Question

What is the name of the reaction given below?
${{C}_{2}}{{H}_{4}}\text{ + }{{\text{H}}_{2}}\text{ }\xrightarrow{Ni/Pt}\text{ }{{C}_{2}}{{H}_{6}}$
A.Substitution
B.Hydrogenation
C.Alkanes formation
D.Dehydrohalogenation

Answer 1

The above reaction is the conversion of an Unsaturated hydrocarbon (alkene) into a saturated hydrocarbon (alkane). Hydrogen is added to the reactant in the presence of the catalyst Nickel and platinum. To identify the type of reaction, observe the new bonds formed in the product.

Complete answer:
First, let us understand the reaction mechanism of the given reaction:
${{C}_{2}}{{H}_{4}}\text{ + }{{\text{H}}_{2}}\text{ }\xrightarrow{Ni/Pt}\text{ }{{C}_{2}}{{H}_{6}}$
In the above reaction, Hydrogen gas is passed over ethene to form ethane. The double bond in ethene is broken and the hydrogen molecules are added at that site. This reaction occurs in the presence of Nickel and platinum which speed up the chemical reaction.
We know that, hydrogenation
Now, in the reaction hydrogen is added to the alkene, which means that alkene is hydrogenated.
A hydrogenation reaction occurs when a hydrogen molecule is added to a double bond. Hydrogenation of alkene is carried out because it is thermodynamically a very stable reaction. The product formed has lower energy as compared to the reactant that makes it stable. With the formation of the alkane high energy is released, so it is an exothermic reaction. Thus this reaction cannot occur under normal conditions. A favourable catalyst is used which speeds up the reaction to the final product.
Therefore, the given reaction: ${{C}_{2}}{{H}_{4}}\text{ + }{{\text{H}}_{2}}\text{ }\xrightarrow{Ni/Pt}\text{ }{{C}_{2}}{{H}_{6}}$ is a Hydrogenation reaction.
Final answer: The correct answer is Option B: Hydrogenation.

Note:
Hydrogenation of alkenes in the presence of a catalyst takes place in two steps. In step 1: the alkene is made to adsorb on the surface of the catalyst along with the hydrogen. In step 2: the hydrogen present on the surface of the metal catalyst shifts from the metal surface to the carbon double. The alkane formed is weakly adsorbed, so it leaves the surface of the catalyst.