For a certain reacation large fraction of molecules has energy more than energy more than the threshold enrgy,yet the rate of reaction is very slow. Why ?

Explanation

To understand why a reaction can be slow even when a large fraction of molecules have energy greater than the threshold energy, we can break down the reasoning into clear steps:

### Step-by-Step Solution:

1. **Understanding Threshold Energy**:
- Every chemical reaction requires a minimum amount of energy, known as the threshold energy, for the reactants to convert into products. Molecules must collide with energy equal to or greater than this threshold to initiate a reaction.

2. **Collision Theory**:
- According to collision theory, two conditions must be met for a reaction to occur:
- The colliding molecules must have energy equal to or greater than the threshold energy.
- The molecules must collide with the proper orientation.

3. **Energy Condition Met**:
- In this scenario, it is stated that a large fraction of molecules possess energy greater than the threshold energy. This means that the first condition for a reaction (sufficient energy) is satisfied.

4. **Orientation Issue**:
- Despite having sufficient energy, the reaction is still slow. This indicates a problem with the second condition—proper orientation.
- Molecules must collide in a specific way for the reaction to proceed. If the orientation is incorrect, even energetic collisions will not lead to a successful reaction.

5. **Example of Improper Orientation**:
- Consider a reaction where CH3Br reacts with OH⁻. If CH3Br collides head-on with OH⁻, they may repel each other without reacting.
- For the reaction to occur, OH⁻ must approach CH3Br in a specific orientation that allows it to attack the carbon atom and displace the bromine atom.

6. **Conclusion**:
- Therefore, the slow rate of reaction, despite a large fraction of molecules having energy above the threshold, is primarily due to improper orientation during collisions. This highlights the importance of both energy and orientation in chemical reactions.