KEY2CHEM

Elementary Reactions

An elementary reaction is one that occurs via a single collision between the reacting particles. Generally, elementary reactions that are unimolecular (one particle reacts) or bimolecular (two particles react) are the most common. Aligning three or particles with appropriate collision energy and orientation is quite rare. As the collision frequency increases, the rate of the reaction increases. Additionally, as the molecular complexity increases (more atoms are contained in the structure), it becomes increasingly challenging to align particles with precisely the correct orientation, as the rate of the reaction decreases.

Example 1.

Which elementary reaction is most likely to occur successfully?

A. $$\require{mhchem}\ce{ 2 NO + Cl2 -> 2 NOCl}$$

B. $$\require{mhchem}\ce{ 2 NO2 -> N2O4}$$

C.  $$\require{mhchem}\ce{ 2 NO + O2 -> 2 NO2}$$

Solution

B. $$\require{mhchem}\ce{ 2 NO2 -> N2O4}$$

Elementary reactions are those that occur in a single collision. Bimolecular reactions (two particles collide) are more likely than termolecular reactions (three particles collide) due to the requirement for correct orientation and energy of all particles for a successful reaction.

Example 2.

Which elementary reaction is unimolecular?

A. $$\require{mhchem}\ce{ CH3CN -> CH3NC}$$

B. $$\require{mhchem}\ce{ NO + O3 -> NO2 + O2}$$

C.  $$\require{mhchem}\ce{ 2 Br -> Br2}$$

Solution

B. $$\require{mhchem}\ce{ CH3CN -> CH3NC}$$

A unimolecular reaction is one where one particle reacts (in this case, it is an isomerization, where the bonds rearrange). A bimolecular reaction is one where two particles (either the same as one another or different) react.

Example 3.

Which elementary reaction is most rare?

A. $$\require{mhchem}\ce{ N2O4 -> 2 NO2}$$

B. $$\require{mhchem}\ce{ 2 NO + O2 -> 2 NO2}$$

C.  $$\require{mhchem}\ce{ CH3Br + OH- -> CH3OH + Br-}$$

Solution

B. $$\require{mhchem}\ce{ 2 NO + O2 -> 2 NO2}$$

Termolecular reactions requiring aligning three molecules with correct orientation; these are rare due to the difficulty in aligning all three particles with exactly the correct orientation.