6. (a) The hydrocarbon molecules in petrol typically contain carbon chains with between five and ten carbon atoms - Leaving Cert Chemistry - Question 6 - 2008

The structures for the hydrocarbons are as follows:

1. Heptane:

   H H H H H H H | | | | | | | H - C - C - C - C - C - C - C - H | | | | | | | H H H H H H H

2. 2,2,4-trimethylpentane:

        CH3
          |
   CH3 - C - CH - CH2 - CH3
          |        |
        CH3   CH2
   

   (Note: Structures can be drawn more neatly using software for clarity.)

The two fractions are:

1. Light gasoline
2. Naphtha

During dehydrocyclisation, the following changes occur:

1. Removal of hydrogen atoms from the hydrocarbon chain, leading to increased ring formation.
2. The creation of aromatic compounds through the restructuring of the hydrocarbon chains.

Another example of an oxygenate is methanol (methyl alcohol).

1. Oxygenates help to reduce carbon emissions by providing a cleaner burn in the engine, contributing to lower pollution levels.
2. They enhance the octane rating of the fuel, which can improve engine performance and efficiency.

The balanced chemical equation for the combustion of ethanol is:

$C_2H_5OH + 3 O_2 → 2 CO_2 + 3 H_2O$

To calculate the heat of combustion, we use the formula:

$\Delta H_{reaction} = \sum \Delta H_f(products) - \sum \Delta H_f(reactants)$

For the products:

• Carbon dioxide: 2 mol × -394 kJ/mol = -788 kJ
• Water: 3 mol × -286 kJ/mol = -858 kJ

For the reactants:

• Ethanol: 1 mol × -278 kJ/mol = -278 kJ

Now substituting into the equation:

$\Delta H_{combustion} = (-788 - 858) - (-278)$ $\Delta H_{combustion} = -1646 + 278 = -1368 kJ$

Thus, the heat of combustion of ethanol is -1368 kJ/mol.