Intermolecular forces

What is an intramolecular force?

• It is the weak force which holds together molecules

What are the 3 types of intermolecular forces?

• London forces

• dipole - dipole forces

• Hydrogen bonds

What are London forces?

• It is when a temporary dipole induces a temporary dipole in a neighbouring which causes a weak attraction force between induced molecules

Why do all molecules have London forces?

• Because they arise from the random movement of electrons in orbitals

What is the difference in the London forces between S8 and Cl2

• S8 has more electrons and therefore there are more random movements of electrons in orbitals which cause the formation of more London forces which cause them to be stronger than CL2

What happens to the BP and MP of atoms and molecules as you increase the Mr and why?

• The MP and BP will increase as you increase the Mr because there are more electrons in the molecules and atoms which cause stronger London forces to form which will require more energy to break

What are te 2 types of dipole - dipole forces?

• Permanent - Permanent dipole interactions

• Permanent - induced dipole interaction

What are permanent dipole interactions?

• They are forces which are formed when 2 permanent dipoles interact and form a weak intermolecular force between the slightly positive and slightly negative part on the 2 dipoles

What is a permanent - induced dipole interaction?

• It is when a permanent dipole comes in contact with a non-polar molecule with dipole, which causes it to become induced and be charged which causes an weak intermolecular force between the molecules

Why might HCl have a higher BP than HI, but why might the opposite also be true?

HCL:

• The difference in electronegativity value of H and Cl is larger than H and I which causes a more pronounced dipole causing the molecule to be more polar which causes stronger permanent dipole-dipole interaction which will require more energy to break than the ones in HI

HI:

• Iodine has more electrons than Chlorine and therefore will form more stronger London forces which will require more energy to break than the London forces the HCL

What is a hydrogen bond?

• it is a permanent dipole permanent dipole interaction which forms between N, O and F and Hydrogen only.(H must be present)

How do they form?

• hydrogens that are bonded to the most electronegative elements: F,N, or O, which interact with a lone pair on another F,N or O

What are 2 anomalous properties which Water has

• High BP

• water being more dense than ice

What is the structure of ice?

• An open lattice which is a regular structure that has a lot space which causes its density to decrease from liquid water

What elements always cause the formation of intermolecular hydrogen bonds

• Compounds containing -N-H bond

What is the order of strength of all 3 intermolecular forces?

1. London forces

2. dipole-dipole forces

3. Hydrogen bonds

What are the 2 exceptions for when London forces are stronger than hydrogen bonds?

• Br2, any molecules with a lot of bromine atoms

• I2, any molecules with a lot of iodine atoms