Explain why the standard enthalpy of combustion of octane (C₈H₁₈) is a negative value, and suggest why liquid octane is considered a more energy-dense fuel than gaseous hydrogen (H₂) on a per-unit-volume basis.

The standard enthalpy of combustion of octane is negative because the combustion reaction is exothermic:

C₈H₁₈(l)+12.5O₂(g)→8CO₂(g)+9H₂O(l)ΔH°≈−5470 kJ mol⁻¹

Energy is released to the surroundings because more energy is released in forming the bonds of the products (CO₂ and H₂O) than is required to break the bonds of the reactants (octane and O₂). Therefore ΔH° is negative.

On a per‑unit‑volume basis liquid octane is a more energy‑dense fuel than gaseous hydrogen because liquid octane has a much higher density. A given volume of liquid octane therefore contains many more moles of fuel than the same volume of gaseous hydrogen, so more energy is released per litre even though hydrogen has a higher energy density per unit mass.