What Is Intermolecular forces

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Introduction

Introduction In molecules, two major forces of interaction can be identified. These are intramolecular forces and intermolecular forces.

Intramolecular forces exist within molecules and influence their chemical properties while intermolecular forces exist within molecules and mediate interaction between molecules. Therefore, intermolecular forces affects the physical properties of molecules. They are weaker than intramolecular forces (i.e the bonding forces that keep molecules together). For example, water can exist as a liquid, solid (ice) and as a gas (steam).

Experiment reveals that the bond angle, the dipole moment, the molecular shape and hydridization of the carbon are the same, irrespective of the physical state the water molecule assumed. These properties are controlled by intramolecular forces. However, the physical properties of water in each of these states differ significantly.

The difference is attributed to intermolecular forces. Consequently, liquid water assumes the shape of the container but possess a definite volume, steam assumes the shape and volume of the container while ice has a definite shape and volume. Other effects of intermolecular forces are deviation of real gases from an ideal behavior, formation of condensed phases, molecular motion, melting and boiling point of some molecules.

Objectives

i. To know and understand the meaning of intermolecular forces and differentiate between inter and intra molecular forces.

ii. To be able to classify intermolecular forces based on different principles

iii/ To understand the origin and the effect of dipole forces, dipole-dipole interaction and hydrogen bonding on molecules

i. To understand the origin and effect of ion-induced dipole force, ion-dipole forces and van der Waals forces on molecules

ii. To understand the different forms of London forces and their effects on molecules

iii. To know the relative strength of intra and inter molecular forces on molecules

Types of intermolecular forces

The major types of intermolecular forces that operate between molecules include the following;

i. Dispersion forces which operate as a London forces (named after Fritz London who first described these forces theoretically in 1930), as a weak intermolecular forces or as van der Waal's forces (named after the person who contributed to our understanding of non-ideal gas behaviour).

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ii. Dipole-dipole interactions: Dipole-dipole is an electrostatic interaction existing between permanent dipoles in molecules. This interaction increases the force of attraction between molecules by reducing their potential energy.

In dipole-dipole interaction, the positive end of one molecule will attract the negative end of the other molecule and influence its position. Therefore, dipole-dipole interaction is common in polar molecules. Hydrogen bonding is a special type of dipole-dipole interaction. However, some molecules are polar but due to their symmetry, the dipole moment cancels each other.

Hence such molecules will not have permanent dipole moment. For example, CCl4 and CO2.

iii. Hydrogen bonds: This is a special case of dipole-dipole interaction that occurs when hydrogen atom is covalently bonded to an electronegative atom such as F, Cl, O, etc. In this case, the hydrogen will carry a partial positive charge while the electronegative element will carry a partial negative charge, leading to an electrostatic attraction between them. Hydrogen bonding has been found to influence properties of some compounds to a large extent.

For example, the high boiling and melting point of water, compared to other hydride in similar groups, the solubility of ethanol in water compared to other alcohols. Also, based on forces of attraction, intermolecular forces can be classified into the following three groups,

Ion-induced dipole forces:

A charged ion, brought near a non-polar molecule can distort its electron cloud and induce a dipole moment. Therefore, ion-induced dipole interaction involves an ion and a non polar molecule interacting with each other.

ii. Ion-dipole forces:

These involve ions and a polar molecule interacting such that the positive and negative groups come closer to each other. The difference between dipole-dipole interaction and ion-dipole interaction is that it involves interaction between ion and a dipole. This interaction is stronger than the one in dipole-dipole because it involves ions.

iii. van der waals forces

(which include Keesom forces, Debye force and London dispersion forces): Van der Waals forces involve interaction between uncharged atoms or molecules. A Van der Waals force can influence the cohesion of condensed phases, physical adsorption of gases and universal force of attraction between macroscopic bodies

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The London forces

The London force is also called induced dipole-induced dipole interaction. Its operates due to the non zero dipole moments in atoms or molecules. It is a dominant contributor to intermolecular forces. It originates from the non zero instantaneous dipole moments of all atoms and molecules. The inducement of the dipole (polarization) can come from the presence of polar molecule or by the repulsion of negatively charged electron clouds in non polar molecules. Since the London forces are caused by fluctuations of electron density in an electron cloud, the greater the number of electrons in an atom, the greater the effect of the London force. The London force is universal because all materials are polarizable and mediates in atom-atom interaction as well as macroscopic bodies in condensed systems.

Solved problem 1 Question 1 (a) (b) Differentiate between inter molecular and intramolecular forces Highlight the effect of inter and intra molecular forces on the various physical states of water .

Solution (a) Intramolecular forces exist within molecules and influence their chemical properties while intermolecular forces exist within molecules and mediate interaction between molecules.

Therefore, intermolecular forces affect the physical properties of molecules. They are weaker than intramolecular forces (I.e the bonding forces that keep molecules together).

(b) Water can exist as a liquid, solid (ice) and as a gas (steam). Experiment reveals that the bond angle, the dipole moment, the molecular shape and hydridization of the carbon are the same, irrespective of the physical state the water molecule assumed.

These properties are controlled by intramolecular forces. However, the physical properties of water in each of these states differ significantly. The difference is attributed to intramolecular forces. Consequently, liquid water assumes the shape of the container but possess as a definite volume. Steam assumes the shape and volume of the container while ice has a definite shape and volume

Solved problem 2 (a) (b) (c) What is dipole-dipole interaction and its effect on the potential energy? Why is hydrogen bonding classified as dipole-dipole interaction?

Explain why dipole-dipole interaction does occur in CCl4 and CO2. Solution (a) Dipole-dipole interactions is an electrostatic interaction existing between permanent dipoles in molecules.

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This interaction increases the force of attraction between molecules by reducing their potential energy.

(b) Hydrogen bonding is a special type of dipole-dipole interaction because the partial positive charge on hydrogen attracts the negative partial charge on the electronegative element that is bonded to hydrogen.

(c) CCl4 and CO2 are non polar and due to their symmetry, the dipole moment cancels each other. Hence they do not have permanent dipole moment and dipole-dipole interaction does not occur. Solved problem 3 (a) Highlight the major features of the following intermolecular forces

i. ion-induced dipole forces

ii. Ion-dipole Forces

iii. Van der Waal Force

Solution i. ion-dipole forces van der Waals forces Ion-induced dipole forces: A charged ion, brought near a non-polar molecule can distort its electron cloud and induce dipole moment.

Therefore, ion-induced dipole interaction involves an ion and a non polar molecule interacting with each other. ii. Ion-dipole forces: These involve ions and a polar molecule interacting such that the positive and negative groups come closer to each other. The difference between dipole-dipole interaction and ion-dipole interaction is that it involve interaction between ion and a dipole. This interaction is stronger than the one in dipole-dipole because it involves ions. iii. van der Waals forces (which include Keesom forces, Debye force and London dispersion force): Van der Waal forces involve interaction between uncharged atoms or molecules. This force can influence the cohesion of condensed phases, physical adsorption of gases and universal force of attraction between macroscopic bodies.

Relative strength of inter and intra molecular forces

Intermolecular forces (dispersion forces, dipole-dipole interactions and hydrogen bonds) are much weaker than intramolecular forces (covalent bonds, ionic bonds or metallic bonds). Dispersion forces are the weakest intermolecular forces (one hundredth-one thousandth the strength of a covalent bond), hydrogen bonds are the strongest of intermolecular forces (about one-tenth the strength of a covalent bond). Generally, the trend representing decreasing trend is dispersion forces < dipole-dipole interactions < hydrogen bonds

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