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Question
‘Stability of a crystal is reflected in the magnitude of its melting points.’ Comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from a data book. What can you say about the intermolecular forces between these molecules?
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Solution
The stability of the crystal increases with its melting point. This is due to the fact that the strength of the intermolecular interactions that hold particles together is correlated with a crystal’s melting point. The intermolecular forces are greater and the crystal is more stable when the melting point is higher.
The melting points of the given substances are as follows:
Solid water = 273 K
Ethyl alcohol = 155.7 K
Diethyl ether = 156.85 K
Methane = 90.5 K
We can infer from the aforementioned statistics that the substances intermolecular force magnitudes follow the following sequence.
Solid water > Diethyl ether > Ethyl alcohol > Methane
Intermolecular forces are strongest in solid water. The existence of hydrogen bonds is the cause of this. Hydrogen bonds are also present in ethyl alcohol, albeit to a smaller degree. Dipole-dipole interactions are the intermolecular forces seen in diethyl ether, a polar molecule. The weakest van der Waals forces, or intermolecular forces, are found in methane. This is due to the non-polar structure of the methane molecule.
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