Molecules are formed by the sharing of electrons between atoms. As the name suggests, intermolecular forces are the electrostatic forces between molecules. These forces can be attractive or repulsive and act between ions or atoms in molecules. The forces are repulsive when atoms are very close to each other. In that case, the nuclei of the two atoms, having the same positive charge, repel each other. Over little longer distances i.e. bond length, the intermolecular forces become attractive. These electrostatic forces of attraction are present between nuclei and shared electron pairs.
As a general measure of the strength of intermolecular forces, these are weaker than intramolecular forces (forces within a molecule) such as a metallic bond, ionic bond, or covalent bond. It is because the intramolecular forces hold the atoms in a molecule together whereas intermolecular forces hold different molecules together.
A comparison of the strength of intermolecular forces against one another would show that ion-dipole interactions are the strongest type of intermolecular force. These are followed by hydrogen bonds, and then by dipole-dipole interactions. The weakest intermolecular forces are London dispersion forces.
The physical properties of all states of matter are influenced by intermolecular forces. The gaseous properties that are affected include boiling point, critical point, and vapor pressure. Liquid properties that depend on intermolecular forces include surface tension, viscosity, and diffusion. The properties of solids that are determined by intermolecular forces are melting and sublimation.
Types of intermolecular forces
1. Ion-dipole interactions
An ion is a positively charged (cation) or negatively charged (anion) species. Dipole refers to the partial negative and positive charges on a molecule. Ion-dipole interactions are electrostatic forces of attraction between an ion and a polar molecule.
Ion dipole interactions are stronger than the dipole-dipole interactions because an ion has a much stronger charge than a dipole when compared. Ion-dipole interactions are even stronger than hydrogen bonds.
The ions align with a polar molecule in such a way that the positive ions are close to the negative part of the dipole and vice versa.
For example
When sodium chloride (NaCl) is dissolved in water, the sodium cation and the partial negative charge on the oxygen atom in a water molecule is the ion-dipole interaction.
Another similar force of attraction is the ion-induced dipole force that occurs when the charges on the ions induce a temporary charge in a nonpolar molecule by distorting its electronic cloud.
2. Dipole-dipole interactions
Dipole-dipole interactions are intermolecular forces of attraction present between two dipoles.
Molecules that have dipoles are called polar molecules. There are electrostatic forces of attraction present between the opposite charges in polar molecules (dipoles). The partial charges in a molecule arise because of the electronegativity differences between the atoms bonded together in molecules.
Electronegativity is the tendency of an atom to attract the bonded pair of electrons towards itself. The more electronegative atom attracts the bonded pair of electrons (electron density) more towards itself. This causes a partial negative charge on the more electronegative atom due to the increased electron density over it. Similarly, a partial positive charge is caused on the atom with lesser electronegativity.
When the polar molecules are close, as in the liquid state, they orient in a manner to maximize the positive-negative attractions and minimize the negative-negative and positive-positive repulsions between them.
In the case of HCl, the partial negative charge lies on the more electronegative chlorine atom and the partial positive charge lies on the lesser electronegative hydrogen atom. The HCl molecules arrange in such a way that the partial negative chlorine end of one HCl molecule faces the partially positive hydrogen end of another HCl molecule and vice versa.
Molecules such as CCl4 do have polar bonds present but the overall dipole moment cancels out because of symmetry in the molecule, this phenomenon is known as geometrical non-polarity.
Strength of Hydrogen bond
A stronger type of dipole-dipole interaction is the hydrogen bond. The requirement for the formation of a hydrogen bond is the presence of a hydrogen atom (bonded to a highly electronegative element such as fluorine, oxygen, or nitrogen) and a lone pair of electrons on another electronegative element.
The highly electronegative F, O, or N elements bonded to the hydrogen atom pulls the electron density away from the hydrogen atom, leaving it like a naked proton with a strong partial positive charge, owing to the small size of the hydrogen atom. This partial positive charge attracts a lone pair of electrons on a nearby electronegative atom, and this electrostatic attraction is the reason for the strength of the hydrogen bond.
In the comparison of boiling points of (H2O) water with (H2S) hydrogen sulfide, H2S is a gas at room temperature and has a boiling point of -60 ºC. Water on the other hand boils at 100 ºC, which is expected to boil at -46 ºC according to other neighbors. Hydrogen bonding is present in H2O while absent in H2S and it is the main reason for this difference in physical nature.
Hydrogen bonding is present in H2O and is the primary reason why ice is less dense than water and floats on top (although solids are usually denser than their respective liquid forms). Substances having hydrogen bonding present also have relatively high boiling points. Hydrogen bonding can be intramolecular in addition to intermolecular.
