Factors That Affect Van der Waals Forces in Molecules
In chemistry, van der Waal force is a kind of interaction that is dependent on the distance between molecules or atoms. It is different from covalent or ionic bonds and is not a resultant attraction of chemical, electronic bonds. Instead, they are relatively weak and hence more vulnerable to disturbance. Also, the van der Waals force vanishes quickly at lengthy distances between the molecules which are interacting with each other.
This force is named after a Dutch scientist Johannes Diderik van der Waals and was discovered by him in the year 1873. However, besides studying the factors that affect van der Waals forces, it is also necessary for you to have other knowledge related to van der Waals definition, characteristics, van der Waals equation, etc.
Let us begin with Van Der Waals’s forces definition.
What is The Definition of Van Der Waals Force?
Van Der Waals bond comprises attraction and repulsion between surfaces, molecules and atoms, and also other forces between particles. It also plays an integral part in diverse fields like:
Supramolecular chemistry
Structural biology
Polymer science
Nanotechnology
Surface science
Condensed matter physics
This force underlies multiple characteristics of molecular solids and organic compounds along with their solubility in non-polar and polar media.
For instance, if no force exists, the distance at which force between the atoms become repulsive instead of attractive is termed as Van Der Waal constant distance. This situation takes place when electron clouds of particles undergo repulsion between them. It has an origin similar to the Casimir effect which arises from quantum interactions accompanied by a zero-point field.
Van Der Waals's forces of attraction are the weakest among the weak chemical forces having a strength that ranges between 0.4 and 4-kilo Joules/mol. However, they may assist a necessary structural load when a host of similar interactions exist. This force is a result of electron density suffering transient shift. Precisely, electron density may shift temporarily to one side of the nucleus. This yields a temporary charge that either attracts or repels a closer atom.
Moreover, when the distance between the atoms is more than 0.6 nm, the force is not that strong that it can be observed. However, when the range is less than 0.4 nm, the force is repulsive.
Furthermore, both dispersion forces and dipole-dipole forces are two types of Van Der Waals forces.
Dipole-Dipole Forces
These are the forces that occur between polar molecules. For instance, one hydrogen chloride molecule contains a hydrogen atom that is partially positive and a partially negative chlorine atom. If many hydrogen chloride molecules are present, they arrange themselves in such a manner that regions of oppositely charged particles are closer to each other.
Note: Dipole-Dipole forces are quite weaker than ionic bonds.
London Dispersion Forces
These forces are also a type of Van Der Waals forces and are considered the weakest of all other intermolecular forces. Often they are termed as London forces after the scientist Fritz London who first discovered it in 1930. London dispersion or Van Der Waals dispersion forces take place between non-polar molecules and atoms due to electron motion.
An example of this force is found in helium. The electron cloud in helium consists of two electrons, which are expected to be distributed equally around the nucleus. However, in some specified moments, the distribution can be uneven, which causes instantaneous dipole. Due to this temporary and weak dipole, the nearby helium atoms are influenced via electrostatic repulsion and attraction. This gives rise to a dipole on neighboring helium atoms.
Moreover, the attraction between induced and instantaneous dipoles is weak, and dispersion force strength increases with the increase in electrons in non-polar molecules and atoms.
Characteristics of Van Der Waals Force
The primary characteristics of Van Der Waals forces are:
These forces are relatively weaker than ionic and covalent bonds.
They are additive forces and cannot be saturated.
They lack directional characteristics.
Van Der Waals forces occur within short ranges, and interactions only occur between the close particles. The force increases if the molecules or atoms are near each other.
They are not dependent on temperature except for dipole-dipole interactions.
Factors Affecting Van Der Waals Forces
Two factors that affect van der Waals forces are:
Number of Electrons Clutched with the Molecules or Atoms
In the modern periodic table, while you traverse down a group, you will notice that atomic radii of elements increase with electron number present in their nuclei. When a comparatively significant electron number is present (with added space so that electrons can disperse), it results in the formation of dipoles which are temporary. The more dipole number is formed; more is the Van Der Waals interactions strength.
An instance of this relation is noticed in different boiling points of neon and xenon. The boiling point of neon is -246 degree Celsius, and that of xenon is -108 degree Celsius. As the atoms of xenon experience strong dispersion forces, it has a significantly low boiling point.
Molecule Shape
Molecules which are long and unbranched tend to exhibit strong forces of dispersion than short and branched molecules. An example of this is the structural isomers of isobutane ( 2 - methyl propane) and butane. Although they have similar chemical formulae, these two isomers have distinct boiling points. Butane has a boiling point of -0.5 degree Celsius and isobutane has a boiling point of -11.7 degree Celsius.
As, boiling points of these two isomers are different, Van Der Waals force is strong in case of unbranched molecules of butane and weak in branched molecules of isobutane.
