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─C─CH₃ 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 because they have similar molecular masses and boiling points due to their structural similarities.

Compound A is likely to auto-ignite because it has the longest carbon chain, making it less branched and therefore more stable under compression.

A: pentane; B: 2-methylbutane; C: 2,2-dimethylpropane.

Catalytic cracking.

The formula for W can be deduced as C₈H₁₈, which is octane.

The advantage of adding tetraethyl lead is to increase the octane rating of the fuel, which reduces knocking and enhances engine performance.

Its use in car engines was discontinued due to the toxic and carcinogenic nature of lead, which poses environmental and health risks.

To calculate the heat of reaction:

1. Use the reaction equation:

   $2(C₈H₉Pb) + 270° → 16CO₂ + 16H₂O + 2PbO$

2. Apply the formula:

   $\Delta H = \Sigma H_{f, products} - \Sigma H_{f, reactants}$

3. Substitute the heats of formation:

   $\Delta H = [16(-393) + 16(-285.8) + 2(-219)] - [2(-527.3)]$

4. Calculate:

   = -6,277.7 \, kJ \, mol^{-1}$$