The following hydrocarbons can all be used as fuels - Leaving Cert Chemistry - Question 8 - 2002

The two structural isomers of butane (C₄H₁₀) are:

1. n-butane:
   CH₃−CH₂−CH₂−CH₃
2. Isobutane (2-methylpropane):
   CH₃−CH(CH₃)−CH₃

Mercaptans are added to natural gas primarily to give it an odor, allowing for detection of gas leaks. This is crucial for safety as natural gas is odorless and can pose explosion risks.

A major source of methane released to the atmosphere is agricultural activities, particularly from livestock digestion (enteric fermentation) and manure management.

The presence of branching in the structure of 2,2,4-trimethylpentane contributes to its high octane rating. Branched hydrocarbons tend to burn more smoothly and are less likely to knock.

The presence of methyl branches or a short carbon chain structure also increases the octane number of hydrocarbons.

The heat of combustion of a compound is the amount of energy released when one mole of the substance undergoes complete combustion with oxygen, forming carbon dioxide and water.

To calculate the heat of combustion, use the formula:

From the balanced equation:

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

The heats of formation are:
ΔH_f(C₄H₁₀) = -125 kJ/mol,
ΔH_f(CO₂) = -394 kJ/mol,
ΔH_f(H₂O) = -286 kJ/mol.

Calculating:

• For products:
  8(-394) + 10(-286) = -3152 - 2860 = -6012 kJ/mol

• For reactants:
  2(-125) = -250 kJ/mol

Thus,

The heat of combustion of butane is -5762 kJ/mol.