The Law of Lindemann is a fundamental principle in chemical kinetics that describes the unimolecular decomposition of molecules. It proposes that a molecule must first absorb energy from collisions with other molecules before it can decompose.
Key Points of the Law of Lindemann:
- Unimolecular Reactions: The Law of Lindemann specifically applies to reactions where a single molecule decomposes into products.
- Activation Energy: The energy absorbed by the molecule must be greater than or equal to the activation energy of the reaction for decomposition to occur.
- Activated Complex: The energized molecule is called an activated complex, which is an unstable intermediate state.
- Collision Frequency: The frequency of collisions between molecules plays a crucial role in determining the rate of decomposition.
- Temperature Dependence: The rate of decomposition is directly proportional to the temperature, as higher temperatures lead to more frequent and energetic collisions.
Example:
Consider the unimolecular decomposition of ozone (O<sub>3</sub>) into oxygen (O<sub>2</sub>) and oxygen atoms (O).
O<sub>3</sub> → O<sub>2</sub> + O
The Lindemann mechanism for this reaction is:
- Collisional Activation: Ozone molecules collide with other molecules, absorbing energy.
- Formation of Activated Complex: Some ozone molecules gain enough energy to form an activated complex, O<sub>3</sub>*.
- Decomposition: The activated complex decomposes into oxygen and oxygen atoms.
Practical Insights:
- The Law of Lindemann helps explain why the rate of unimolecular reactions increases with temperature.
- It provides a framework for understanding the role of collisions in chemical reactions.
Conclusion:
The Law of Lindemann is a crucial concept in understanding the kinetics of unimolecular reactions. It highlights the importance of collisions and activation energy in the decomposition of molecules.
