Ethene is prepared industrially by

Analysis

The question asks for the industrial method used to produce ethene ($C_2H_4$), which is a simple alkene. We need to identify which of the listed processes is responsible for breaking down larger hydrocarbon molecules into smaller, more useful ones like ethene.

Concept Explanation

In the petrochemical industry, crude oil is processed into various fractions. However, the demand for smaller, highly reactive molecules like ethene (used to make plastics like polythene) often exceeds the amount naturally present in crude oil.

Cracking is the process of breaking down large, long-chain hydrocarbon molecules (found in heavy oil fractions) into smaller, more useful molecules (like ethene and petrol) by heating them to high temperatures, often in the presence of a catalyst.

• Reforming: This is used to convert low-octane naphtha into high-octane gasoline (petrol) by changing the structure of the molecules (e.g., making them branched or cyclic).
• Polymerization: This is the opposite of cracking; it is the process of joining many small monomer units (like ethene) together to form a long-chain polymer (like polyethene).
• Distillation: This is a physical separation process used to separate components of crude oil based on their different boiling points. It does not create new chemical substances.

Step-by-Step Solution

1. Identify the goal: We need to produce ethene ($C_2H_4$), a small molecule.
2. Evaluate the options: • Distillation only separates existing substances; it doesn't create ethene. • Polymerization consumes ethene to make plastics. • Reforming changes the shape of molecules to improve fuel quality. • Cracking breaks large, heavy molecules into smaller, lighter ones.
3. Conclusion: Since ethene is a small molecule derived from larger hydrocarbons, cracking is the correct industrial process.

Correct Answer

The correct option is D.

Exam Tip

Think of Cracking as "breaking." Whenever you see a question about converting long-chain hydrocarbons into shorter, more useful ones (like ethene or petrol), the answer is almost always Cracking. Remember: Cracking = Breaking down.