction rate?

Molecularity affects the units of reaction rate through its influence on the rate law. For a first-order (unimolecular) reaction, the rate units are concentration/time (e.g., M/s). For a second-order (bimolecular) reaction, the units are (concentration)²/time (e.g., M²/s).

34. Can a reaction with molecularity 3 have a rate-determining step with molecularity 2?

Yes, a reaction with an overall molecularity of 3 can have a rate-determining step with molecularity 2 if it proceeds through multiple elementary steps. The overall molecularity is determined by the stoichiometry, while the rate-determining step might involve fewer molecules.

35. What is the relationship between molecularity and the order of reaction with respect to each reactant?

In elementary reactions, the molecularity equals the sum of the orders with respect to each reactant. For example, in a bimolecular reaction A + B → C, the molecularity is 2, and the reaction is first order with respect to both A and B.

36. How does molecularity affect the half-life of a reaction?

Molecularity affects half-life through its influence on the rate law. For first-order (unimolecular) reactions, half-life is independent of concentration. For second-order (bimolecular) reactions, half-life is inversely proportional to initial concentration. Higher molecularities lead to more complex half-life relationships.

37. Can a reaction with molecularity 1 have a reaction order greater than 1?

In elementary reactions, a reaction with molecularity 1 (unimolecular) will have a reaction order of 1. However, in complex reactions with multiple steps, it's possible for a reaction with an apparent molecularity of 1 to have a higher overall order due to the influence of other steps in the mechanism.

38. What is the significance of molecularity in gas-phase reactions?

In gas-phase reactions, molecularity is particularly important because it directly relates to the number of gas molecules that must collide for the reaction to occur. Higher molecularity reactions are less favored in gases due to the lower probability of multi-molecule collisions in the less dense gaseous state.

39. How does molecularity relate to the concept of reaction profiles or energy diagrams?

Molecularity influences the shape of reaction profiles or energy diagrams. Higher molecularity reactions typically have more complex energy diagrams due to the need for multiple species to come together. The activation energy barrier often increases with higher molecularity due to the entropy cost of bringing more molecules together.

40. Can molecularity help in distinguishing between parallel and consecutive reactions?

Molecularity can help distinguish between parallel and consecutive reactions by providing information about individual elementary steps. Parallel reactions may have different molecularities for each pathway, while consecutive reactions might show a progression of molecularities through the reaction sequence.

41. What is the relationship between molecularity and the frequency factor in the Arrhenius equation?

Molecularity affects the frequency factor in the Arrhenius equation. Higher molecularity reactions generally have larger frequency factors because they represent the frequency of collisions between reactant molecules. However, they also tend to have higher activation energies, which can counteract this effect on reaction rates.

42. How does molecularity impact the concept of reaction flux?

Molecularity directly impacts reaction flux, which is the rate of flow of molecules through a reaction. Higher molecularity reactions typically have lower fluxes due to the decreased probability of simultaneous collisions between multiple reactant molecules.

43. How does molecularity relate to the principle of microscopic reversibility?

The principle of microscopic reversibility states that the mechanism of the forward and reverse reactions must be the same at a molecular level. This means that the molecularity of each elementary step in the forward direction must match the molecularity of the corresponding step in the reverse direction.

44. What is the significance of molecularity in understanding reaction networks?

In reaction networks, molecularity helps in understanding the complexity and interconnectedness of reactions. It provides insight into the likelihood of different pathways and can help in predicting dominant reaction routes based on the probability of molecular collisions.

45. How does molecularity affect the sensitivity of a reaction to isotope labeling studies?

Molecularity can affect the sensitivity of isotope labeling studies. Higher molecularity reactions may show more complex isotope effects due to the involvement of multiple labeled species. This can provide more detailed information about the reaction mechanism but may also complicate the interpretation of results.

46. Can molecularity help in predicting the effect of pressure on reaction rates in gas-phase reactions?

Yes, molecularity can help predict pressure effects on gas-phase reaction rates. Higher molecularity reactions are generally more sensitive to pressure changes because they involve the collision of more gas molecules. Increasing pressure typically increases the rate of higher molecularity reactions more significantly.

47. How does molecularity relate to the concept of reaction order in complex reaction mechanisms?

In complex reaction mechanisms, the overall reaction order may not directly correspond to the molecularity of any single step. The molecularity of the rate-determining step often influences the overall reaction order, but other factors like steady-state approximations and rate laws of individual steps also play a role.

48. What is the significance of molecularity in understanding autocatalytic reactions?

In autocatalytic reactions, molecularity helps understand how the product influences the reaction rate. The molecularity of steps involving the autocatalyst can explain the characteristic sigmoidal rate profile of these reactions, as the effective molecularity changes as the product accumulates.

49. How does molecularity affect the interpretation of kinetic isotope effects?

Molecularity affects the interpretation of kinetic isotope effects by influencing the number of isotopically sensitive positions in the reaction. Higher molecularity reactions may show more complex isotope effects due to multiple isotopically sensitive bonds or positions being involved in the rate-determining step.

50. Can molecularity provide insights into the entropy changes during a reaction?

Yes, molecularity can provide insights into entropy changes. Higher molecularity reactions generally involve a greater decrease in entropy in the transition state, as more molecules must come together in a specific orientation. This entropy consideration contributes to the rarity of high-molecularity reactions in nature.