what happens when the long chained hydrocarbon vapour passes over powdered aluminium gas
The correct answer and explanation is:
When long-chained hydrocarbon vapors pass over powdered aluminium oxide (Al₂O₃), not aluminium gas, a cracking reaction occurs.
🔹 Correct Answer:
Cracking takes place, breaking long-chain hydrocarbons into shorter-chain alkanes and alkenes.
🔬 Detailed Explanation (300 words):
Cracking is a chemical process used in the petrochemical industry to break down large, saturated hydrocarbon molecules (usually from crude oil) into smaller, more useful molecules like alkanes and alkenes. These smaller molecules are important as fuels and as raw materials in the production of plastics and other chemicals.
There are two main types of cracking:
- Thermal cracking – uses high temperatures and pressure.
- Catalytic cracking – uses a catalyst to speed up the reaction and reduce the required temperature and pressure.
In the case described in the question, catalytic cracking is being referenced. The powdered substance used is not aluminium gas (which doesn’t exist in practical lab settings), but typically powdered aluminium oxide (Al₂O₃) or silicon dioxide (SiO₂) — both act as solid catalysts.
🔥 What Happens:
When hydrocarbon vapors are passed over heated powdered aluminium oxide at temperatures around 500°C, the long hydrocarbon chains break apart into:
- Smaller alkanes (e.g., octane, useful as petrol)
- Alkenes (e.g., ethene, propene, used in plastic production)
This is important because:
- Long-chain hydrocarbons (like heavy oils) are not very useful as fuels since they burn poorly.
- Cracking converts them into more volatile, flammable, and valuable compounds.
Example Reaction:
A long-chain alkane like C₁₅H₃₂ might crack into:
- C₈H₁₈ (octane) and
- C₇H₁₄ (heptene)
Cracking is also an endothermic process, meaning it absorbs heat, which is why high temperatures are necessary.
Summary:
Passing long-chain hydrocarbon vapor over powdered aluminium oxide results in cracking, producing shorter, more useful hydrocarbons, especially alkenes and alkanes.