Van Der Waals Equation
According to the ideal gas law, the molecules are treated as point particles which do not interact with others. They only interact with the containers they are placed in. The equation for ideal gas law is as follows:
PV = nRT
Where V = Volume occupied by n moles of a gas
P = Pressure
T = Temperature
R = Gas constant
The volume taken up by the real gas molecule V is replaced by Van Der Waals equation with ( Vm - b ) where b is the volume occupied by 1 mole of molecules, and Vm is the molar volume of gas. Hence, the equation can be stated as:
P ( Vm - b ) = RT
Another change made in the ideal gas law for the reason that molecules of gas interact with one another (they experience repulsion and attraction at high pressure and low pressure respectively), and real gases show distinct compressibility than ideal gases. For interaction between the molecules, Van Der Waals added a term a / V2m, where a is equal to a constant whose value is dependent on gas. Therefore, Van Der Waals equation example can be written as:
( P + a (1 / V2m )) ( Vm - b ) = RT
And is further rearranged in the form:
( P + a (n2 / V2)) (V - nb) = n RT
Where V = gas volume
a = specific value of gas
P = pressure
T = temperature
R = gas constant ( 0.08206 L - atm / mol K )
Uses of Vander Waals Forces by Geckos and Arthropods
Geckos have the ability to hang on to a glass surface with the help of only one toe. This ability of geckos to hang on to glass surfaces to climb up on sheer surfaces and so on has been attributed to the Vander Waals forces for many years. The Vander Waals force for geckos exists between these surfaces and the spatula of the geckos. Spatulae are microscopic projections on the footpads of geckos. These spatulae cover the hair-like setae which are found on the footpads of the gecko. A few years later, some studies suggested that the reason for geckos being able to hang on to glass surfaces and climb up may be due to the role of capillary adhesion. This hypothesis was soon rejected by more recent studies.
In a recent study, it has been observed that the adhesion of gecko to smooth Teflon and polydimethylsiloxane is chiefly determined by electrostatic interaction. This electrostatic interaction is caused by contact electrification. This rebuked the theory that the cause of adhesion is Vander Waals or capillary forces.
Do it Yourself
Question: Determine which of the following gases have (i) smallest Van Der Waals constant "a" and (ii) highest Van Der Waals constant "b"?
1) NH3
2) N2
3) CH2 Cl2
4) Cl2
5) CCL4
Join the answer like for example if gas 1) NH3 fits (i) and 4) CL2 fits (ii) then the answer is 14.
Your options are:
a) 23
b) 12
c) 21
d) 53
e) 45
f) 25
g) 15
h) 13
Van Der Waals Force Applications
Applications of Van Der Waals forces are widely found in Geckos and Arthropods. Geckos can hang on surfaces made of glass with just one toe and can also climb on steep surfaces. This ability of them is because of Van Der Waals forces that attract spatulae and the surfaces.
Few spiders also make use of van der Waals force to hang upside down or climb on smooth surfaces like porcelain or glass.
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FAQs on Van der Waals Forces and Their Affecting Factors
1. What are van der Waals forces in chemistry?
Van der Waals forces are weak intermolecular forces that act between molecules due to temporary or permanent dipoles. These forces are weaker than covalent or ionic bonds but significantly affect physical properties like boiling point and melting point. They include:
- London dispersion forces (instantaneous dipole–induced dipole)
- Dipole–dipole interactions (between polar molecules)
- Dipole–induced dipole forces
2. What factors affect the strength of van der Waals forces?
The strength of van der Waals forces mainly depends on molecular size, shape, and polarity. Key affecting factors include:
- Molecular mass: Larger atoms or molecules have stronger London dispersion forces.
- Number of electrons: More electrons increase polarizability.
- Surface area: Long, straight molecules experience stronger attractions than branched ones.
- Polarity: Polar molecules exhibit additional dipole–dipole interactions.
3. How does molecular size influence van der Waals forces?
Van der Waals forces increase as molecular size and molar mass increase because larger molecules have more electrons and greater polarizability. For example:
- F2 (gas) has weaker dispersion forces than I2 (solid).
- Iodine molecules contain more electrons, leading to stronger temporary dipoles.
4. How does molecular shape affect van der Waals forces?
Molecular shape affects van der Waals forces by altering the surface area of contact between molecules. Straight-chain molecules have:
- Greater surface area
- Stronger dispersion forces
- Higher boiling points
5. Why does polarity increase van der Waals forces?
Polarity increases van der Waals forces because polar molecules exhibit dipole–dipole interactions in addition to London dispersion forces. In polar molecules:
- Partial positive (δ+) and partial negative (δ-) charges exist.
- Opposite charges attract between neighboring molecules.
6. What is polarizability and how does it affect van der Waals forces?
Polarizability is the ease with which an electron cloud can be distorted, and higher polarizability leads to stronger London dispersion forces. It increases with:
- Larger atomic radius
- Greater number of electrons
7. What is the difference between London dispersion forces and dipole–dipole forces?
The key difference is that London dispersion forces occur in all molecules due to temporary dipoles, while dipole–dipole forces occur only in polar molecules with permanent dipoles. Comparison:
- London dispersion: Present in all atoms and molecules; weakest type.
- Dipole–dipole: Present only in polar molecules; generally stronger.
8. How do van der Waals forces affect boiling point?
Stronger van der Waals forces result in a higher boiling point because more energy is required to separate molecules. When intermolecular forces increase:
- Molecules attract more strongly.
- More heat energy is needed to overcome these attractions.
9. Are hydrogen bonds considered van der Waals forces?
Hydrogen bonds are sometimes classified separately, but they are a strong special case of dipole–dipole interaction. Hydrogen bonding occurs when hydrogen is bonded to N, O, or F and interacts with a lone pair on another molecule. For example, strong hydrogen bonding exists between H2O molecules, significantly increasing its boiling point compared to similar-sized molecules.
10. Why are van der Waals forces important in real life and chemistry?
Van der Waals forces are important because they determine physical properties and molecular interactions in many chemical systems. They influence:
- Boiling and melting points of substances
- Solubility of nonpolar compounds
- Protein folding and DNA base pairing
- Liquefaction of gases
