Intermolecular forces: Types, Explanation, Examples

‘Inter‘ means ‘between’ as happening between two things, showing that the term ‘intermolecular forces’ refers to the electrostatic forces of attraction or repulsion between molecules. Another similar term, ‘intramolecular forces’ means the forces present between atoms to constitute molecules. . An example is the word ‘international’ meaning, between two or more nations, and ‘intranational’ meaning, within a nation.

Intermolecular forces (IMF) also known as secondary forces are the forces of attraction that exist between molecules. These forces include dipole-dipole interactions, ion-dipole interactions, ion-induced dipole interactions, van der Waals forces, and hydrogen bonding. These forces are required to determine the physical properties of compounds such as melting point, boiling point, density, vaporization, and enthalpies, etc. Intermolecular forces are much weaker than intramolecular forces (forces of attraction that hold atoms into a molecule) like ionic bonds or covalent bonds, etc.

Types of intermolecular forces

Mainly, intermolecular forces are divided into three types; dipole to dipole interactions, van der Waals forces (Londen Dispersion forces), and Hydrogen bonding. But the term induced dipoles, referring to Londen dispersion forces (LDF) is divided into further types. This makes five types of intermolecular forces.

• Dipole-dipole forces
• Ion-dipole interaction
• Ion-induced dipole interaction
• Van der Waal forces (Keesom forces, Debye forces, London dispersion)
• Hydrogen bonding

Information on intermolecular forces can be obtained by the study of macroscopic properties like pressure, volume, temperature (PVT) data using instrumental techniques. Such macroscopic properties are quantified by virial coefficients and Lennard-Jones potential.

1. Dipole-dipole interaction

Dipole to dipole interaction is due to the partially positive and negative poles across molecules. The negative poles of molecules attract the positive poles of other molecules and result in the establishment of electrostatic forces of attraction between molecules known as dipole-dipole interaction.

This type of electrostatic force of attraction is weaker than ion to ion interaction. This is because the interaction between dipoles is based on partial charges while that of ionic interactions is based on permanent positive and negative charges called cations and anions.

An example of dipole-dipole forces is the interaction between the HCl molecules. There is a partial positive charge on the hydrogen atom and a partial negative charge on the chlorine atom. The partially negative chlorine attracts partially positive hydrogen from other molecules giving rise to a huge network of such interactions.

Dipole to dipole interactions are only possible in polar molecules like water (H₂O), ammonia (NH₃), sulfur dioxide (SO₂), hydrogen disulfide (H₂S), nitrogen dioxide (NO₂), EDTA (Ethylene diamine tetraacetic acid), etc. The properties of polar compounds are factors of these interactions.

Read more about Polar and Non-polar bonds.

2. Ion-dipole interaction

Ion to dipole interactions occur when ions and polar molecules interact and an interactive force is established between them. The polar molecules contain partial positive and negative charges, and as a consequence, when an ion with a positive or negative charge comes closer to such molecules, an electrostatic force of attraction is set between them. As similar charges repel and opposite ones attract, an ion always faces the oppositely charged end of a polar molecule.

When sodium chloride (NaCl) is added to water, the water molecules make interact with the sodium ions and chloride ions.

This property of water to separate ionic compounds into individual ions by making ion-dipole interactions is sometimes referred to as solvation or hydration.

3. Ion-induced dipole interaction

Ions are responsible for ion-induced interactions. When an ion comes near a nonpolar molecule, it induces partial positive and negative poles generation on that molecule, converting it into a dipolar structure. In other words, an ion induces a dipole moment on nonpolar molecules and converts them into polar molecules. Such interactions are named ion-induced dipole interactions.

Examples of ion-induced dipole interactions include metal ions approaching oxygen and other molecules and creating a net dipole moment in them, etc.

If this induction of dipoles is done by ions, the resultant forces are called ion-induced dipole interaction. But, if these interactions result from already polar molecules to nonpolar molecules, these interactions are called dipole-induced dipole interactions, also known as Debye forces as explained below.

4. Van der Waals forces

Named after Dutch physicist, Johannes Diderik van der Waals, these interactions are the attractive forces between all atoms and molecules. These forces quickly vanish upon the increase in the distance between individual atoms or molecules and the threshold distance for such forces is called van der Waals contact distance.  Unlike other types of bonding interactions, van der Waals interactions do not have an electrostatic or chemical base.

