Chapter 10.2, Problem 10.5P

(a)

Interpretation Introduction

Interpretation:

The substance that has the largest dipole-dipole forces has to be given.

Concept Introduction:

Intermolecular forces:

Intermolecular forces are like cohesive forces, acting between the molecules.  The overall stability of a compound depends on how strong the molecules are held together.  Intermolecular force is concerned about the overall stability of a substance.  A stable substance has stronger intermolecular forces.

Dipole-dipole forces:

A covalent bond is formed by mutual sharing of electrons between atoms.  The two distinct types of covalent compounds are non-polar covalent and polar covalent compounds.

Atoms of the same element, particularly non-metals, bond which each other through covalent bond.  There is no polarity between the atoms connected by the bond since the atoms have same electronegativity.  Such type of compounds is non-polar covalent compounds.  Hydrogen molecule is best example.

If atoms of slightly different electronegativity are covalently bonded, polarity arises spontaneously in the molecule due to the slight electronegativity difference between the atoms.  Such compounds are polar covalent compounds.  A $\text{HCl}$ molecule can be considered as example for this type.

 In each molecule of a polar covalent compound, the electron cloud is displaced from the atom of low electronegativity to the atom of relatively high electronegativity through the covalent bond.  As a result a “dipole” – a species containing weak partial positive and negative charge due to the unsymmetrical distribution of bonding electrons between atoms, is formed.  Each dipole orient itself in such a direction that its positive end lies in close proximity to the negative end of the other dipole.  The interaction between the dipoles is called “dipole-dipole forces”.

(b)

Interpretation Introduction

Interpretation:

The substance that has the largest Hydrogen bond forces has to be given.

Concept Introduction:

Intermolecular forces:

Intermolecular forces are like cohesive forces, acting between the molecules.  The overall stability of a compound depends on how strong the molecules are held together.  Intermolecular force is concerned about the overall stability of a substance.  A stable substance has stronger intermolecular forces.

Hydrogen bond forces:

Hydrogen bonding is formed in polar covalent compounds containing hydrogen and other high electronegativity like fluorine, oxygen or nitrogen.  These atoms in a molecule partially bond to hydrogen of the other same molecule or within a molecule.  This type of bonding is called hydrogen bonding and it is stronger than dipole-dipole forces.  Hydrogen bonding has significant impact on stability, density and other properties of matter.  Water is a best example of hydrogen bonding, in which each oxygen atom of a water molecule forms hydrogen bond with hydrogen of another water molecule.

(c)

Interpretation Introduction

Interpretation:

The substance that has smallest dispersion force has to be given.

Concept Introduction:

Intermolecular forces:

Intermolecular forces are like cohesive forces, acting between the molecules.  The overall stability of a compound depends on how strong the molecules are held together. Intermolecular force is concerned about the overall stability of a substance.  A stable substance has stronger intermolecular forces.

London dispersion force:

London dispersion forces exist in non-polar compounds whereas dipole-dipole forces exist in polar covalent compounds.  Dipole-dipole force is stronger than London dispersion force.  Both polar and non-polar covalent compounds have London dispersion forces.  These forces are due to temporary dipoles and do not exist permanently.  The molecules convert to dipoles instantly and disappear.  This is due to the uneven distribution of electrons between their atoms occurs momentarily when the bonded electrons approach nucleus.  Thus it is a weakest force.

Larger size molecules have lesser interaction between nuclei and electrons.  Thus the electrons are free from nuclear force of attraction and easily form dipoles.  Thus, larger the size of the molecules, higher is the strength of London dispersion force.