# Van der Waals Force

**Van der Waals forces** are specific intermolecular interactions observed in liquids and solids. They are electrostatic in nature, arising from the interactions of positively and negatively charged species.

intermolecular forces hold molecules together (in contrast to *intra*molecular forces, which hold atoms together *within* a molecule). They help determine bulk properties such as boiling point and melting point.

There are two intermolecular forces that are collectively referred to as Van der Waals Forces: London Dispersion Forces and dipole-dipole interactions.

## Types of Van der Waals Forces

In atoms, the electrons are continuously orbiting in shells. It is possible that at some point all the electrons come to one side of the atom, making it an instantaneous dipole that repels the electrons of neighboring atoms, making an *induced dipole.* This interaction between instantaneous dipole-induced dipole is known as the London dispersion force.

Dipole-dipole forces are similar to London Dispersion forces, but they occur in molecules that are permanently polar versus momentarily polar.

## Van der Waals Equation

When we have some special cases, like nonideal(real) gases. We can use the equation to predict gas properties:

$(P+\frac{n^2a}{V^2})(V -nb) = nRT$

The V in the formula refers to the volume of gas, in moles n. The intermolecular forces of attraction are incorporated into the equation with the $\frac{n^2a}{V^2}$ term where $a$ is a specific value of a particular gas. $P$ represents the pressure measured, which is expected to be lower than in usual cases. The variable $b$ expresses the eliminated volume per mole, which accounts for the volume of gas molecules and is also a value of a particular gas. $R$ is a known constant, $0.08206\frac{L*atm}{mol*K}$, and $T$ stands for temperature.

Unlike most equations used for the calculation of real, or ideal, gases, Van der Waals equation takes into account, and corrects for, the volume of participating molecules and the intermolecular forces of attraction.

$1. \text{NH}_3 \qquad 2. \text{N}_2 \qquad 3. \text{CH}_{2}\text{Cl}_2 \qquad 4. \text{Cl}_2 \qquad 5. \text{CCl}_4$

Predict which of the above gases have:

(i) the smallest van der Waals "a" constant

(ii) the largest "b" constant.

Concatenate the answer, for an example if gas $1. \text{NH}_3$ fits part (i) and gas $4. \text{Cl}_2$ fits the part (ii), then enter the answer as 14. For a and b refer van der waal

**Cite as:**Van der Waals Force.

*Brilliant.org*. Retrieved from https://brilliant.org/wiki/van-der-waals-force/