3. Van der Waals forces
There are molecular attractions that may or may not require the presence of charges. These are known as the Van der Waals forces of attraction and are present in all molecules.
The Van der Waals forces are a combination of Keesom force (permanent molecular dipole-dipole), Debye forces (permanent dipole-induced dipole), and London dispersion forces (instantaneous-induced dipole), of which the London dispersion forces are the weakest.
London dispersion forces, a type of Van der Waals’ forces
The most common among Van der Waals forces is the London dispersion force (fluctuating dipole-induced dipole) that arises due to the non-zero instantaneous dipole moments in molecules.
Electrons, apparently uniformly distributed, are in constant movement around the nuclei and sometimes happen to come close in a certain space. The electron density increases at that certain space and creates a partial negative charge there. On the contrary, a partial positive charge is formed where the electron density is decreased.
This non-symmetrical electron distribution produces temporary dipoles. The formation of a temporary dipole in one molecule can induce dipoles in neighboring molecules. This phenomenon creates an intermolecular attraction that is weak and short-lived.
However, it is more effective in larger molecules with more electrons, as it increases the chance of the formation of dipoles. This is better explained by the concept of polarizability (the ability of an electronic cloud to be distorted to give dipolar charge distribution). Hence larger molecules with more electrons exhibit higher polarizability than smaller molecules.
For example, halogens are non-polar molecules. Cl2 has a total of 34 electrons. It has very weak Van der Waal interactions, compared with I2 which has 144 electrons in one molecule.
In addition, butane and isobutane have the same molecular formula but different structures. The boiling point of butane is -1 °C while isobutane has -11.7 °C. In simple words, the butane molecule has 4 faces to interact with other molecules while isobutane only has 3 points. Hence, the greater the number of electrons in a molecule stronger will be the produced Van der Waal forces. Also, with more faces of molecules available, higher would be the chances to interact with other molecules.
Concepts Berg
How do forces of attraction affect the properties of compounds?
The properties of all states of matter are influenced by intermolecular forces of attraction. These properties depend primarily on the strength of intermolecular forces, and the types of intermolecular forces present since they vary in strength. This “strength” affects the “sticking together” of molecules, which is in turn responsible for the change in physical properties.
The gaseous properties that are affected include critical point, boiling point, and vapor pressure.
The liquid properties that depend on intermolecular forces include surface tension, diffusion, and viscosity.
Solid properties that are determined by intermolecular forces are melting and sublimation.
What is the order of strength for intermolecular forces?
The order of strength for intermolecular forces, ranked from weakest to strongest is:
London dispersion forces < Dipole-dipole interactions < Hydrogen bonding < Ion-dipole interactions
How do you know which intermolecular force is strongest?
Ion dipole interaction is the strongest intermolecular force of attraction. This is because it involves the electrostatic interaction between completely charged ions and partially charged dipoles.
What causes the strength of intermolecular forces?
The strength of intermolecular forces is caused by electrostatic attraction between the charges, whether they may be partial charges, permanent dipoles, induced dipoles, or ions.
What are 4 types of intermolecular forces?
The four types of intermolecular forces are ion-dipole interactions, hydrogen bonding, dipole-dipole interactions, and London dispersion forces, ranked from strongest to weakest in strength.
What intermolecular force is the weakest?
London dispersion forces are the weakest type of intermolecular forces because this electrostatic attraction is weak, short-ranged, and short-lived. It occurs between the temporarily induced dipoles.
What is the strongest and weakest type of bond?
Bonds in general refer to intramolecular forces of attraction that hold a molecule together. There are a few types of bonds. Firstly, the metallic bond, in which the positive nuclei are fixed in position in a sea of delocalized electrons. Another type of bond is covalent, formed by the mutual sharing of electrons. Then, there’s the ionic bond, where the transfer of electron(s) takes place from one atom to another.
Usually, the covalent bond is considered to be the strongest, especially in the cases of giant covalent structures like diamond or graphite, having a network of covalent bonds. At other times, it is the ionic bond that is considered stronger. As for the metallic bond, it is deemed somewhat weaker than the other two.
Strength of intermolecular forces in gases.
Since molecules in the gas state are far apart, the intermolecular forces are negligible and weak in strength. This is the case especially for ideal gases.
Strength of intermolecular forces in liquids.
The strength of intermolecular forces in liquids is intermediate since the molecules are relatively closer.
Strength of intermolecular forces in solids.
Out of all states of matter, solids have the strongest intermolecular forces as the strength depends on the distance between the molecules. The molecules/ formula units are closely packed in the solid-state.
What factors affect the strength of intermolecular forces?
