CHM 110 - CHEMISTRY AND ISSUES IN THE ENVIRONMENT

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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 andthe compound is a gas.

The reason that longer chain molecules have higher boiling points is that longer chain molecules become wrapped around and enmeshed in each other much like the strands of spaghetti. More energy is needed to separate them than short molecules which have only weak forces of attraction for each other.

FOCUS ON FOSSIL FUELS

Petroleum refining is the process of separating the many compounds present in crude petroleum. The principle which is used is that the longer the carbon chain, the higher the temperature at which the compounds will boil. The crude petroleum is heated and changed into a gas. The gases are passed through a distillation column which becomes cooler as the height increases. When a compound in the gaseous state cools below its boiling point, it condenses into a liquid. The liquids may be drawn off the distilling column at various heights.

Although all fractions of petroleum find uses, the greatest demand is for gasoline. One barrel of crude petroleum contains only 30-40% gasoline. Transportation demands require that over 50% of the crude oil be converted into gasoline. To meet this demand some petroleum fractions must be converted to gasoline. This may be done by "cracking" - breaking down large molecules of heavy heating oil; "reforming" - changing molecular structures of low quality gasoline molecules; or "polymerization" - forming longer molecules from smaller ones.

For example if pentane is heated to about 500 C the covalent carbon-carbon bonds begin to break during the cracking process. Many kinds of compounds including alkenes are made during the cracking process. Alkenes are formed because there are not enough hydrogens to saturate all bonding positions after the carbon-carbon bonds are broken.