Question

Name one anti – knocking agent. What are its merits and demerits?

Answer 1

Hint: Anti – knocking agents are added to a fuel that helps in the effective use of the fuel. They help in the prevention of the knocking sound that engines make due to less effective combustion of the fuels. They are therefore termed as fuel additives. The knocking sound is produced in the engine due to the low octane number of the gas.

Complete answer:
When the fuel in the engine of any vehicle is not properly subjected to combustion then it produces the knocking sound in the working of the engine. Usually fuels (petrol) that have low octane numbers create a knocking sound. Octane number is the measure of the knocking resistance.
To prevent these knocking sounds, anti – knocking agents are used. These are chemical compounds added in the fuels that help in effective combustion. The most common anti – knocking agent used is tetraethyl lead (TEL) that is $Pb{{({{C}_{2}}{{H}_{5}})}_{4}}$.
The merit of tetraethyl lead is that it is used to enhance the octane number of a fuel like gasoline. The more the octane number, higher is the efficiency of the fuel and lower probability to produce knocking sound.
The demerit of tetraethyl lead is that, on combustion reactions in the fuels, it produces lead oxide that is considered as a harmful gas and a pollutant. It is a poisonous catalytic converter and contributes to air pollution.
Hence, an anti – knocking agent is tetraethyl lead (TEL).

Note:
The octane number (ON) is represented between the values from 0 to 100. The higher octane number means the fuel is more compressible and can be effective in the combustion. For instance, petrol can be 95 petrol and 98 petrol, but petrol with ON 98 is best because it is effective against the knocking of the engine.