For which type of the reactions, order and molecularity have the same value ?

Explanation

To determine for which type of reactions the order and molecularity have the same value, we can follow these steps:

### Step 1: Define Order and Molecularity
- **Order of a Reaction**: The order of a reaction is defined as the sum of the powers of the concentration terms in the rate law expression for that reaction.
- **Molecularity of a Reaction**: Molecularity refers to the number of reactant molecules involved in a single step of the reaction mechanism.

### Step 2: Identify the Type of Reaction
- The type of reaction we are interested in is called an **elementary reaction**.

### Step 3: Characteristics of Elementary Reactions
- An elementary reaction is a reaction that occurs in a single step and involves a single transition state. In these reactions, the rate law can be directly written from the balanced equation.

### Step 4: Example of an Elementary Reaction
- Consider a hypothetical elementary reaction where two reactants A and B combine to form a product:
\[ A + B \rightarrow Products \]
- The rate law for this reaction can be expressed as:
\[ \text{Rate} = k[A]^1[B]^1 \]
- Here, the order of the reaction is the sum of the exponents in the rate law:
\[ \text{Order} = 1 + 1 = 2 \]

### Step 5: Calculate Molecularity
- The molecularity of this reaction is determined by counting the number of reactant molecules involved:
- In this case, there are 2 reactant molecules (A and B).

### Step 6: Conclusion
- Since both the order (2) and molecularity (2) are equal for this elementary reaction, we conclude that for **elementary reactions**, the order and molecularity have the same value.

### Final Answer
**Elementary reactions are the type of reactions for which the order and molecularity have the same value.**

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