Why is it often difficult to determine the actual pathway for a chemical reaction?

Each step in a mechanism will have a transition state. The transition states are often very difficult to identify and during a reaction the molecules exist in this state for essentially zero time. ... It is more of a theoretical idea for the configurations the chemical species must pass through during the reaction.

Explanation:

a gas, having a molecular weight of 13 kg/kmol and a kinematic viscosity of 0.25 cm2/s, flows through a pipe 0.25 m internal diameter and 5 km long at the rate of 0.4 m3/s and is delivered at atmospheric pressure.