Video Transcript
Hydrogen gas and heat can be used
in the following reaction scheme to convert ethyne into ethene, then into
ethane. What catalyst must also be used in
steps A and B? (A) Aluminum mesh, (B) copper(II)
chloride solution, (C) finely divided nickel, (D) magnesium turnings, (E) powdered
vanadium pentoxide.
In this question, we are being
asked to identify the catalyst that can be used to facilitate the conversion of
ethyne into ethane. Let’s begin by taking a closer look
at the reaction scheme.
Ethyne, which contains a
carbon–carbon triple bond, is an alkyne. In step A, ethyne is converted into
ethene with the help of a catalyst. Ethene contains a carbon–carbon
double bond and is an alkene. Ethene contains two more hydrogen
atoms than ethyne. Next, in step B, ethene is
converted into ethane. Because ethane contains a
carbon–carbon single bond, it is an alkane. Ethane contains two more hydrogen
atoms than ethene.
So now we can see that in each step
of the reaction scheme, two more hydrogen atoms are being added to the molecule. Both alkenes and alkynes can react
with diatomic hydrogen in a hydrogenation reaction. This is a type of addition reaction
in which hydrogen reacts with an unsaturated hydrocarbon to make a larger molecule
and no by-product.
During step A of the reaction
scheme, the first hydrogenation reaction takes place. During the reaction, one 𝜋 bond
between the carbon atoms in ethyne and the single bond in the hydrogen molecule
break. Two new bonds form between carbon
and hydrogen atoms, resulting in a molecule of ethene. In step B, another hydrogenation
reaction occurs, and the 𝜋 bond in ethene and the single bond in the hydrogen
molecule break. Two new bonds form between carbon
and hydrogen atoms, and the result is a molecule of ethane.
The hydrogenation of alkynes and
alkenes requires the presence of a metal catalyst. The most common metal catalysts
used are platinum, palladium, and nickel. And they’re often in powdered form
to increase the surface area where the reaction takes place. Since nickel is one of the possible
catalysts for these reactions, we can select answer choice (C) as the correct
answer.
In conclusion, the catalyst that
must be used in steps A and B of the reaction scheme to convert ethyne to ethane is
finely divided nickel, or answer choice (C).