These are the weakest force of attraction between the molecules and are usually seen as Keesom forces, Debye forces, and London dispersion forces (LDF).

Keesom forces

Keesom forces are the temperature dependent and very weak van der Waals forces that originate between permanent dipoles. As, molecules rotate constantly and it is generally assumed that they cannot be locked in one place, but at some points, they do get locked. These interactions arise at those moments and are seen in all quadrupoles and multipoles.

Keesom forces are named after William Hendrik Keesom and arise due to interactions between permanent polar structures. An aqueous solution that contains electrolytes, does not have Keesom interaction.

Keesom interaction energy depends upon the inverse sixth power of distance between individual polar structures. As the permanent spatially fixed dipole structures depend on inverse third power of distance, Keesom forces are the interactions among multiples.

where,

• m₁ and m₂ = dipole moment of individual molecules
• ε₀ = permittivity of free space
• ε_r = dielectric constant
• T = temperature
• k_B = Boltzmann constant (1.38064852 × 10^-23 m² kg s^-2 K^-1)
• r = distance between polar molecules

Debye forces

When permanent dipoles rotate and move, non-polar molecules in the vicinity become polarized. This polarization is induced especially by the electronic repulsion of individual molecules. The result of such disturbance of electronic clouds is a molecular attraction called Debye forces.

Debye forces are also termed permanent dipole-induced dipoles interactions. These forces do not depend on temperature as much as that of Keesom forces. These forces are molecular interactions and are not seen between atoms.

Polarization induced by polar molecules is the main cause behind the existence of such forces. These attractive forces are stronger than the London dispersion forces but are weaker than dipole-dipole interactions.

For example,

• Argon molecule experiences dipolar characteristics when attracted by the hydrogen atom of HCl or repelled by the chlorine atom of HCl.
• Polar molecules such as water, create dipoles in neutral molecules like oxygen, etc.

London dispersion forces (LDF)

London dispersion forces are named after a German-American physicist, Fritz London. They are also known as London forces, dispersion forces, and Instantaneous dipole-induced dipole forces.

These forces are the result of random fluctuation in electronic density. Atom-atom interaction also contains London dispersion forces. These forces depend upon the number of electrons in an atom. The greater the number of electrons, the stronger, the London dispersion forces are there.

Hydrocarbons show weak van der Waals forces, although, this effect can be increased by the introduction of a heteroatom. For example, alkyl halides have considerably more van der Waals interactions than the parent hydrocarbons. It is actually the polarizability of atoms, which determines how easy it is to disturb the electronic cloud of atoms, to introduce LD forces.

5. Hydrogen bonding

Hydrogen bonding is a type of dipole-dipole interaction but it is placed separately as another type of intermolecular force due to its extremely strong nature. The hydrogen bond is the only intermolecular force to have the word “bond” in its name because it resembles intramolecular forces’ strength.

The attractive forces between a hydrogen atom from a molecule with a highly electronegative atom like nitrogen (N), oxygen (O), and fluorine (F). Hydrogen bonds are the strongest of the intermolecular forces with bond energies of 4 kJ to 50 kJ per mole.

Hydrogen atom has an electronegativity value of (2.2) which when bound with highly electronegative atoms generate these type of bonds. Sometimes, it is referred to as a covalent bond but it is not entirely covalent, it just resembles covalent bond energy.

Hydrogen bonding is responsible for the higher boiling and melting point of different compounds like water (H₂O), alcohols (R-OH), and hydrogen fluoride (HF), etc.

The more electronegative atoms attract the electronic cloud of the hydrogen atoms and a partial positive charge is formed there as (-H^δ+). Electronegative atoms such as oxygen, nitrogen, and fluorine atoms are electron lone pair donors, which donate to this partial positive hydrogen. This way, a partial negative charge is generated on the more electronegative atoms.

Examples of hydrogen bonds in molecules are;

Attractive forces among the following molecules are termed hydrogen bonding.

• Water
• Ammonia
• Alcohols
• Some alkyl halides
• Acetylacetone
• Esters
• Amides
• DNA
• RNA
• Protiens
• Sugars, etc

Some polymers also have hydrogen bonding, such as,

• Nylon
• Kevlar, etc

Hydrogen bonding is the strongest of intermolecular forces. This comparison of strength of intermoleculatr forces can well be understood at; Polar vs Nonpolar bonds.