The strength of intermolecular forces is affected by the distance between the molecules. As the distance between the molecules is increased, the intermolecular forces rapidly decrease in strength.
Another important factor is the strength of charges present for the electrostatic attraction between molecules.
For London dispersion forces, it means an increase in the number of electrons for ease in the polarization of the electron cloud.
In the case of dipole-dipole interactions, atoms with more difference in electronegativity will cause stronger dipoles, and stronger will be the intermolecular force of attraction.
As for ion-dipole interactions, their strength depends on the electrostatic attraction between the ions and dipoles. Stronger charges on ions as well as dipoles would increase the strength of the intermolecular force.
How do you determine the relative strength of intermolecular forces of alcohols?
The types of intermolecular forces found in alcohols are hydrogen bonding and London dispersion forces. The longer chains (more surface area) allow more London dispersion forces and the functional -OH group allows hydrogen bonding. Multiple -OH groups that are “exposed” (able to form hydrogen bonds) contribute greatly to an increase in the intermolecular forces and in turn, the boiling point.
So the longer the carbon chain, the more will be the London dispersion forces, and hence the stronger the intermolecular forces of attraction will be.
Branching in alcohols decreases the surface area, allows for lesser London dispersion forces, and therefore lowers the boiling point.
Also, the more -OH groups that can participate in hydrogen bonding, the stronger the intermolecular forces of attraction.
Does the strength of the intermolecular forces correlate with the specific heat capacity?
Yes, the stronger the intermolecular forces, the greater the heat capacity (ability to absorb energy). This is also the reason why water has a relatively high heat capacity: the presence of hydrogen bonding.
How are intermolecular forces important to humans?
Intermolecular forces affect the physical properties of many substances essential to humans and other living beings. An example is the high heat capacity of water which is due to hydrogen bonding, a type of intermolecular force of attraction. Since many organisms are composed mainly of water, a high heat capacity allows regulated body temperature.
The DNA structure of humans is a double helix. The two phosphate backbone strands are connected through hydrogen bonds.
Why are intermolecular forces so important?
Intermolecular forces are important because they can alter the physical properties of substances made of similar molecules. The properties include melting and boiling points, viscosity, surface tension, vapor pressure, and enthalpy of vaporization.
What is the relationship between boiling points and intermolecular forces?
The stronger the intermolecular forces, the more is the boiling point of substances. This is due to an increase in the enthalpy of vaporization.
Why does the higher the molecular mass the stronger the intermolecular force we have?
Because higher molecular mass corresponds to a greater number of electrons. This means increased London dispersion forces.
How is melting point affected by intermolecular forces?
More energy is required to overcome stronger intermolecular forces in a solid and so higher the melting point observed.
How is cooling related to intermolecular forces?
When a substance is cooled down, its internal energy is decreased i.e. random motion of gas molecules becomes slower as kinetic energy decreases. Its molecules are now more prone to interact with other molecules, to induce new dipoles. Thus, gases also become solid at low temperatures. E.g CO2 (dry ice) at -80°C.
What is the relation between intermolecular forces and distance?
All Intermolecular forces are electrostatic forces. So, by Coulomb’s law, F is proportional to 1/r2
Distance is inversely proportional to intermolecular forces, whether attractive or repulsive.
What is the difference between intermolecular and electrostatic forces?
Electrostatic forces refer to forces of attraction and repulsion between electric charges. There are forces of attraction between opposite charges and repulsion between similar charges.
Intermolecular forces are attributed to forces between the molecules. These are repulsive when molecules are very close to each other and are attractive when the molecules are relatively farther. Be it ion-dipole interactions, dipole-dipole interactions, or Van der Waals’ forces, all intermolecular forces of attraction more or less rely on electrostatic forces to keep the molecules close.
What type of intermolecular forces does SO3 have? If it has hydrogen bonding where are its potential bonding sites?
The only intermolecular force SO3 has is the Van der Waals’ force of attraction. As the molecule is symmetrical, the dipoles cancel out.
Do amides have stronger intermolecular forces than amines?
Yes, amides do have stronger intermolecular forces than amines because, in addition to a nitrogen atom, there is also a carbonyl (C=O) present which can help in additional hydrogen bonding in amides. But it does depend on other factors such as the surface area of the molecule or the ability of the groups to form hydrogen bonds.
What intermolecular forces are displayed by HBr?
There are dipole-dipole interactions and van der Waals’ forces of attraction between HBr molecules.
How do intermolecular forces affect a liquid’s heat of vaporization?
The stronger the intermolecular forces, the more is the heat required to overcome them. This corresponds to increased heat of vaporization.
How does vapor pressure relate to intermolecular forces?
