Final answer:
The intermediate formed during thermal cracking is a free radical, produced when high temperatures break C-C and C-H bonds. The free radical participates in further reactions to yield smaller hydrocarbons, like ethene and propene, in a process called a decomposition reaction.
Step-by-step explanation:
The type of reactive intermediate formed during thermal cracking is called a free radical. This reactive intermediate is formed when a high temperature, ranging from 450 °C to 900 °C, provides enough thermal energy to break the stable C-C and C-H bonds homolytically. In thermal cracking, these bonds break to form radical species that continue to react and typically result in the formation of smaller chain alkanes and alkenes.
In a typical free radical reaction mechanism, there are three main steps: initiation, where the radical is formed; propagation, where the radical reacts with other molecules to form new radicals; and termination, where radicals combine to form stable products. The free radical formed during the cracking of octane, for example, is part of the propagation step and would lead to smaller hydrocarbons such as ethene, C₂H₄, and propene, C₃H₆, though the particular products depend on the specific hydrocarbon being cracked and the conditions. The reaction in thermal cracking is classified as a decomposition reaction because a larger molecule is broken down into smaller molecules. This contrasts with a synthesis reaction where smaller molecules combine to form larger ones.