The fuel in camping gas cylinders, like the one pictured on the right, is a liquefied mixture of propane, butane, and another compound which is a structural isomer of butane - Leaving Cert Chemistry - Question 6 - 2014

Propane (C₃H₈) can be represented as:

    H   H   H
    |   |   |
H - C - C - C - H
    |   |   |
    H   H   H

IUPAC also accepts the commonly used structural formula as:

   CH₃-CH₂-CH₃

Propane has a lower boiling point than butane due to its smaller molecular size. The intermolecular forces present in butane are stronger due to its larger electron cloud, which results in greater van der Waals forces. Hence, more energy is required for butane to change from liquid to gas compared to propane.

Structural isomers are compounds that have the same molecular formula but differ in the arrangement of atoms. This means that they contain the same number and types of atoms, but the structure and connectivity of these atoms differ.

The isomer of butane (C₄H₁₀) is called isobutane or methylpropane. Its structural formula can be represented as:

       H   H
       |   |
   H - C - C - H
       |   |
   H - C - H
       |
       H

Or the condensed structural formula:

   CH₃-CH(CH₃)-CH₂-CH₃

The heat of combustion is defined as the amount of energy released when one mole of a substance is completely burned in excess oxygen. It is expressed in kJ/mol.

The balanced equation for the complete combustion of butane (C₄H₁₀) is:

2 C₄H₁₀ + 13 O₂ → 8 CO₂ + 10 H₂O

To calculate the heat of combustion, we will use the formula:

ΔH_combustion = ΣΔH_f(products) - ΣΔH_f(reactants)

Substituting the values into the equation:

• Products: 8 CO₂ and 10 H₂O
  • 8 mol CO₂ = 8 x -393.5 kJ/mol = -3148 kJ
  • 10 mol H₂O = 10 x -285.8 kJ/mol = -2858 kJ

Total of products:

Total = -3148 - 2858 = -6006 kJ

• Reactants: 2 C₄H₁₀
  • 2 mol butane = 2 x -125.7 kJ/mol = -251.4 kJ

Total of reactants:

Total = -251.4 kJ

So:

ΔH_combustion = -6006 - (-251.4) = -6006 + 251.4 = -5754.6 kJ

Thus, the heat of combustion of butane is approximately -2877.3 kJ/mol.