question_answer 30)

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 the data book. What can you say about the inter-molecular forces between these molecules? 

Answer:

(a) The stability of a crystal depends upon the magnitude of forces of interaction in the constituting particles. Greater the force of attraction present, more will be the stability of the crystal. For example, ionic salts such as NaCI, KCI etc. have very high melting and boiling points while the molecular crystals such as naphthalene. iodine etc. have low values of melting and boiling points. (b) The melting points of different substances are: (i) Water = 273 K (ii) Ethyl alcohol = 155 7 K (iii) Diethyl ether = 156 8 K (iv) Methane 90 5 K. The intermolecular forces in molecules of water and ethyl alcohol are mainly hydrogen bonding. The magnitude is more in water than in alcohol as is clear from the values of melting points. The forces in the molecules of diethyl ether are dipolar forces while in methane these are mainly the van der Waal's forces which is quite evident from its melting point (least among the compounds listed).