Question

Tetra ethyl lead is added to gasoline to _______

• A. reduce gum formation
• B. increase its octane number
• C. reduce the pour point
• D. increase its smoke point

Hint:

Tetraethyl Lead (TEL) is a classic anti-knock agent used to boost the octane rating of gasoline.
Remember that "octane number" applies to gasoline (spark-ignition engines), whereas "cetane number" applies to diesel (compression-ignition engines).
Currently, non-toxic alternatives like MTBE or ethanol are used to increase octane numbers.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary purpose of adding Tetraethyl Lead (TEL) to gasoline in petroleum refining and fuel formulation.
Developing high-performance fuels for internal combustion engines requires controlling the combustion rate to prevent engine knocking.

Step 2: Key Formula or Approach:
1. Engine knocking (auto-ignition) occurs when the unburned fuel-air mixture in the cylinder ignites spontaneously before the spark plug's flame front reaches it.
2. The octane number is a standard measure of a fuel's ability to resist knocking.
3. Additives that increase this resistance are called anti-knock agents.
4. Tetraethyl Lead, \(\text{Pb}(\text{C}_2\text{H}_5)_4\), is an organometallic compound that historically served as a highly effective anti-knock additive.

Step 3: Detailed Explanation:
Let us examine the mechanism of Tetraethyl Lead in gasoline:
- When gasoline containing TEL burns inside the engine, the weak \(\text{Pb}-\text{C}\) bonds break at high temperatures, releasing lead oxide (PbO) and ethyl radicals.
- The lead species act as radical scavengers, terminating the radical chain reactions that lead to rapid, pre-flame auto-ignition of the hydrocarbon fuel.
- By delaying auto-ignition, TEL allows the fuel to burn smoothly and progressively, which significantly increases the fuel's octane rating (octane number).
- This allows engines to operate at higher compression ratios, improving thermal efficiency and engine power.
- Although highly effective, TEL has been phased out globally in commercial motor gasoline due to the high toxicity of lead emissions and because lead poisons the catalytic converters used for emissions control.
- Nevertheless, its primary historical and chemical function is to increase the octane number of the fuel.

Step 4: Final Answer
Therefore, Tetra ethyl lead is added to gasoline to increase its octane number, corresponding to option (B).