Chapter 9, Problem 137CP

Interpretation Introduction

Interpretation:

The variation in the melting points of the given set of compounds has to be rationalized.

Concept Introduction:

• Melting point of any compound, depends upon its strength of Intramolecular force and Intermolecular force present in it.
• Intramolecular force refers to type of bonding between the atoms.
• Intermolecular forces are the forces that bind the molecules together to attribute to a stability of a compound.
• They are collectively known as “Interparticle forces”. If the strength of interparticle forces is high, melting point will be high and if it is low, melting point will be low.
•  The classification can be summarized as follows –

                      

                                                              Figure 1

                                                                                  

                                                              Figure 2

                                             

                                                                   Figure 3

The type of bonding between atoms or ions is Intramolecular force. The intramolecular force in ionic compounds is electrostatic force of attraction between the ions of opposite charges.  Usually ionic compounds are solids with high melting points. Covalent bonds are of two types, that is polar covalent bond and non-polar covalent bond. Covalent compounds are found as solids and liquids with moderate melting point. Metallic bond is formed between the metal atoms of an element. Metals have high melting points.

Intermolecular forces are weaker than Intramolecular forces. They are of three types - London dispersion forces, dipole-dipole forces and hydrogen bonding. They are collectively known as Van der Waals forces.

London dispersion forces exist in all types of molecules. This is the force responsible for the condensation of non-polar compounds into liquids or solids under low temperature.

Dipole-dipole forces exist in polar covalent compounds. Hydrogen bonding exists in polar covalent compounds containing Fluorine, Oxygen or Nitrogen directly bonded to Hydrogen.

These atoms in a molecule partially bond to hydrogen of the other molecule of the same compound. It is relatively the strongest one. Hydrogen bonded compounds are usually liquids. They exhibit high boiling point.