Chemistry 1030 - Intermolecular Forces

What are Van Der Waals forces?

Van Der Waals forces are when Nonpolar molecules stick together due to magnetic attractions.

Intermolecular forces are generally much ______ than covalent bonds

Weaker

What are dipole-dipole forces?

When two molecules that behave like little magnets that stick to each other.

Does a lone pair of electrons make the molecule polar?

yes

How to tell the type of intermolecular force in a molecule?

1. Draw the lewis structure for the molecule. 2. Examine the lewis structure and see if there's a hydrogen atom stuck DIRECTLY to a fluorine, nitrogen, or oxygen atom. If it is, the molecule experiences *hydrogen bonding*. An important note, however, is that there may be molecules that contain F,N, or O as well as hydrogen that AREN'T involved in hydrogen bonding. An example would be *HCN*, which has a Lewis Structure that looks like H-C:::N: Remember, hydrogen bonding only takes place when the H is stuck DIRECTLY to F, N, or O. 3. If the molecules doesn't have a H-N, H-F, or H-O bond, you need to keep looking. The next question: Is the molecule polar? If it is, the molecule experiences dipole-dipole forces, and if it isn't, it experience Van Der Waals forces.

What intermolecular forces are present in: *ammonia (NH₃)*

Ammonia is a polar molecule. Dipole-Dipole and London Dispersion forces, and hydrogen bonding.

What *intermolecular forces* are present between the following pair of molecules: *CH₃F and CH₄*

CH₄ has London dispersion forces

What *intermolecular forces* are present between the following pair of molecules: *Cl⁻ and H₂O*

Cl⁻ is and H₂O is hydrogen bonding.

What intermolecular forces are present in: *dichloromethane (CH₂Cl₂)*

Dipole-Dipole interactions and dispersion forces.

What are the three types of *intermolecular* forces?

Dipole-Dipole, Hydrogen Bonds, and Van Der Waals forces.

Why do CH₃OH molecules from hydrogen bonds, but CH₄ molecules do not?

Hydrogen Bonding only occurs when a Hydrogen is stuck directly to fluorine, oxygen, or nitrogen. In CH₃OH, there is an oxygen atom. Whereas in CH₄ there is not, which is why it isn't a hydrogen bond.

What are Hydrogen Bonds?

Hydrogen bonds also take place in polar molecules. However, the molecules that undergo hydrogen bonding all contain a hydrogen atom that's bonded to a nitrogen atom, oxygen atom, or fluorine atom.

How are hydrogen bonds and dipole-dipole forces different?

In the Hydrogen-Oxygen bond (or H-N or H-F bonds), the hydrogen is LESS electronegative than the other atom, causing the electrons in the bond to be pulled away from it. However, in a hydrogen bond, you'll notice that the atoms that hydrogen is bonded to each have *lone pairs*. Because the oxygen, fluorine, and nitrogen atoms all have these lone pairs, the lone pairs tend to be attracted toward the partial-positively charges hydrogen atoms on nearby molecules.

Differences between Intramolecular and Intermolecular Forces

Intramolecular forces are the forces that keep the atoms in a compound stuck to each other - in other words, they're just chemical bonds. Intermolecular forces, on the other hand, are the forces that hold two covalent molecules to one another.

What intermolecular forces are present in: *n-Hexane (CH₃CH₂CH₂CH₂CH₂CH₃)*

It is a non-polar molecule. The only bond is C-H which is non-polar due to Carbon and Hydrogen having very similar electronegativities. Secondly, hexane is symmetric so any polarity in the molecule would cancel out. Therefore, the only intermolecular force acting in Hexane are induced *dipole-dipole forces* or *Van Der Waals forces/London Dispersion Forces*

What *intermolecular forces* are present between the following pair of molecules: *H₂ and CO₂*

They both have London Dispersion Forces.