Van der Waals Forces: Discovery, London Forces, Hydrogen Bond

Van der Waals Forces: The existence of weak attractive intermolecular forces was first proposed by Johannes van der Waals, a Dutch scientist who researched the effects of these molecular forces on the behaviour of real gases. These forces are known as van der Waals forces in honour of this honour. The interaction of positive and negative charges between surrounding atoms of molecules when they are very close together causes such forces to exist. Various van der Waals forces have been explored in this article.

Dispersion Forces or London Forces

Because their electronic charge cloud is symmetrically distributed, atoms and non-polar molecules are electrically symmetrical and have no dipole moment. However, intermolecular forces may exist in non-polar atoms like noble gases or non-polar molecules like \({{\rm{O}}_2},{\rm{ }}{{\rm{N}}_2},{\rm{ }}{{\rm{H}}_2}\), etc. This is because noble gases and non-polar molecules can be liquified, implying that intermolecular attraction exists. In other words, a dipole can form in such atoms or molecules at any time.

Consider two noble gas atoms that are very close to one other in order to understand the genesis of these forces. Because the electrical charge cloud of each atom is symmetrically spread around the nucleus, each atom is uncharged. However, due to the motion of the electron, we can suppose that the electronic charge distribution in one of the atoms becomes asymmetrical for a short period of time. The unsymmetrical centres of positive and negative charges do not coincide during this brief period. There is a small transitory dipole known as instantaneous dipole as a result of the instantaneous distribution of electrons in the atom. This instantaneous dipole causes an instantaneous induced dipole by distorting the electronic distribution in the neighbouring atom.

Both atoms’ transient dipoles attract one other. Instantaneous dipole and instantaneous induced dipole forces are other names for these attractive forces. Non-polar molecules are also driven to form transient dipoles. The German physicist Fritz London was the first to study these forces, and so they are known as London forces. These forces are also known as dispersion forces.

Dipole-Dipole Forces

Molecules with permanent electric dipoles, such as \({\rm{HCl}},{\rm{ N}}{{\rm{H}}_3}\), and \({{\rm{H}}_2}{\rm{O}}\), experience such forces. Positive and negative charge centres are present in a polar molecule, and the dipole ends have “partial charges.” The partial charges are always lower than the electrical charge as a whole. These forces are caused by interactions between the polar molecules’ oppositely charged ends. The negatively charged end of one molecule attracts the positively charged end of the other, and vice versa. The dipole-dipole interactions between hydrogen chloride molecules, for example, are depicted here.

The attraction forces increase as the dipole moment of the molecules increases. The Keesam initially researched this effect in \(1912\), and it’s also known as the orientation effect. These forces are also referred to as Keesam forces, and they have a net attractive effect. These forces are more powerful than those in London. However, because only partial charges are involved, these interactions are weaker than ion-ion interactions. It’s worth noting that, in addition to dipole-dipole interactions, polar molecules can also interact via London forces. As a result, the overall intermolecular forces in polar molecules.

Dipole-Induced Dipole Forces

These sorts of attraction forces operate between a polar molecule with a permanent dipole and a molecule with no permanent dipole. A substance’s molecules, for example, do not have any dipoles if it is non-polar. When these molecules come into contact with other molecules that have a permanent dipole, an induced dipole forms on the non-polar molecules.

This is because the electric field of the polar molecule deforms the electron cloud of the non-polar atom or molecule. This results in a shift in the negative charge’s centre of gravity relative to the nuclear charge, producing an induced dipole moment. As the size of the atom or molecule increases, the influence of electric dipole on it also increases.

Dipole-induced dipole forces are the forces that exist between a dipole and an induced dipole. The more easily molecules may be distorted, the stronger the attractive forces become. Debye studied this effect in \(1920\), and it’s also known as the induction effect. There is also a cumulative effect of dispersion forces in this scenario and dipole-induced dipole interactions. The van der Waals forces are the three types of forces mentioned above. Depending on the type of molecule, the magnitude of each type of interaction will vary.

