6. (a) (i) What term is used to describe compounds like A, B and C below that have the same molecular formula but different structural formulae? H3C—CH2—CH2—CH3 H3C—CH—C—CH3 | H A B C (ii) Why do A, B and C separate in the same fraction in the distillation of crude oil? (iii) Explain which of A, B and C is most likely to auto-ignite in a petrol engine - Leaving Cert Chemistry - Question 6 - 2019

A, B, and C separate in the same fraction during distillation due to their similar boiling points. Since they have similar relative molecular masses, their interactions with the distillation apparatus result in them being vaporized and condensed together at similar temperatures.

Compound A is most likely to auto-ignite in a petrol engine. This is because it has the longest carbon chain, which generally correlates with a higher octane rating, making it less prone to knocking. In contrast, compounds B and C may have branched structures that can influence their ignition timing adversely.

The systematic IUPAC names are as follows:

• A: pentane
• B: 2-methylbutane
• C: 2,2-dimethylpropane

The oil refining process is known as catalytic cracking.

The molecular formula of W can be deduced as C8H18. This is inferred from the reaction where one octane (C8H18) molecule produces two propene (C3H6) molecules, resulting in the consumption of 8 hydrogen atoms and thus maintaining balance in the chemical equation.

The advantage of adding tetraethyl lead to petrol is to increase the octane rating of the fuel. This reduction in the auto-ignition tendency of the fuel helps to prevent knocking and enhances engine performance.

Its use in car engines was discontinued because tetraethyl lead is toxic, poisonous, and potentially carcinogenic. Additionally, it can cause significant environmental damage, particularly to the atmosphere and water supply.

To find the heat of reaction for tetramethyllead, we use the enthalpy change equation:

$\Delta H = \sum H_{formation \, products} - \sum H_{formation \,, reactants}$

Substituting the respective values:

$\Delta H = [16(-393) + 20(-285.8) + 2(-219)] - [2(-314.8) + 4(-527.3)] \approx -6277.7 \, kJ \, mol^{-1}$

Thus, the heat of reaction for the combustion of one mole of tetramethyllead is approximately -6277.7 kJ mol–1.