Effects of intermolecular forces on chemical compounds

Intermolecular forces deal with the physical effects in chemical substances like boiling and melting point, etc. The stronger these forces are, The more energy is required there, to break the bonds. They considerably increase the melting and boiling point of the compounds.

Parameters affected by intermolecular forces are;

• Melting points
• Boiling points
• Freezing points
• Vapour pressure
• Surface tension, etc

Strength of intermolecular forces

Applications of intermolecular forces of attraction

Some practical applications of intermolecular forces are as:

• Adhesives and glue materials
  • Adhesives are extensively used binding chemicals which use intermolecular forces for binding purpose.
• Construction materials
  • Contact cement, tile grouts, and  paint industry is full with intermolecular forces applications.
• Epoxy resins
  • Resins use intermolecular forces to become so dense and sometimes decreased water content epoxy resins freeze at room temperatures.
• Electrical devices
  • Bakelite, a common electrical switch making material comprises of Novolac and formaldehyde chains bound by hydrogen bonding. which gives characteristic strongness to this material, etc.

Intermolecular forces of attraction can be used statistically to determine melting point, boiling point, vaporization, density, enthalpies of fusion. Furthermore, hydrogen bonds are responsible to hold the nucleotide bases in DNA and RNA, etc.

Concepts Berg

Are ionic bonds intermolecular forces?

Sometimes, ionic bonds are referred as intermolecular forces because of the ability of ionic compounds to create forces with outside molecules similar to inside ones. In other words, ionic compounds are very polar, which makes them able to attract other atoms, ions or molecules.

How do forces of attraction affect the properties of compounds?

Forces of attraction affect the properties of compounds by increasing the boiling points, melting points, enthalpies of vaporization and fusion, etc.

What are the 5 types of intermolecular forces?

• Dipole-dipole forces
• Ion-dipole interaction
• Ion-induced dipole interaction
• Van der Waals forces
• Hydrogen bonding

What is the meaning of intermolecular forces?

Intermolecular forces means the forces between the molecules like HCl, HI, and water, etc.

What is the strongest intermolecular force?

Among five types of intermolecular forces,

• Dipole-dipole interaction
• Ion-dipole force
• Ion-induced dipole force
• van der Waals interaction
• Hydrogen bonding

The dipole-dipole interaction is the strongest of all these forces and van der Waals is the weakest one.

What type of intermolecular forces does HBr have?

HBr contains dipole-dipole interaction and it can also make hydrogen bonding with more electronegative atom containing chemicals.

What type of intermolecular force holds ethanol molecules together?

Ethanol contains a hydroxyl group that has hydrogen atoms, making it able to make hydrogen bonds. So, hydrogen bonding is the force that holds the ethanol molecules together.

How do intermolecular forces work?

Intermolecular forces are the forces of attraction among molecules. As same charges repel and opposite ones attrcat, the oppositely charged ions or molecules create attractions called intermolecular forces.

Why are intermolecular forces so important?

To study the physical properties of chemical compounds like their melting and boiling points, enthalpies of fusion and vaporization, etc.

What are the intermolecular forces of HF?

Hydrogen fluoride has dipole-dipole interaction. These dipole dipole intractions in this case are internal hydrogen bonding.

Do ionic compounds have intermolecular forces?

Ionic compounds have dipole dipole interactions.

Can intermolecular forces be repulsive?

If the same poles come to contact with each other, repulsive occurs.

What is the difference between intermolecular and electrostatic forces?

Intermolecular forces are discussed in chemistry. These are the forces that hold the molecules together whereas electrostatic forces are dependent on the charges. The greater the charge, the higher the electrostatic forces.

What intermolecular forces are present between two molecules of CF₃CF₃?

Ethane has weak London dispersion forces (van der Waals forces).

What is the strongest intermolecular force in ammonia?

NH₃ has hydrogen and highly electronegative atom nitrogen, so hydrogen bonding.

Do intermolecular forces influence viscosity?

Intermolecular forces are directly proportional to viscosity. Greater the intermolecular forces, greater is the viscosity.

Reference books

• Organic Chemistry (4th edition), Francis A. Carey, University of Virginia

Reference links

• Introduction to Intermolecular Forces (University of California, Merced)
• Hydrogen bonding (Purdue University, West Lafayette, Indiana)
• Explanation of intermolecular forces – KhanAcademy
• Intermolecular forces of attraction – Lumenlearning