Hydrogen Bond

A hydrogen bond is a sort of dipole-dipole interaction that is unique. When a hydrogen atom bonds to an atom of a highly electronegative element like fluorine, oxygen, or nitrogen, the hydrogen atom creates a weak connection with the other molecule’s electronegative atom. The hydrogen bond is the name for this weak bond. For example, in hydrogen fluoride \(\left( {{\rm{HF}}} \right)\), the hydrogen atom makes a weak connection with the fluorine atom of a neighbouring molecule while remaining attached to the fluorine atom. This might be displayed as:

In other words, the hydrogen atom works as a link between two atoms, forming a covalent bond with one and a hydrogen bond with the other. A dotted line represents the hydrogen bond, whereas a solid line represents the covalent bond. \({{\rm{HF}}}\) behaves as a cluster of hydrogen fluoride molecules due to hydrogen bonding. Although hydrogen bonding is thought to be limited to the elements \({\rm{N}},{\rm{ O}}\), and \({\rm{F}}\), some compounds containing \({\rm{Cl}}\) may also participate in hydrogen bonding. The hydrogen bond’s energy ranges from \(10\) to \(100\, {\rm{ kJ\, mo}}{{\rm{l}}^{ – 1}}\). This is quite a significant amount of energy, and therefore hydrogen bonds are a powerful force in determining the properties of many compounds. These play a significant role in the proteins and nucleic acids. The strength of the hydrogen bond is determined by the coulombic interactions between the lone pair of electrons of the electronegative atom of one molecule and the hydrogen atom of another molecule.

Summary

1. Intermolecular forces may exist in non-polar atoms like noble gases or non-polar molecules like \({{\rm{O}}_2},{\rm{ }}{{\rm{N}}_2},{\rm{ }}{{\rm{H}}_2}\), and others.
2. There is a small transitory dipole known as instantaneous dipole as a result of the instantaneous distribution of electrons in the atom. This instantaneous dipole causes an instantaneous induced dipole by distorting the electronic distribution in the neighbouring atom. Both atoms’ transient dipoles attract one other. These forces are called dispersion forces or London forces.
3. Molecules with permanent electric dipoles, such as \({\rm{HCl}},{\rm{ N}}{{\rm{H}}_3}\), and \({{\rm{H}}_2}{\rm{O}}\), experience dipole-dipole forces.
4. These forces are caused by interactions between the polar molecules’ oppositely charged ends. The negatively charged end of one molecule attracts the positively charged end of the other, and vice versa.
5. Between a polar molecule with a permanent dipole and a molecule with no permanent dipole, dipole-induced dipole forces operate.
6. This is because the electric field of the polar molecule deforms the electron cloud of the non-polar atom or molecule. This results in a shift in the negative charge’s centre of gravity relative to the nuclear charge, producing an induced dipole moment.
7. A hydrogen bond is a sort of dipole-dipole interaction that is unique. When a hydrogen atom bonds to an atom of a highly electronegative element like fluorine, oxygen, or nitrogen, the hydrogen atom creates a weak connection with the other molecule’s electronegative atom.
8. Hydrogen bonds are a powerful force in determining the properties of many compounds. These play a significant role in the proteins and nucleic acids.

FAQs on Van der Waals Forces

Q.1. What are Van der Waals Forces?
Ans: The existence of weak attractive intermolecular forces was first proposed by Johannes van der Waals, a Dutch scientist who researched the effects of these molecular forces on the behaviour of real gases. These forces are known as van der Waals forces in honour of this honour. The interaction of positive and negative charges between surrounding atoms of molecules when they are very close together causes such forces to exist. Various van der Waals forces are dispersion forces, dipole-dipole forces and dipole-induced dipole forces.

Q.2. Are hydrogen bonds a type of van der Waals Force?
Ans: A hydrogen bond is a special type of dipole-dipole force. So it is considered a van der Waals force. A hydrogen bond is a sort of dipole-dipole interaction that is unique. When a hydrogen atom bonds to an atom of a highly electronegative element like fluorine, oxygen, or nitrogen, the hydrogen atom creates a weak connection with the other molecule’s electronegative atom. The hydrogen bond is the name for this weak bond.

Q.3. Briefly describe London forces.
Ans: Because their electronic charge cloud is symmetrically distributed, atoms and non-polar molecules are electrically symmetrical and have no dipole moment. However, intermolecular forces may exist in non-polar atoms like noble gases or non-polar molecules like \({{\rm{O}}_2},{\rm{ }}{{\rm{N}}_2},{\rm{ }}{{\rm{H}}_2}\), and others. This is because noble gases and non-polar molecules can be liquified, implying that intermolecular attraction exists. In other words, a dipole can form in such atoms or molecules at any time.

Q.4. What is a hydrogen bond?
Ans: A hydrogen bond is a sort of dipole-dipole interaction that is unique. When a hydrogen atom bonds to an atom of a highly electronegative element like fluorine, oxygen, or nitrogen, the hydrogen atom creates a weak connection with the other molecule’s electronegative atom. The hydrogen bond is the name for this weak bond.

Q.5. Which type of force is present between two non-polar molecules?
Ans: Dispersion force is present between two non-polar molecules.

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