(a) Define (i) heat of formation of a substance, (ii) octane number of a fuel - Leaving Cert Chemistry - Question 6 - 2004

The octane number of a fuel is a measure of its resistance to knocking or pinging during combustion, which is associated with the phenomenon of auto-ignition. A higher octane number indicates better performance of the fuel in preventing premature ignition, making it suitable for high-compression engines.

Using the given balanced equation, we can calculate the heat of formation of methane as follows:

1. Given:

   • ΔH for the reaction is -890.4 kJ mol⁻¹.
   • Standard heats of formation:
     • CO₂ = -394 kJ mol⁻¹
     • H₂O = -286 kJ mol⁻¹.

2. Using Hess’s law:

   $ΔH_{reaction} = Σ ΔH_f(products) - Σ ΔH_f(reactants)$

   $; ΔH_{reaction} = [(-394) + 2 imes (-286)] - [ΔH_f(CH₄) + 0]$

   Solving for ΔH_f(CH₄):

   $-890.4 = (-394 - 572) - ΔH_f(CH₄)$

   $ΔH_f(CH₄) = -394 - 572 + 890.4$

   $ΔH_f(CH₄) = -76.6 ext{ kJ mol}^{-1}$

1. High energy content: Methane has a high calorific value, meaning it releases a significant amount of energy upon combustion, making it an efficient fuel.

2. Low emissions: Methane produces fewer pollutants compared to other fossil fuels when burned, such as lower levels of carbon dioxide and particulates, contributing to cleaner air.

Mercaptans are added to natural gas to provide an odour to the otherwise odorless gas. This is essential for safety reasons, as any leaks can be easily detected, preventing accidents.

The fractionation process, often used in oil refineries, involves the following steps:

1. Heating: Crude oil is heated in a distillation column to separate components based on boiling points.

2. Vaporization: As the crude oil heats, it vaporizes and rises through the column.

3. Condensation: The vapors cool down at different heights in the column, condensing into liquid at various fractions based on their unique boiling points.

4. Collection: Each fraction is collected separately at specific trays or points in the column, allowing for the isolation of lighter hydrocarbons (e.g., gasoline) at the top and heavier fractions (e.g., lubricating oils) at the bottom.

1. Chain Length: Longer hydrocarbon chains generally have lower octane numbers compared to branched chains due to increased tendency to undergo auto-ignition.

2. Branching: The presence of branching in the hydrocarbon structure improves the octane number by increasing resistance to knocking due to more stable structures.