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What causes the differences in the Melting and Boiling Points of these molecules? Water...

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kazz711 | Student, Grade 12 | (Level 1) Honors

Posted September 4, 2013 at 11:36 AM via web

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What causes the differences in the Melting and Boiling Points of these molecules? Water (H2O), Ammonia (NH3), Hydrogen Fluoride (HF), Methane (CH4) and Neon (Ne)

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llltkl | College Teacher | (Level 3) Valedictorian

Posted September 4, 2013 at 6:15 PM (Answer #1)

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Boiling point (and also melting point) is a function of intermolecular forces of attraction within molecules. Among the given molecules, H2O and HF can form reasonably strong intermolecular hydrogen bonds. These two molecules are expected to have higher boiling (and melting) points. Water molecule has twice as many hydrogen atoms as does HF and, therefore, water can theoretically form more hydrogen bonds than hydrogen fluoride. Thus water is expected to have the highest boiling point of them all, followed by HF.   In ammonia (NH3) molecule, chance of a very weak intermolecular hydrogen bond is present there, through the intervention of weakly electronegative nitrogen atoms. So, NH3 will follow HF in that sequence. In the molecules of methane and neon, however, dispersion forces are the only possible intermolecular forces and methane having a greater surface area, will definitely outweigh neon in the order of magnitude of such forces. Hence the order of melting and boiling points of the given molecules is expected to follow the sequence:

H2O>HF>NH3>CH4>Ne.

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