The vapor pressure is a measure of the pressure exerted by a gas on its liquid in a closed container. Stronger intermolecular forces correspond to lower vapor pressure as the liquid does not evaporate easily.
How does a covalent bond have a strong bond within the molecule but a weak intermolecular force?
The covalent bond is a force present within a molecule (intramolecular). Intramolecular forces are stronger than intermolecular forces.
The intermolecular forces present can be dipole-dipole, ion-dipole, or Van der Waals’ forces (not the covalent bond since it is an intramolecular force), and these are all weaker than the covalent bond.
How is the surface tension related to the intermolecular forces?
Surface tension is defined as the energy required to increase the surface area of a liquid by a unit area. More energy is required to overcome stronger intermolecular forces of attraction, so the stronger the intermolecular forces, the higher the surface tension.
Also, there are cohesive forces that correspond to intermolecular forces.
Why are intermolecular forces decreasing during heating?
The kinetic energy of molecules increases by heating and so does the intermolecular distance. With the increase in the distance between the molecules, the intermolecular forces decrease.
How does the melting point of a substance indicate the strength of its intermolecular force of attraction?
Substances melt when their intermolecular forces of attraction are overcome. Therefore, a higher melting point would predict stronger intermolecular forces of attraction present.
Are intermolecular forces between covalent molecules strong?
The intermolecular forces are not as strong as the intramolecular forces (covalent bond, ionic bond, metallic bond) holding the atoms in molecules together. However, the strength of intermolecular forces varies depending on what type of intermolecular forces are present and to what extent.
The covalent molecules can have hydrogen bonding and dipole-dipole interactions, which are stronger intermolecular forces. On the other hand, a large covalent molecule (a less branched polymer) can have lots of Van der Waals’ forces which attribute to an even stronger intermolecular force.
What types of intermolecular forces exist between HCl molecules?
Van der Waals’ forces most definitely exist between HCl molecules. Then there are the dipole-dipole interactions also present due to the electronegativity difference between H and Cl.
However, no hydrogen bonding takes place in HCl molecules due to the absence of a highly electronegative atom that can form hydrogen bonds with ease (F, O, N).
What intermolecular forces operate between two CBr4 molecules?
The intermolecular forces that operate between two CBr4 molecules are London dispersion forces (Van der Waals’ forces).
Due to the tetrahedral symmetry of the molecule, the dipoles cancel out, and so the possibility of dipole-dipole interactions is ruled out.
What types of intermolecular forces occur between two polar molecules?
Dipole-dipole interactions are the intermolecular forces that occur between two polar molecules due to their polarity. Van der Waals’ forces are also present, but the presence of hydrogen bonding is determined by the availability of hydrogen atoms as well as a highly electronegative atom (F, O, N).
Why is the boiling point of methanoic acid higher than ethanol though they have the same molecular mass?
It is due to the fact that there is stronger, more extensive hydrogen bonding present in methanoic acid as compared to ethanol. This is because of an extra oxygen atom present in the functional group of the carboxylic acid than the alcohol, which increases the strength of the dipole. It corresponds to a stronger intermolecular force and so higher boiling point.
Therefore, it is about the number of hydrogen bonds and not about stronger Van der Waals’ forces that a similar molecular mass would play a part in.
Which intermolecular forces do methanol molecules have?
In addition to the hydrogen bonding, methanol also contains Van der Waals’ forces of attraction as its intermolecular forces, which can be shown with the help of dotted lines between the molecules.
Why does the boiling point of a homologous series increase as molecular mass increases?
The reason is the increase in intermolecular forces, specifically the London dispersion forces (Van der Waals’ forces). This is because the surface area of the molecules increases as the molecular mass increases. More surface area corresponds to extended London dispersion forces and thus, the higher boiling points.
How are evaporation rate and temperature related to the strength of an intermolecular force?
Stronger intermolecular forces mean a lower vapor pressure and so a lesser rate of evaporation. As for temperature, increasing it would weaken the intermolecular forces by increasing the distance between the molecules. So evaporation rate and temperature are inversely proportional to the strength of the intermolecular force.
Can the melting and boiling point of substances be accurately predicted if we have the strength of their intermolecular forces?
Yes, the melting and boiling points of substances can be predicted with the help of intermolecular forces, because these properties depend on intermolecular forces.
Explain the correlation between molecular size and the strength of the instantaneous dipoles holding them together?
The larger a molecule is, the more polarizable is its electron cloud and the stronger the dipoles formed. This refers to increased electrostatic forces of attraction. The strength of instantaneous dipoles depends primarily on the polarizability of the electron cloud.
References
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Intermolecular and Surface Forces by Jacob N.
