Question

An unsaturated hydrocarbon having two carbon atoms was made to react with two moles of hydrogen gas in the presence of a catalyst to give a saturated hydrocarbon. Using the above information, answer the following questions.
(a) The type of reaction that occurred is __________ (addition / substitution)
(b) Name a suitable catalyst which is used in the reaction
(c) Write a balanced chemical equation for the above reaction

Hint:

If the hydrocarbon had only reacted with one mole of hydrogen, the starting material would have been Ethene (C_2H_4)!

Answer 1

This question describes an unsaturated hydrocarbon with two carbon atoms reacting with two moles of hydrogen in the presence of a catalyst to form a saturated hydrocarbon.

Let us understand it step by step.

An unsaturated hydrocarbon containing two carbon atoms and capable of adding two moles of hydrogen must be ethyne (C₂H₂).
This is because ethyne has a triple bond between the two carbon atoms, and a triple bond can add two molecules of hydrogen to become a saturated hydrocarbon.

After addition of two molecules of hydrogen, ethyne gets converted into ethane (C₂H₆), which is a saturated hydrocarbon.

(a) The type of reaction that occurred is:
The correct answer is addition.
In this reaction, hydrogen is added across the multiple bond of an unsaturated hydrocarbon.

(b) Name a suitable catalyst used in the reaction:
A suitable catalyst is nickel (Ni).
Other catalysts that may also be used are platinum (Pt) or palladium (Pd), but nickel is the commonly expected answer.

(c) Balanced chemical equation:

C₂H₂ + 2H₂ → C₂H₆

In the presence of catalyst:

C₂H₂ + 2H₂ \(\xrightarrow{\text{Ni}}\) C₂H₆

Detailed Explanation:
Unsaturated hydrocarbons contain double or triple bonds.
These multiple bonds can break and allow atoms like hydrogen to add to the molecule.
Since two moles of hydrogen are added here, the original compound must have had a triple bond.
Therefore, the hydrocarbon is ethyne, and the final product is ethane.

Final Answers:
(a) Addition
(b) Nickel
(c) C₂H₂ + 2H₂ → C₂H₆