Question

Liquid propane, a widely used fuel, is produced by compressing gaseous propane. During the process, approximately 15 kJ of energy is released for each mole of gas liquefied. Where does this energy come from?

Short answer

The energy released during the liquefaction process (i.e. 15 kJ) is the kinetic energy of gaseous molecules which get condensed to the liquid form which possesses negligible kinetic energy.

Step-by-step solution

Difference in the intermolecular forces

Due to the intermolecular forces, there is a relative difference in the kinetic energy of the molecules.

The gas phase of any compound has weak intermolecular forces of attraction and thus the kinetic energy of gaseous molecules is high, whereas liquids have a comparatively strong force of attraction and thus less kinetic energy.

Liquefaction process

Liquid propane when compressed to produce gaseous propane releases 15kJ of energy. During this condensation process, the kinetic energy of the gaseous molecules decreases.

This energy which is released during this process is the liquefaction energy(equivalent to 15kJ) for each mole of gas liquefied.