Video Transcript
One molecule of octane is broken
down into two product molecules according to the given chemical equation. Compound X is a saturated
hydrocarbon. What is the molecular formula of
compound X? (A) C6H12, (B) C6H14, (C) C7H14,
(D) C7H16, or (E) C8H14.
Octane is an alkane with eight
carbon atoms. The question describes this alkane
undergoing a cracking reaction. A cracking reaction is a type of
decomposition reaction where larger organic molecules are broken down into smaller
molecules.
A cracking reaction can be
simplified into three steps. First, carbon-to-carbon single
bonds are broken. Then, carbon-to-hydrogen bonds are
broken and reformed to rearrange the organic molecule’s structure. Then, a carbon-to-carbon double
bond is formed. So the cracking reaction of an
alkane will form two products, one of which will contain a carbon-to-carbon double
bond and is called an alkene. The other product will be a
saturated hydrocarbon called an alkane.
In a cracking reaction, as we would
e𝑥pect for any other chemical reaction, the total number of carbon atoms remains
the same. Since octane has eight carbon
atoms, the total number of carbon atoms in the products must be eight as well. The first product molecule given is
a compound with two carbon atoms and a carbon-to-carbon double bond. It is a useful organic compound
called ethene.
We can determine that to find the
number of carbon atoms present in compound X, we can subtract the number of carbon
atoms in ethene from the total number of carbon atoms in the products. We get that eight minus two equals
six. So, compound X must have six carbon
atoms.
Compound X must also be an alkane,
and the general formula for alkanes is C𝑛H2𝑛+2, where 𝑛 is the number of carbon
atoms. By substituting six as 𝑛, we can
calculate the number of hydrogen atoms in compound X. First, we find two times six gives
us 12. Then, 12 plus two gives us 14.
Therefore, the molecular formula of
compound X is answer choice (B), C6H14.
