In an elementary reaction, A + 2B represents the molecularity of the reaction, meaning the number of reacting species that collide simultaneously in a single step.
Key Meaning:
- A + 2B means:
- One molecule of A
- Two molecules of B
- These three particles collide at the same time for the reaction to occur.
Molecularity:
- Molecularity = total number of reacting species
- Here, Molecularity = 1 (A) + 2 (B) = 3
- So, this is a termolecular reaction
Important Points:
- Elementary reactions occur in a single step
- The stoichiometric coefficients directly represent molecularity
- Rate law can be written directly as:
Rate = k [A][B]²
Practical Insight:
- Termolecular reactions are rare because the probability of three particles colliding simultaneously with proper orientation is low
- Most real reactions occur through multiple elementary steps
Conclusion:
The expression A + 2B in an elementary reaction indicates a termolecular process, where one molecule of A reacts with two molecules of B in a single collision step.