The octane number of a fuel is described as a measure of the tendency of the fuel to cause knocking, or as a measure of the tendency of the fuel to resist auto-ignition - Leaving Cert Chemistry - Question 6 - 2005

1. Short chain length: Hydrocarbons with shorter chains tend to have higher octane numbers due to lower volatility and reduced tendency to knock.

2. Branching: The presence of branching in hydrocarbons increases the octane number, as branched molecules are more stable during combustion.

Lead compounds are unsuitable because they act as catalyst poisons and can damage the catalytic converter in modern vehicles, leading to increased emissions and harmful environmental effects.

Common additives include oxygenates such as ethanol or methanol, which help to enhance the octane rating of fuels.

1. Pentane: CH3-CH2-CH2-CH2-CH3 (Pentane)

2. 2-methylbutane: (CH3)2CH-CH2-CH3 (2-methylbutane)

3. 2,2-dimethylpropane: (CH3)3C-CH3 (2,2-dimethylpropane)

Using the heat of formation data:

We have:

• For 2C6H6: $2 imes 49 ext{ kJ/mol} = 98 ext{ kJ/mol}$
• For 12CO2: $12 imes -394 ext{ kJ/mol} = -4728 ext{ kJ/mol}$
• For 6H2O: $6 imes -286 ext{ kJ/mol} = -1716 ext{ kJ/mol}$

The enthalpy change is:

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

Calculating: $\Delta H = (-4728 - 1716) - 98 = -3271 \text{ kJ/mol}$

Thus, the heat of combustion of liquid benzene is -3271 kJ/mol.