TEL, tetraethyl lead, acts as antiknocking agent. It acts as……………………… catalyst.

Answer

**Step-by-Step Solution:**

1. **Understanding the Role of Tetraethyl Lead (TEL):**
- Tetraethyl lead (TEL) is a compound that has been historically used as an anti-knocking agent in petrol (gasoline). Its primary function is to improve the combustion characteristics of the fuel.

2. **Defining Anti-Knocking Agents:**
- Anti-knocking agents are substances added to fuels to prevent knocking during combustion. Knocking is an undesirable phenomenon that occurs when fuel ignites prematurely in the engine cylinder, leading to inefficient engine performance and potential damage.

3. **Identifying the Type of Catalyst:**
- Catalysts are substances that increase or decrease the rate of a chemical reaction without being consumed in the process. They can be classified as:
- **Positive Catalysts:** These increase the rate of reaction.
- **Negative Catalysts:** These decrease the rate of reaction.

4. **Analyzing the Function of TEL:**
- In the context of its use as an anti-knocking agent, TEL works by reducing the rate of ignition of the fuel vapors. This means that it helps to control the combustion process, preventing premature ignition.

5. **Conclusion:**
- Since TEL reduces the rate of reaction (in this case, the ignition of fuel vapors), it is classified as a **negative catalyst**.

**Final Answer:**
Tetraethyl lead (TEL) acts as a **negative catalyst**.

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