The rate of reaction for a bimolecular elementary reaction is directly proportional to the product of the concentrations of the two reacting species.
Understanding Bimolecular Elementary Reactions
A bimolecular elementary reaction is a chemical reaction step in which two reactant molecules collide and combine to form products. This is one of the most common types of elementary steps in a reaction mechanism. The molecularity of a reaction step refers to the number of molecules participating in that specific step. For a bimolecular reaction, the molecularity is two.
Determining the Rate Law
For an elementary reaction, the rate law can be directly determined from its stoichiometry. This is a unique characteristic of elementary steps, as opposed to overall reactions which may involve multiple steps.
Consider a general bimolecular elementary reaction:
A + B → Products
In this reaction, one molecule of reactant A collides with one molecule of reactant B. The rate law for this specific elementary step is given by:
rate = k[A][B]
Where:
- rate represents the speed at which the reaction proceeds.
- k is the rate constant, a proportionality constant that depends on temperature and the specific reaction.
- [A] is the molar concentration of reactant A.
- [B] is the molar concentration of reactant B.
If the two reacting molecules are the same, such as in the elementary step:
2A → Products (or A + A → Products)
The rate law would similarly be:
rate = k[A]2
This signifies that the reaction rate is proportional to the square of the concentration of reactant A.
Summary of Elementary Reaction Rate Laws
The following table illustrates how the rate law is derived directly from the molecularity of elementary reactions:
| Molecularity | Elementary Step Example | Rate Law |
|---|---|---|
| Unimolecular | A → Products | rate = k[A] |
| Bimolecular | A + B → Products | rate = k[A][B] |
| 2A → Products | rate = k[A]2 | |
| Termolecular | A + B + C → Products | rate = k[A][B][C] |
| 2A + B → Products | rate = k[A]2[B] | |
| 3A → Products | rate = k[A]3 |
It is crucial to remember that these simple rate laws derived directly from stoichiometry only apply to elementary reactions, not necessarily to overall complex reactions which consist of multiple elementary steps.
