What Determines the Strength of Van Der Waals Forces?
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: Influencing Factors in Chemistry
1. What are the two main factors that affect the strength of Van der Waals forces?
The strength of Van der Waals forces is primarily influenced by two key factors:
- Number of Electrons (or Molecular Size): Molecules with a larger number of electrons have larger, more dispersed electron clouds. This makes them more polarisable, leading to the formation of stronger temporary dipoles and thus, stronger London dispersion forces. This is why the boiling points of noble gases increase down the group.
- Shape of the Molecule: Long, unbranched molecules have a larger surface area for interaction compared to compact, branched molecules of the same molecular mass. This increased contact area allows for stronger Van der Waals forces, which typically results in higher boiling points (e.g., n-butane vs. isobutane).
2. What are the different types of intermolecular attractions that are collectively known as Van der Waals forces?
Van der Waals forces are a general term for the weaker intermolecular attractions and typically include two main types:
- Dipole-Dipole Forces: These are electrostatic attractions that occur between polar molecules that have permanent positive and negative ends (dipoles). The positive end of one molecule attracts the negative end of a neighbouring molecule.
- London Dispersion Forces: These are the weakest intermolecular forces and exist in all molecules, including non-polar ones. They arise from the temporary, instantaneous dipoles created by the random movement of electrons, which in turn induce dipoles in adjacent molecules.
3. How does the shape of a molecule influence the strength of its Van der Waals forces?
The shape of a molecule has a significant impact on its surface area, which directly affects the strength of Van der Waals forces. Long, chain-like molecules (like n-pentane) have a larger surface area available for intermolecular contact than spherical or branched molecules (like neopentane). This greater contact area allows for more numerous and stronger London dispersion forces, leading to a higher boiling point for the unbranched isomer.
4. Why are Van der Waals forces important in chemistry, even though they are considered very weak?
Although individually weak, the cumulative effect of Van der Waals forces across a vast number of molecules is very significant. They are crucial for determining many physical properties of substances, such as:
- Boiling and melting points: Stronger Van der Waals forces require more energy to overcome, leading to higher boiling and melting points.
- Solubility: They explain why non-polar substances can dissolve in non-polar solvents.
- Structure of molecular solids and polymers: These forces hold molecules together in a solid or liquid state and are vital for the structure of polymers and biological macromolecules like DNA.
5. Which is stronger: a hydrogen bond or a Van der Waals force? Explain why.
A hydrogen bond is significantly stronger than a typical Van der Waals force. A hydrogen bond is a special, stronger type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. This creates a very strong, permanent dipole. In contrast, the most common type of Van der Waals force, the London dispersion force, arises from weak, temporary dipoles. The permanent and stronger nature of the dipole in hydrogen bonding results in a much stronger attraction.
6. How do London dispersion forces arise even in completely non-polar atoms like Helium (He)?
Even in a non-polar atom like Helium, the electrons are in constant, random motion. At any given instant, the electron distribution can be uneven, creating a temporary, instantaneous dipole where one side of the atom is slightly negative and the other is slightly positive. This fleeting dipole can then induce a dipole in a neighbouring Helium atom, leading to a weak, short-lived electrostatic attraction. These attractions, occurring constantly between atoms, are the London dispersion forces.
7. Can you provide a real-world example where Van der Waals forces are clearly observable?
A classic real-world example is the ability of a gecko to climb smooth surfaces like glass. A gecko's feet are covered in millions of microscopic hairs called setae, which are further split into even smaller tips called spatulae. This structure massively increases the surface area in contact with the wall. The cumulative effect of the weak Van der Waals forces between the spatulae and the surface molecules is strong enough to support the gecko's entire body weight.
