The evaluation of petrol from crude oil before it is put on sale is to prevent

Analysis

The question asks why petrol (gasoline) obtained from crude oil must be evaluated or treated before it is sold for use in internal combustion engines. We need to identify the specific phenomenon that occurs when low-quality petrol is used in an engine.

Concept Explanation

When petrol is used in an engine, it should burn smoothly. However, if the petrol has a low octane rating (meaning it contains a high proportion of straight-chain alkanes), it tends to ignite prematurely due to high pressure and heat before the spark plug fires.

This premature, explosive combustion causes a metallic knocking or rattling sound in the engine, which can lead to engine damage and loss of power. This phenomenon is known as knocking or pinking. To prevent this, petrol is treated (often by adding anti-knock agents like tetraethyl lead or by reforming the hydrocarbons into branched-chain alkanes) to ensure it burns smoothly.

Step-by-Step Solution

1. Analyze the options: • Cracking (D): This is the process of breaking down large hydrocarbon molecules into smaller, more useful ones. It is a production method, not a prevention method. • Pyrolysis (C): This is the thermal decomposition of substances at high temperatures in the absence of oxygen. It is a chemical process, not a problem to be prevented. • Reforming (B): This is a process used to improve the quality of petrol by converting straight-chain alkanes into branched-chain alkanes or cyclic hydrocarbons to increase the octane rating. It is a solution to the problem, not the problem itself. • Pinking (A): This is the common term used to describe "knocking" in an engine. It is the undesirable phenomenon that occurs when petrol quality is poor.

2. Conclusion: The evaluation and treatment of petrol are specifically carried out to prevent the engine from "pinking" (knocking).

Exam Tip

Remember the "Octane Rating" rule: High octane = smooth burning (no pinking). Low octane = "pinking" or "knocking." If you see "pinking" or "knocking" in a JAMB question, it is almost always referring to the quality of petrol in an internal combustion engine.