10.1 Intermolecular Forces

The behavior of liquids may be explained by the kinetic molecular theory.

The popular phrase "Intermolecular attraction" is used to refer to attractive forces between the particles of a substance.

Particles in a solid are tightly packed together and often arranged in a regular pattern, while in a liquid they are close together and in a gas they are far apart.

Particles in a solid vibrate about fixed positions and do not generally move in relation to one another; in a liquid, they move past each other but remain in constant contact; and in a gas, they move independently of one another.

The strengths of the attractive forces that make up each phase are reflected in the properties of solid, liquid, or gas.

The various forces of attraction that may exist between the atoms and the molecule of a substance will be detailed in this module.
The attractive forces hold particles close together, whereas the particles' KE increases the distance between particles.

Transitions between solid, liquid, and gaseous states of a substance occur when conditions of temperature or pressure favor the associated changes in intermolecular forces.

Consider a sample of water as an example of the processes depicted in this figure.

When water is cooled sufficiently, the attractions between H2O molecule will be able to hold them together when they come into contact with each other.

When water in the air is cooled enough to form liquid water, it can be seen on the outside of a cold beverage glass or in the form of fog.

If the temperature is not too high, we can liquefy gases.

The attraction between the molecules of the gas becomes stronger when the pressure is increased.
They form liquids.
C4H10 is the fuel used in disposable lighters and is a gas at standard temperature and pressure.

Gaseous butane condensation occurs when it is compressed within the storage compartment of a disposable lighter.

If the pressure on the liquid becomes too high or the temperature becomes too low, the molecule of the liquid no longer has enough KE to overcome the IMF between them.

A more thorough discussion of these and other changes of state, or phase transitions, is provided in a later module of this chapter.

This simulation can be used to visualize concepts introduced in this chapter.

The attractions between gas molecules will cause them to form liquids.

The strength of the attractive forces varies widely, with the weak IMFs between small molecules being the most common.

To convert one mole of liquid HCl into gaseous HCl requires 17 kilojoules.
It takes 25 times more energy to break the bonds between the hydrogen and chlorine atoms.

In the next three sections of this module, we will look at the various types of IMFs.

The van der Waals forces are present in all phases of the substance.

The London force dispersion is named after Germanborn American physicist Fritz London who first explained it in 1928.
The formation of temporary dipoles is what causes dispersion forces.

The two molecule can be attracted to each other by the dispersion forces.

The forces are relatively weak and only become significant when the molecules are very close.

The dispersion forces of larger and heavier atoms are stronger than those of smaller and lighter atoms.
Weaker attractive forces are reflected by F2 and Cl2 at room temperature.

The strength of dispersion forces is affected by the electronic structure of the atoms or molecule in the substance, so the increase in melting and boiling points with increasing atomic/molecular size may be rationalized.

The valence electrons are closer to the nucleus in a larger atom.

They are easier to form the temporary dipoles that produce the attraction.
A molecule that has a charge cloud that is easily distorted is said to be very polarizable and will have large dispersion forces; one with a charge cloud that is difficult to distort is not very polarizable and will have small dispersion forces.

The smaller the molecule, the less polarizable and the weaker the dispersion forces, the larger the dispersion forces, are predicted by the skills acquired in the chapter on chemical bonding and molecular geometry.

There are 16 g/mol, 32 g/mol, 77 g/mol, and 123 g/mol of CH4, SiH4, GeH4, and SnH4 in the mass.
The lowest boiling point is expected to be CH4 and the highest is expected to be SnH4.
The highest boiling point is expected to be CH4 SiH4 GeH4.

From lowest to highest boiling point, order the following hydrocarbons: C2H6, C3H8, and C4H10.

The larger the molecule, the larger the dispersion forces and the higher the boiling point are.

C2H6 is the lowest boiling point and C3H8 is the highest.

The shape of the molecule affects the dispersion forces between them.

There are three types of neopentane, shown in are 36 degC, 27 degC, and 9.5 degC.
pentane gives a greater surface area for contact between molecules.

The weakest dispersion forces can be found in Neopentane molecule, which is the most compact of the three.

The greater the area of the strip's contact, the stronger the connection.

The boiling points of the isomers show the strength of the dispersion forces.

Geckos can adhere to almost any surface.

They can run up smooth walls and across ceilings that have no toe-holds, and they don't have to worry about sticky things on their toes.
A gecko sticks to the surface if you attempt to pick it up.
Scientists have been investigating this phenomenon for hundreds of years, but only recently discovered the details of the process that allows geckos' feet to behave this way.

Kellar Autumn, who leads a multi-institutional gecko research team, found in 2000 that geckos adhere equally well to both polar and non polar dioxide.

Weak intermolecular attractions arise from temporary, synchronized charge distributions between adjacent molecules, and this proved that geckos stick to surfaces.
The total attraction over millions of spatulae is large enough to support many times the weight of the gecko.

