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what are the main factors on which melting point and boiling point depend [in some cases order of mp and bp are diff] (explain in brief) take hf h2o and nh3 as eg

what are the main factors on which melting point and boiling point depend [in some cases order of mp and bp are diff] (explain in brief)
take hf h2o and nh3 as eg

Grade:12

1 Answers

Aarti Gupta
askIITians Faculty 300 Points
9 years ago
The melting point of a substance is defined as the temperature at which pure solid is in equilibrium with pure liquid at atmospheric pressure.It depends both on the structure and the bonding in substance.Since both these factors changes abruptly across a period so,there is abrupt changes in melting point also.For ex- when we move from left to right m.p rises through the metals and metalloids and then drops abruptly to low-values for non-metals.
Boiling point depend on the strength of bonds which gets broken when liquid converted to a vapour.B.p of metals are higher than that of m.p.this is because in metals metallic bonding still exist in liquid phase and atoms have to be separated to a considerable distance in forming the vapour phase which involves the complete breakage of the metal bonds.Thus b.p of metals are higher than their m.p.However in case of carbon and silicon having giant structures ,most of the bonds get broken during melting.So,b.p are high but not much higher than the m.p.On the other hand in case of simple molecule structures like P4,S8 involves breaking of weak Van der Waals bonds,so b.p occur at low temperatures,not much above the m.p.
Both 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 so these two molecules are expected to have higher boiling as well as 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.Now in case of ammonia (NH3) molecule, chance of a very weak intermolecular hydrogen bond is present there,because of weakly electronegative nitrogen atoms. So, NH3 will follow HF in that sequence.

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