Liquids and Solids

Chapter 4 Liquids and Solids

Question 4.1: Why are intramolecular forces weaker than intermolecular forces?

Answer: Intermolecular forces are forces of attraction between molecules of same kind or different kinds. Intramolecular forces are forces of attraction with in a molecule. Intramolecular forces are much stronger because these are due to mutual sharing of electrons but intermolecular forces have nothing to do with sharing of valence electrons. So that is the reason that intramolecular forces are weaker than intermolecular forces.

Question 4.2: What are dipole–dipole Forces? Why are these forces weaker in gaseous phase than in liquid phase?

Answer: The force of attraction between positive end of one molecule and negative end of other molecule is called dipole-dipole force.

The strength of these forces depends upon the electronegativity differences between bonded atoms and distance between molecules. The distance between molecule in gaseous phase are greater, so these forces are very weaker in this phase. In liquids, these forces are reasonably strong due smaller distance between molecules.

Question No. 4.3: What are dipole –induced dipole forces?

Answer: The force of attraction between positive end of a permanent dipole and negative end of a induced dipole is called dipole-induced dipole force. This is also known Deby force. Induced dipole is due to collision of non polar molecule with polar molecule.

Question No. 4.4: Why are dipole – dipole forces are much stronger than dipole induced dipole forces?

Answer: Dipole–dipole forces are much stronger because these forces are among permanent dipoles. On the other hand, in dipole–induced dipole forces are weaker, these force are among the permanent and induced dipoles. This induced dipole is formed for a short time and that is why the attraction between a dipole and induced dipole is comparatively less strong.

Question No. 4.5: What are London dispersion forces?

Answer: The momentary force of attraction created between instantaneous dipole and induced dipole is called instantaneous dipole–induced dipole force or London dispersion force. Here instantaneous dipole is formed when electrons of one non polar molecule come close to the electrons of other non polar molecule. These electrons push each other and in this way instantaneous dipole is formed. This instantaneous dipole then disturbs the electron cloud of other nearby molecule and converts it into induced dipole.

Question No. 4.6: Why are London dispersion forces weaker than dipole-dipole forces?

Answer: London forces are very short lived and vanish quickly as they are formed. This is because dipole are created by temporary disturbance of electronic cloud. So London forces are weaker. The dipole–dipole forces maintain and do not vanish because of permanent dipole. So dipole –dipole forces are comparatively stronger.

Question No. 4.7: Why the melting and boiling points of halogens increase down the group?

Answer: In halogens London forces predominate. These forces depend upon the size of atoms. Greater the size of atoms, greater is polarizability and stronger is the London force. The atomic sizes of halogens increase gradually from fluorine to iodine. This is due to addition of an extra shell of electrons in each halogen from Fluorine to iodine. Hence, London forces increase from fluorine to Iodine so melting and boiling points also increase.
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