Boiling Point Trends
boiling point trends
boiling point trends
Introduction to Boiling point trends
As atomic number rises, boiling point/K increases due to there being more electrons, which create a larger strength of negative charge around the nucleus of the atom. This affects the forces of attraction between molecules or atoms i.e. van der Waals' forces, and they become stronger. With stronger forces, it needs more energy input to change the state of the element - resulting in a higher boiling/melting point.
Trends in melting and boiling points
Usually Boiling points of alcohols increase regularly with increase in the number of carbon atoms. Alcohols usually have higher boiling points than the corresponding hydrocarbons and alkyl halides. For example methyl alcohol has much higher B.P. as compared to methyl halide and methane.
Compounds Boiling points in (K)
CH_4 111
CH_3C_l 249
CH_3BR 278
CH_3OH 337.5
The trends in melting points and boiling points going across, the reason for the large difference is that the O-H bond in methyl alcohol is involved in hydrogen bonding. Alkanes do not have hydrogen bonding and so their boiling points are low. Similarly the aromatic alcohols such as Ethylene glycol are a colorless, viscous liquid having B.p. of 470 K. it is poisonous when taken orally. It is miscible with water and ethanol in all proportions.
A second example is Benzyl alcohol is a colorless oily liquid having B.p. 478 K. It is thinly soluble in water but dissolves readily in alcohol and ether.
Phenol has high boiling points (B.P. 455 K) due to intermolecular hydrogen bonding. Boiling points normally raise going from sodium to aluminum, then decline to argon (again with a bump at sulphur).
Ethers show a gradual increase with the increase in molecular weight. Ethers have lower boiling point than the corresponding isomeric alcohols. For e.g. dimethyl ether (B.p. 249 K) has lower boiling points than ethyl alcohol. This is because ether molecules cannot form hydrogen bonding like that of alcohol.
Explanation of the Boiling Point Trends
Melting points: When a substance melts, some of the attractive forces holding the particles together are busted or loosened so that the particles can move liberally around each other but are still close together. The stronger these forces are, the more energy is required to defeat the higher the melting temperature.
Boiling points: similarly when a substance boils, most of the outstanding striking forces are broken so that the particles can move freely and far apart. Stronger the attractive forces are, the more energy is required to overcome them and the higher the boiling temperature.
Examples of boiling points of the metals such as Sodium, magnesium and aluminum are all metals. They usually tend to have metallic bonding, in which case positive metal ions are attracted to delocalized electrons. Going away from sodium to aluminum:
This is because there is an increase in the charges of the metal ions which ranges from +1 to +3 especially for aluminum +2.
The increase in M.p. and B.P. is due to increase in number of delocalized electrons. Thus the strength of the metallic bonding increases with increase in melting and boiling point.
Silicon: it is a metalloid which means the elements behaves both as a metal and non-metal. Silicon has high melting and boiling point because of the strong covalent bonding and looks like a giant molecule. Due to this strong force of attraction silicon metal needs so much of energy to break down the molecules.
Phosphorus, sulphur, chlorine and argon: The phosphorus, sulphur, chlorine and argon are all non-metals, they usually be present as small, separate molecules. In this case Phosphorus, sulphur and chlorine survive as plain molecules, with tough covalent bonds among their atoms. Whereas the Argon exists as separate atoms (it is monatomic).this elements usually have very low boiling and melting points because, when these four substance melt or boil, the van der Waals forces exist between the molecules gets broken . These molecules usually have very weak bonds so very little energy is needed to up come this bond breaking.
Boiling Point Trends of Alkali Metals
The alkali metals are all extremely reactive and be never found in basic forms in nature. Because of this, they are generally stored immersed in mineral oil or kerosene (paraffin oil). They also smudge easily and have low melting points and densities. Physically, the alkali metals are regularly silver-colored, except for metallic cesium, which has a golden tint. These elements are all soft metals of low density. Chemically, all of the alkali metals react insistently with the halogens to form ionic salts.
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The alkali metals show a number of trends when moving down the group - for instance: decreasing electro negativity, increasing reactivity, and decreasing melting and boiling point.
by: Smith
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