Question

Which one of these was added in Petrol as anti-knocking.
a) Pb
b) Tetraethyl lead
c) Hg
d) Ni

Answer 1

Hint: An Anti-knocking agent is a gasoline additive used to decrease an engine's unpleasant sound or knocking tendency.

Complete step by step answer:
Let's start with the understanding of knocking.It is a kind of explosion produced in the internal combustion engine due to the immature ignition of the fuel and therefore it slows down the reaction which creates unpleasant sound from the internal combustion engine known as Knocking.

Chemical structure and knocking tendency depend upon nature and structure of molecules of hydrocarbon present in the fuel.
Knocking tendency decreases in the following order:
Straight chain alkane > Branched chain alkane > Alkene > Aromatics

There are major groups of antiknock agents.
1.) Aromatics:Toluene and xylene are used as anti knock additives. The properties of toluene and xylene are almost identical. They both raise the Octane number of fuel and they are non-carcinogenic compounds unlike benzene so they are not harmful for humans.
2.) Oxygenates: Those compounds which contain oxygen in their chemical structure. The usually known as oxygenated chemical compounds.There are some compounds of alcohol are used as anti knocking agents. Ethanol is the most widely used alcohol to reduce the knocking.
3.) Organometallic compounds: It contains a carbon-metal bond.They are toxic in nature. Dicylopentadienyl iron, nickel carbonyl and tetraethyl are used as metal antiknock agents.

The octane number of fuels can be raised by the addition of certain compounds like Tetraethyl lead. Such compounds are called Antiknock compounds and the process is called doping in the fuel which is made by above molecules possessing anti knock properties.
Tetraethyl lead is an organometallic compound which is formed when Ethyl Bromide reacts with sodium lead alloy.
Other options are incorrect as they all are metals and do not show a property for reducing knocking sound individually so they are not included in any group which are mentioned.
So option 2 (Tetraethyl lead) is correct.
Note: Octane number is a measurable property of a fuel’s ability to resist knocking or unpleasant sound. During the reaction occurs in the internal engine. Higher the Octane number, Lower the knocking.