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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
CH3CH2CH2CH3 Butane -0.5
CH3CH2CH2CH2CH3 Pentane 36 liquid
CH3(CH2)6CH3 Octane 125
QUES. State whether the compounds above will be a gas or liquid state at room temperature
(20 C). Hint: If the boiling point is below 20 C, then the liquid has already boiled
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