Tetraethyl lead TEL is added to petrol to minimise knocking in internal combustion engines. In terms of reaction rate, TEL mainly acts as which type of catalyst like additive?

Introduction / Context:
In older petrol engines, knocking or detonation was a serious problem that reduced efficiency and could damage the engine. Tetraethyl lead, abbreviated TEL, was once widely added to petrol as an anti knocking agent to improve fuel performance. Conceptually, its action can be compared to catalytic effects on reaction rates. This question asks you to identify whether TEL behaves like a positive catalyst, a negative catalyst, an auto catalyst, or an induced catalyst with respect to the reactions responsible for knocking.

Given Data / Assumptions:

• Knocking arises from uncontrolled, premature combustion and undesirable chain reactions in the fuel air mixture.
• Anti knocking agents aim to slow or prevent these undesirable reactions.
• TEL is added in small amounts to petrol for this purpose.
• The options describe types of catalysts in terms of whether they speed up or slow down reactions and how they appear in the system.

Concept / Approach:
A positive catalyst increases the rate of a desired reaction. A negative catalyst, also called an inhibitor or retarder, decreases the rate of a reaction, particularly an undesirable one. Auto catalysts are formed in the reaction mixture itself, and induced catalysts act only in the presence of another catalyst. TEL does not speed up combustion; instead, it suppresses uncontrolled chain reactions that cause knocking, effectively slowing those undesirable side reactions. In this sense, it behaves like a negative catalyst, decreasing the rate of knocking reactions and allowing smoother, controlled combustion of petrol.

Step-by-Step Solution:
Step 1: Understand that engine knocking results from rapid, uncontrolled combustion and explosive pressure waves in the cylinder. Step 2: Recognise that TEL is added in small quantity to petrol to reduce this knocking, leading to smoother burning of the fuel air mixture. Step 3: Note that TEL does not aim to speed up the combustion reaction; rather, it interferes with the chain processes that lead to premature detonation. Step 4: Relate this role to the concept of a negative catalyst, which retards or inhibits unwanted reactions and stabilises the system. Step 5: Conclude that TEL is best classified, in this context, as a negative catalyst like inhibitor with respect to knocking reactions.

Verification / Alternative check:
Engine and fuel chemistry texts describe anti knocking agents as substances that raise the octane rating of petrol by making it more resistant to premature ignition. TEL exerts its effect by interacting with radicals and intermediates in the combustion process, thereby slowing down uncontrolled chain branching reactions that cause knocking. This behaviour is analogous to that of inhibitors used in chemical industry to prevent runaway polymerisation or decomposition, which are classic examples of negative catalysts. TEL is not produced automatically by the fuel, so it is not an auto catalyst, and it does not rely on a second catalyst to work, so it is not an induced catalyst in normal classification.

Why Other Options Are Wrong:
A positive catalyst that speeds up all reactions would actually make knocking worse by accelerating combustion indiscriminately, which is the opposite of what anti knocking agents are intended to do. Auto catalysts are substances formed during the reaction that then catalyse it further, like manganese in some oxidation reactions; TEL is deliberately added from outside and is not generated by petrol combustion. Induced catalysts are catalysts whose activity appears only in the presence of another catalyst; there is no such second catalyst involved in the basic explanation of TEL action. Therefore, these options do not correctly describe TEL role in reducing knocking.

Common Pitfalls:
Learners sometimes see the word catalyst and assume that any useful additive must be a positive catalyst that speeds reactions up, so they choose that option without considering whether faster reactions would actually help. Another confusion is treating all specialised additives as auto catalytic simply because they are related to the reaction system. To answer correctly, focus on the effect on knocking. Because TEL reduces the rate of undesirable reactions and increases fuel stability, it behaves like a negative catalyst in this context.

Final Answer:
TEL minimises knocking when mixed with petrol by acting effectively as a negative catalyst that retards undesirable reactions causing knocking.