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 is as a fuel for heating and electricity generation.

The concentration of methane in the atmosphere is a cause of concern because it is a potent greenhouse gas that contributes to global warming. Its release affects climate change, leading to rising sea levels, increased frequency of extreme weather events, and harmful impacts on agriculture and biodiversity.

High molecular mass alkanes have high boiling points due to stronger intermolecular forces, specifically van der Waals forces, which increase with molecular size. Larger molecules have more electrons, leading to greater polarizability and hence stronger dispersion forces, requiring more energy (higher temperature) to break these interactions.

The balanced equation for the dehydrocyclisation of heptane is:

$C_7H_{16} \rightarrow C_7H_8 + H_2$

where methylbenzene is C7H8 (toluene).

The purpose of dehydrocyclisation in oil refining is to convert linear alkanes into cyclic hydrocarbons, which improves the quality and octane rating of gasoline. This process is essential for producing higher-value products and optimizing fuel efficiency.

1. But-2-ene: $\text{CH}_3-\text{C} (\text{H})=\text{C}(\text{H})-\text{CH}_3$

2. 2-methylbutane: $\text{CH}_3-\text{C}(\text{H})-\text{CH}_2-\text{CH}_3$

3. Second alkene (X) could be vinylcyclopropane: $\text{C}_5\text{H}_8$ (or other isomers).

Since the reaction starts with a dodecane (C12H26) molecule and produces but-2-ene and 2-methylbutane, the remaining product can be deduced. Total carbon for but-2-ene (C4H8) and 2-methylbutane (C5H12) is:

$12 - (4 + 5) = 3$

Thus, the molecular formula of product X is C3H6.

Hess's law states that the total enthalpy change for a reaction is the same, regardless of the number of steps the reaction takes, as long as the initial and final states are the same.

We can use the heats of formation to calculate the heat change for the reaction:

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

The reaction is:

$C_{12}H_{26} \rightarrow C_4H_8 + C_5H_{12} + C_3H_6$

Therefore:

$\Delta H = [(-93.1) + (-178.4) + (20.0)] - [(-350.9)] = -183.2 kJ$