Question

Alkaline earth metals form carbides of the type \(\overline{M C}_{2}\). But \(\mathrm{MgC}_{2}\) on heating converts into \(\mathrm{X}\), which on hydrolysis give (1) Methane (2) Acetylene (3) Propyne (4) Ethane

Short answer

Acetylene

Step-by-step solution

- Identify the formula of the compound

The given compound is \( \mathrm{MgC_{2}} \).

- Determine the type of carbide

The compound \( \mathrm{MgC_{2}} \) is a carbide of the alkaline earth metal magnesium.

- Identify the reaction on heating

On heating, \( \mathrm{MgC_{2}} \) decomposes to form magnesium and acetylene gas: \[ \mathrm{MgC_{2}} \rightarrow \mathrm{Mg} + \mathrm{C_{2}H_{2}} \]

- Consider hydrolysis of the product

When the acetylene gas \( \mathrm{C_{2}H_{2}} \) formed upon heating is subjected to hydrolysis, it does not significantly change in composition but just reacts to form simple acetylene gas again.

- Find the correct product on hydrolysis

Since \( \mathrm{C_{2}H_{2}} \) is acetylene, the correct answer is acetylene.

Key concepts

Magnesium Carbide

Magnesium carbide, with the chemical formula \(\mathrm{MgC_{2}}\), is a compound formed from the reaction of magnesium with carbon. It belongs to the family of carbides, which are compounds composed of carbon and a less electronegative element—often a metal. Magnesium, being an alkaline earth metal, forms different kinds of carbides, and in this case, it forms the ionic compound \(\mathrm{MgC_{2}}\). Understanding magnesium carbide is fundamental in grasping how it reacts under different conditions, as it sheds light on the behavior of other similar alkaline earth metal carbides.

Acetylene Formation

When magnesium carbide (\(\mathrm{MgC_{2}}\)) is heated, it undergoes a decomposition reaction. This reaction breaks down the original compound into simpler substances. In this case, \(\mathrm{MgC_{2}}\) decomposes to produce magnesium and acetylene gas (\(\mathrm{C_{2}H_{2}}\)). This can be described by the equation:

\[ \mathrm{MgC_{2} \rightarrow Mg + C_{2}H_{2}} \]

Acetylene is a hydrocarbon with the formula \(\mathrm{C_{2}H_{2}}\), characterized by a triple bond between the two carbon atoms. Acetylene is highly flammable and used in various industrial applications, including as a fuel for welding torches.

Hydrolysis Reaction

Hydrolysis is a chemical reaction involving the breaking of a bond in a molecule using water. In the context of acetylene, when acetylene gas \(\mathrm{C_{2}H_{2}}\) is subjected to hydrolysis, the reaction typically involves water as a reactant but does not significantly alter the composition of the acetylene. Essentially, acetylene remains as acetylene.

• This means acetylene (\(\mathrm{C_{2}H_{2}}\)) does not transform into other compounds like methane, propyne, or ethane upon hydrolysis.


• The primary product of hydrolysis remains acetylene.

This understanding is crucial in exercises involving the identification of reaction products when dealing with metal carbides. Hence, the correct answer to the original exercise is acetylene (the correct option being option 2).