${\mathrm{CH}}_2={\mathrm{CH}}_2+{\mathrm{H}}_2\stackrel{\mathrm{Ni} \mathrm{or} \mathrm{Pt} }{⟶}{\mathrm{CH}}_3-{\mathrm{CH}}_3$ in the reaction which is mentioned above, the reactant is Ethene and the product formed is [[1]]

Addition reactions with hydrogen take place in unsaturated hydrocarbons which are Alkenes and Alkynes. In general, alkanes and alkynes can be written as-
Alkenes: ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}}$
Alkynes: ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}-\text{2}}$
The following are the necessary conditions for an additional reaction to occur:
1. The existence of multiple bonds (double and triple bonds) between carbon atoms in a hydrocarbon chain.
2. The hydrogen addition should take place in the presence of a catalyst, such as nickel or platinum.
Here is the chemical equation which would explain the mentioned process:
$\underset{\mathrm{Ethene}}{\mathrm{C}{\mathrm{H}}_{\mathrm{2}}=C{\mathrm{H}}_{\mathrm{2}} }+ {\mathrm{H}}_{2 \mathrm{ }\mathrm{ }}\mathrm{ }\stackrel{\mathrm{Ni} \mathrm{or} Pt}{\to }\mathrm{ }\mathrm{ } \underset{\mathrm{Ethane}}{\mathrm{C}{\mathrm{H}}_{\mathrm{3}} -\mathrm{C}{\mathrm{H}}_{\mathrm{3}}}$
In the reaction which is mentioned above, the reactant is Ethene and the product formed is Ethane. 

The reaction which is mentioned also requires a lot of heat for the occurrence.