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Boiling Points and Structures of Hydrocarbons

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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. ...read more.


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. ...read more.


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. ...read more.

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