Video Transcript
Decane is a relatively large,
saturated hydrocarbon that can be broken down into smaller molecules during a
cracking reaction. Which of the following pairs of
molecules could be produced when a single molecule of decane is broken down during a
cracking reaction? (A) C8H18 plus C2H4. (B) C8H18 plus C2H6. (C) C8H16 plus C8H14. (D) C2H4 plus C9H20. Or (E) C2H4 plus C10H22.
To answer this question, we need to
know how many carbon atoms are in a molecule of decane. From the -ane portion of its name,
we know that decane is an alkane, and its prefix dec- indicates that it contains 10
carbon atoms. When decane undergoes cracking, it
is broken down into smaller molecules. After the cracking process, the
total number of carbon atoms in the two product molecules will be the same as the
total number of carbon atoms in the large decane molecule.
As decane has 10 carbon atoms, the
total carbon atoms for the two smaller molecules should also be 10. With this in mind, we can have a
look at the answer choices.
For answer choice (A), the total
number of carbon atoms is eight plus two, which gives us 10 and is the same for
answer choice (B). Answer choice (C) is eight plus
eight, which gives us a total of 16 carbon atoms, 11 carbon atoms for answer choice
(D), and 12 carbon atoms for answer choice (E). Since the products in answer
choices (C), (D), and (E) do not have a total of 10 carbon atoms, we can eliminate
them.
To determine the chemical formulas
for the two product molecules, we must consider the type of hydrocarbon that decane
is. The question tells us that decane
is a saturated hydrocarbon, which means that in its structure the carbon atoms
engage in only single bonds with each other. During the cracking reaction of a
saturated hydrocarbon, smaller alkanes and alkenes are produced. The general formula for an alkane
is C𝑛H2𝑛+2, where 𝑛 is a whole number representing the number of carbon atoms in
the structure. We can see that the first product
listed in both answer choices (A) and (B), C8H18, fits this general formula and thus
is our alkane product.
Therefore, the second product must
be an alkene and must fit the general formula for alkenes, which is C𝑛H2𝑛, where
𝑛 again is a whole number representing the number of carbon atoms in the
structure. We can see that the second product
in answer choice (B) does not fit the general formula for an alkene. We can eliminate answer choice
(B). The second product listed in answer
choice (A) does fit this general formula and is thus an alkene.
Therefore, the pair of molecules
that could be produced when a single molecule of decane is broken down during a
cracking reaction is answer choice (A) C8H18 plus C2H4.
