09. Forces between molecules - VDW forces

What are the 2 types of intermolecular forces?

VDW and hydrogen bonding

What is the difference between intramolecular and intermolecular forces?

Intramolecular -> Affect chemical properties of molecules (cause molecule to react in certain way) Intermolecular -> Affect physical properties of molecules (affect state molecule exists in)

What are the 2 types of VDW?

Forces between molecules having induced and permanent dipoles

Give an example of an induced dipole?

He-He

Why does the helium atom in He-He have no permanent dipole?

Because helium atoms have same electronegativity value

Induced dipoles: What happen in all molecules?

Natural fluctuations of electron density as electrons are always moving

Induced dipoles: What can arise from natural fluctuations?

Temporary or instantaneous dipoles can arise

Draw a diagram showing helium atoms with temporary dipole between them

Yes

Induced dipoles: Explain the electrons in the covalent bond that link the atoms?

Electrons in the covalent bond that link the atoms move more to one end of the molecule than the other

Induced dipoles: Describe the electron cloud

The electron cloud isn’t always evenly distributed and negative and positive centres of charge do not coincide

Induced dipoles: What happens as a result of the temporary dipole?

One end carries small positive charge and other end small negative charge

Induced dipoles: describe the fact that one side of molecule has small negative charge whilst other has small positive charge

This small charge separation is not permanent -> Charges can switch ends

Induced dipoles: What happens when molecules come close together?

Temporary dipoles induce dipoles in adjacent molecules. Continuous movement of electrons in molecules leads to breaking of these dipoles but elsewhere in the molecule other instantaneous dipoles are formed

What factors affect strength of VDW?

Size of molecule (Mr) and SA (shape)

Why is the size of molecule a factor?

Large molecules have more electrons so more VDW

Why is surface area a factor?

Long thin molecules have larger SA so stronger forces

Give an example of a permanent dipole?

H-Cl

Permanent dipoles: What happens due to the small delta positive and negative charges on H-Cl

They attract oppositely charged dipoles on other polar molecules

Permanent dipoles: What attract oppositely charged dipoles on other polar molecules?

Small delta negative and delta positive charges on a polar molecule

Permanent dipoles: Draw a diagram as to how HCl molecules interact?

Yes