Video Transcript
What is the best description for
the cracking process? (A) Cracking is a dehydrogenation
process that forms alkenes as a main product. (B) Cracking is a dehydration
process that forms alkenes as a main product. (C) Cracking is a distillation
process that separates hydrocarbons into alkanes and corresponding alkenes. (D) Cracking is a thermal
decomposition reaction that breaks large hydrocarbon molecules into more useful
shorter ones. Or (E) cracking is a thermal
decomposition reaction that breaks large hydrocarbon molecules into alkenes and
corresponding alkynes.
Let’s clear some space on screen so
we can discuss cracking. A cracking reaction is the process
of taking larger organic molecules and turning them into smaller ones. So a cracking reaction is a
decomposition reaction, which is a type of reaction where a substance breaks down to
form two or more other substances. We can classify cracking reactions
as decomposition reactions since generally one reactant is breaking down to give
multiple smaller products.
Cracking reactions can occur for
both saturated and unsaturated hydrocarbons. Saturated hydrocarbons contain only
carbon-to-carbon single bonds, while unsaturated hydrocarbons contain at least one
multiple bond between carbon atoms. We will focus on the cracking
process of saturated hydrocarbons illustrated here in the diagram.
These large, saturated hydrocarbons
are often undesirable components of crude oil. There are three basic steps in a
cracking reaction process. Generally, a carbon-to-carbon
single bond in the long saturated molecule breaks. A lot of thermal energy is required
to break the stable carbon-to-carbon single bond. Additionally, sometimes high
pressure is applied, or a catalyst is used. Next, a carbon-to-hydrogen bond is
broken, and hydrogen atoms rearrange to form new carbon-to-hydrogen bonds. Finally, a new carbon-to-carbon
double bond forms. This process results in the
formation of smaller alkanes and alkenes. Smaller alkenes like ethene and
propene are extremely useful for the production of polymers.
Now that we have seen what the
cracking process is and its purpose, let’s have a look at the answer choices.
Since cracking is a type of
decomposition reaction, we can eliminate answer choices (A), (B), and (C) as they do
not describe decomposition reactions. Let’s consider answer choices (D)
and (E). Answer choice (E) states that the
products in a cracking reaction are alkenes and alkynes. However, it is alkanes and alkenes
that are formed from a cracking reaction. Therefore, we can also eliminate
answer choice (E).
Therefore, the best description for
the cracking process is answer choice (D). Cracking is a thermal decomposition
reaction that breaks large hydrocarbon molecules into more useful shorter ones.
