HS Chemistry - Intermolecular Forces

Types of Intermolecular Forces:

Overview of The Page

This page will cover:

• What are the different types of intermolecular forces?

There are 4 types of intermolecular forces. From weakest to strongest they are: 1. Dispersion forces $aka London dispersion$ 2. Dipole-dipole forces 3. Hydrogen Bonding $a special type of dipole\-dipole force$ 4. Ion-dipole forces

The first two are also called van der Waals forces.

Note: Ionic and metallic bonding are not covered here, as distinct molecules are not formed in ionic and metallic bonding.

Dispersion forces

The polarizability of an atom is the tendency of its electron cloud to distort. A nonpolar particle can be temporarily polarized to allow dispersion force to form.

We can use monatomic Helium atoms to show how dispersion force forms, but it works for other molecules too:

Two Helium atoms approach each other.

Their electrons are randomly spinning. Due to this random spin, at one point one atom's electrons both face the other atom.

Negative charges repel each other. The other atoms' electrons rush to the other side of that atom.

The sides of the atoms facing each other are now oppositely charged, and attract each other. A force of attraction is formed. In this final image, the Helum atoms are enlarged so that the bond between the Helium monoatomic molecules can be seen. The atoms did not actually grow bigger.

This form of attraction is called an induced-dipole force, which is another term for dispersion force. Dispersion force is actually kind of a misnomer - it doesn't cause particles to disperse. In fact, it does quite the opposite.

Induced-dipole bond more accurately describes what is happening. The molecules are non-polar to begin with; their polarity is less than 0.5, and thus they have no significant dipole. When the electrons come into the right positions, a dipole is momentarily created, or induced, between the two molecules, creating a temporary attraction between the molecules.

Since the electrons are moving very fast, dispersion forces only last very momentarily $like very, very momentarily \- think millionth of a second$. If the electrons slow down, then the attraction between the atoms/molecules due to dispersion force stays longer $like maybe a hundred-thousandth of a second instead$. If the thermal energy drops, the electrons will move slower. This will allow the dispersion force to remain longer, as the electrons will retain their positions for a longer time.

If the atoms/molecules have more electrons, that will also increase the dispersion force/cause more dispersion forces to form. Additionally, bigger atoms/molecules will cause more dispersion forces because there's more space where the electrons can be, and therefore it will take more time before the electrons face each other and break the induced-dipole bond.

Lastly: if something is easier to polarize, it has a lower boiling point, which means the intermolecular forces are easier. This is because if something is easier to polarize, then it is not only easier for the dispersion forces to form between atoms/molecules, but also easier for them to break apart.

Dipole-Dipole Interactions

As opposed to dispersion forces, which form between non-polar molecules, dipole-dipole forces are a permanent dipole force. This means that they can only form between molecules which already have a significant dipole $i.e. molecules which have a polarity greater than 0.5$. The more polar the molecule is, the greater its dipole is, and thus the greater the intermolecular dipole-dipole forces will be.

In dipole-dipole forces, as in dispersion forces, the oppositely charged ends of molecules attract each other. The only difference is that the ends will retain their charges even after the dipole-dipole bonds have been broken, as the molecules were already polar to begin with. Otherwise they are identical.

Induced-dipole forces and permanent dipole forces occur between different types of molecules. Induced-dipole forces occur between non-polar molecules, and permanent-dipole forces occur between polar molecules. This gives us information about two important physical properties of substances:

1. For molecules of approximately equal mass and size, the more polar molecule will have the higher boiling point, as the dipole-dipole interactions will be stronger.

2. Which of the van der Waals forces is stronger? It depends. If 2 molecules have roughly the same size and shape, dipole-dipole interactions $permanent dipole forces$ will most likely be the stronger intermolecular force. If they have different sizes and shapes, then it will probably be dispersion forces $induced dipole forces$.

Hydrogen Bonding

Hydrogen bonding, just like dipole-dipole interactions, is a permanent dipole force. In fact, it's a special type of dipole-dipole force that's unusually strong. It forms when a Hydrogen atom bonded to a N, O, or F atom is brought near another electronegative atom $more electronegative than Hydrogen$ in another molecule. The Hydrogen atom is attracted to this electronegative atom in the other molecule.

Why is this bond stronger than other dipole-dipole forces? When a Hydrogen atom bonds with a N, O, or F atom $atoms which are highly electronegative$, they pull the shared electrons closer towards themselves, leaving the Hydrogen atom with a slight positive charge. Hydrogen only needs two electrons to fill its outer shell - both of those electrons are being shared between the atoms. Thus, the Hydrogen atom's nucleus is practically bare, with no electrons surrounding it. When another electronegative atom $more electronegative than Hydrogen$ in another molecule interacts with the practically bare nucleus, it creates an unusually strong force of intermolecular attraction. This is known as Hydrogen bonding, and it is illustrated below in the example of H₂O:

In the drawing above, the Oxygen atom is able to pull the shared electrons $circled$ closer to itself than to Hydrogen. This leaves Hydrogen with a slightly positive charge and Oxygen with a slightly negative charge $denoted above$. The other end of Hydrogen, away from the bond, is positively charged now - there are no electrons there.

The Hydrogen, with a slight positive charge, bonds with an electronegative atom $one with a slightly negative charge$ from another molecule. Thus, a dipole-dipole bond is formed. In this case, it is unusually strong because the other electronegative atom $in this case the Oxygen from another H~2~O molecule$ is interacting with a practically bare nucleus.

Ion-Dipole Interactions

Ion-dipole interactions are found in solutions of ionic compounds.

When an ionic compound dissolves, its ions split apart due to ion-dipole interactions. The solvent, usually water, is made of molecules with some polarity, even if the polarity isn't large. Multiple molecules of the solvent attach onto the ionic compound.

The molecules of the solvent have some polarity, which means they have a dipole with a positive and negative end. Several molecules of the solvent surround the ionic compound. The molecules that are nearer to the negatively charged ions re-orient themselves so that their positive ends are facing the negatively charged ions, and pull on them. The molecules that are nearer to the positively charged ions re-orient themselves so that their negative ends are facing the positively charged ions, and pull on them. With so many molecules pulling on each of the ions, the ions split apart, and the ionic compound dissolves.

Ion-dipole interactions are typically the strongest type of intermolecular forces outside of ionic or metallic bonding.

Practice

What kind of intermolecular bonding is strongest between CO₂ molecules? Induced-dipole bonding Permanent-dipole bonding Hydrogen bonding Ion-dipole bonding

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