6. (a) Identify the hydrocarbon gas produced by anaerobic bacterial decomposition of either animal waste or vegetation - Leaving Cert Chemistry - Question 6 - 2017

A major use of methane (CH4) is as a fuel, particularly in heating and electricity generation. It is also used in the production of hydrogen through steam reforming.

The concentration of methane in the atmosphere is a cause of concern because it is a potent greenhouse gas. Methane has a global warming potential many times greater than carbon dioxide, contributing to climate change and rising sea levels. Also, it affects livestock and crop yields, posing risks to food security.

High molecular mass alkanes have higher boiling points due to stronger intermolecular forces, such as van der Waals (London dispersion) forces. As the molecular size increases, there are more electrons available, which leads to greater dispersion forces between the molecules, thereby requiring more energy (heat) to break these interactions.

The balanced equation for the dehydrocyclisation of heptane can be written as:

The purpose of dehydrocyclisation in oil refining is to convert straight-chain alkanes into cyclic hydrocarbons, improving the octane rating of fuels. This process also enhances the yield of higher value aromatic compounds used in various chemical processes.

The structural formulas are as follows:

1. But-2-ene:

   

2. 2-Methylbutane:

   

3. X (undetermined alkene): The structure will depend on the specific alkene produced during the cracking process.

Given that the products of the catalytic cracking of dodecane (C12H26) include but-2-ene and 2-methylbutane, we can deduce that X should balance the equation by containing the remaining carbon atoms. Therefore, assuming but-2-ene (C4H8) and 2-methylbutane (C5H12) account for 9 carbons, the third product (X) must be C3H6.

Hess's law states that the total enthalpy change for a chemical reaction is the same, regardless of the number of stages or steps in which the reaction occurs, as long as the initial and final conditions are the same.

The heat change for the cracking reaction can be calculated using Hess's law as follows:

Given:

• ∆H_f (dodecane) = -350.9 kJ/mol
• ∆H_f (but-2-ene) = -93.1 kJ/mol
• ∆H_f (2-methylbutane) = -178.4 kJ/mol
• ∆H_f (X) = 20.0 kJ/mol

The equation is:

$ext{Heat Change} = ext{Sum of heats of formation of products} - ext{Heat of formation of reactant}$

Calculating:

$\Delta H = [(-93.1) + (-178.4) + (20.0)] - (-350.9)$

$\Delta H = -251.5 + 350.9 = 99.4\ ext{ kJ}$