Necessity for third bodies in termination

Answer. When the unimolecular reaction is in the first order region the ratedetermining step is reaction, rather than activation. The reverse reaction must also have reaction as rate determining and this corresponds to the second order region of the recombination. Recombination and decomposition will be in the high pressure region. 

At low pressures the unimolecular decomposition will have activation rate determining, and will be second order with a third body required for energy transfer. 

The recombination must also have energy transfer as rate determining, and for this removal of energy a third body is required, leading to third order kinetics (see Section 4.5.12). 

These generalizations about the need for a third body can be explained physically in terms of the three types of recombination  

Third bodies are thus required for atom-atom recombinations. 

---