Alex Greaney and Congcong Hu at Oregon State University talked about how geckos can change the angle of their spatulae.

When a small shear force is applied to the feet, they become sticky.
By curling and uncurling their toes, geckos can alternate between sticking and unsticking from a surface, and thus easily move across it.
It is possible that further investigations will lead to the development of better applications.

The setae are small hairs that branch into many triangular tips.

Kellar Autumn's research shows that van der Waals forces are responsible for a gecko's ability to cling and climb.

Consider a polar molecule.

The less positive H atom bears a partial negative charge in the HCl molecule.
The force between the positive and negative ends of a polar molecule is illustrated.

This image shows two arrangements of polar molecules that allow an attraction between the positive and negative ends of a molecule.

The effects of a dipole-dipole attraction can be seen when we compare the properties of the two compounds.

Both HCl and F2 have the same number of atoms.

The average KE of the two substances would be the same at a temperature of 150 K. This substance is gaseous at this temperature because the dipoledipole attractions between HCl and F2 are enough to cause them to stick together.

The higher normal boiling point of HCl compared to F2 is a reflection of the greater strength of dipole-dipole attractions.
The relative strengths of attraction present within different substances will often be shown by values such as boiling or freezing points.

Both CO and N2 have similar London dispersion forces.

CO is a polar molecule.
N2 can't exhibit dipole-dipole attractions because it's nonpolar.
CO is expected to have a higher boiling point because the dipole-dipole attractions between CO and N2 are stronger.

The London dispersion forces are similar due to the fact that ICl and Br2 have the same mass.

ICl is polar and also exhibits dipole-dipole attractions.

ICl will have the higher boiling point because the stronger attractions require more energy to overcome.

There is a gas at room temperature.

Even though it has a lower mass, water is a liquid.
The difference between the two compounds cannot be attributed to dispersion forces.
ONF is the heavier molecule and both have the same shape.
It is expected to experience more dispersion forces.
We can't attribute the difference in boiling points to the differences in the dipole moments of the molecule.
Both molecules have the same dipole moments.
The large difference between the boiling points is due to a strong dipole-dipole attraction that may occur when a molecule contains a hydrogen atom.

Water molecule interact with nearby water molecule.

Hydrogen bonds are generally stronger than other dipole-dipole attractions and dispersion forces.

The properties of liquid and solid are affected by hydrogen bonds.

Consider the trends in boiling points for the hydrides of group 15 (NH3, PH3, AsH3, and SbH3), group 16 (H2O, H2S, H2Se, and H2Te), and group 17 The polarities of the molecule decrease slightly as we progress down any of these groups.
The effect of increasingly stronger dispersion forces dominates that of increasingly weaker dipole-dipole attractions, and the boiling points are observed to increase steadily.

The boiling points for each class of compounds increase with increasing mass for elements in periods 3, 4, and 5.

If we use this trend to predict the boiling points for the lightest hydride, we would expect NH3 to boil at about -110 degC, H2O to boil at about -80 degC, and HF to boil at about -110 degC.

The stark contrast between our naive predictions and reality provides compelling evidence for the strength of hydrogen bonding, as shown in The stark contrast between our naive predictions and reality.

The hydride of the 2 elements in groups 17 and 16 exhibit high boiling points due to hydrogen bonding.

Consider the compounds propane and dimethyl ether.

The shapes of CH3OCH3, CH3 CH2 OH, and CH3 CH2 CH3 are similar, so they will exhibit the same dispersion forces.

dipole-dipole attractions will be experienced by CH3OCH3 because it is polar.
The uniquely strong dipole-dipole attraction known as hydrogen bonding will be experienced by CH3 CH2 OH.
The boiling points are CH3 CH2 CH3 CH3OCH3 CH3 CH2 OH.
The boiling point of propane is -42.1 degC, the boiling point of dimethyl ether is -24.8 degC, and the boiling point of ethanol is 78.5 degC.

The melting point and boiling point for ethane are predicted to be less than the melting point and boiling point for methylamine.

CH3CH3 and CH3NH2 are similar in size and mass, but the -NH group in methylamine may cause hydrogen bonding.
Its melting and boiling points are increased by this.
It is difficult to predict values, but the known values are a melting point of -93 degC and a This OpenStax book is available for free.

The genetic information that determines the characteristics of the organisms is contained in deoxyribonucleic acid, which serves as a template to pass this information on to the offspring.

A double-stranded helix is formed by two separate DNA molecules that are held together by hydrogen bonding.

One of the nitrogenous bases is bound to a phosphate group on the other side.

pyrimidines are single-ringed structures found in two of the bases.
Two of the other structures, adenine and guanine, are double-ringed.
One purine and one pyrimidine, with adenine and thymine, form a pair of bases.
The base pairs are held together by hydrogen bonding.

A and T share two hydrogen bonds, C and G share three, and both pairs have a similar shape and structure.