Fractional Distillation of Crude Oil Boiling Points and Structures of Hydrocarbons

Fractional Distillation of Crude Oil

BOILING POINTS AND STRUCTURES OF HYDROCARBONS

The boiling points of organic compounds can give important clues to other physical properties. A liquid boils when its vapor pressure is equal to the atmospheric pressure. Vapor pressure is determined by the kinetic energy of molecules. Kinetic energy is related to temperature and the mass and velocity of the molecules. When the temperature reaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the forces of attraction that hold molecules in the liquid state. Then these molecules break away from the liquid forming the gas state.

Vapor pressure is caused by an equilibrium between molecules in the gaseous state and molecules in the liquid state. When molecules in the liquid state have sufficient kinetic energy, they may escape from the surface and turn into a gas. Molecules with the most independence in individual motions achieve sufficient kinetic energy (velocities) to escape at lower temperatures. The vapor pressure will be higher and therefore the compound will boil at a lower temperature.

BOILING POINT PRINCIPLE:

Molecules which strongly interact or bond with each other through a variety of intermolecular forces can not move easily or rapidly and therefore, do not achieve the kinetic energy necessary to escape the liquid state. Therefore, molecules with strong intermolecular forces will have higher boiling points. This is a consequence of the increased kinetic energy needed to break the intermolecular bonds so that individual molecules may escape the liquid as gases.

THE BOILING POINT CAN BE A ROUGH MEASURE OF THE AMOUNT OF ENERGY NECESSARY TO SEPARATE A LIQUID MOLECULE FROM ITS NEAREST NEIGHBORS.

MOLECULAR WEIGHT AND CHAIN LENGTH TRENDS IN BOILING POINTS

A series of alkanes demonstrates the general principle that boiling points increase as molecular weight or chain length increases (table 1.).

Table 1. BOILING POINTS OF ALKANES

Formula Name Boiling Point C Normal State at Room Temp. +20 C

CH4 Methane -161 gas

CH3CH3 Ethane - 89

CH3CH2CH3 Propane - 42