Question

What type of reaction occurs between ethene and hydrogen?
A.Addition
B.Dehydration
C.Substitution
D.Oxidation

Answer 1

Hint:Ethene has the molecular formula \[{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}\] and there is a double bond between the two carbon atoms. So this makes it add up different molecules to the double bond.

Complete answer:
The alkenes and the alkynes are often referred to as the unsaturated hydrocarbons due to the presence of the double and triple bonds, respectively which are pi-bonds formed by the sidewise overlap of the atomic orbitals and are hence weaker than the sigma bonds which are formed by the head-on overlap of the atomic orbitals. Hence when the hydrogen molecules come in contact with the ethene molecules in the presence of the platinum or the palladium catalyst then the hydrogen molecules attack the pi-bond of the ethene molecule and form the ethane molecule.

So, this is an example of the addition reaction and the answer is option A.

Note:
The reaction between the ethene molecules and the hydrogen molecules proceed as follows. The hydrogen molecules attach themselves to the platinum or the palladium metal surface and then when these molecules come in contact with the ethene molecule, then the pi-bond between the C atoms is broken and the hydrogen atoms formed from the hydrogen molecule on the surface of the metal catalyst attacks the pi electron cloud and form the saturated hydrocarbon ethane. The saturated hydrocarbons undergo substitution reactions while the unsaturated ones undergo the addition reactions.