m2 - intermolecular forces

Question 1

intermolecular forces names

Answer 1

permanent dipole-dipole interactions (eg H δ+ - Cl δ-)
induced dipole-dipole interactions (London forces)
hydrogen bonding

Question 2

intermolecular forces definition

Answer 2

an attractive force between neighbouring molecules

Question 3

don’t quote these numbers, but relative strength of bond types

Answer 3

ionic and covalent 1000
hydrogen bonds 50
dipole-dipole forces 10
london forces 1

Question 4

permanent dipole-dipole interactions

Answer 4

a permanent dipole-dipole force is a weak attractive force between permanent dipoles in neighbouring polar molecules
(H δ+ - Cl δ - - - - - - H δ+ - Cl δ-) (IMF is the Cl attracted to next H)

Question 5

induced dipole-dipole interactions

Answer 5

attractive forces between induced dipoles in neighbouring molecules
(a form of dipole-dipole interaction)
- transient dipoles present in atoms because the electron cloud is in constant motion
- (δ- δ+) (no dipole) -> original temporary dipole will induce the other molecule as the + end of the left molecule will attract electrons from the right molecule, so it becomes (δ- δ+) - - - - (δ- δ+). the + end of one attracts the - end of the other

Question 6

london forces

Answer 6

• caused by constant random movement of electrons in atoms shells. this movement unbalances the distribution of charge within the shells
• at any moment, there will be an instantaneous dipole across the molecules
• this induces a dipole in neighbouring molecules and so on
• the small induced dipoles attract one another causing weak IMF (london forces)
• more electrons = more fluctuations in electron cloud possible…so greater london forces

Question 7

hydrogen bonding is

Answer 7

a strong dipole-dipole interaction between
- an e- deficient hydrogen atom (O-Hδ+) (N-Hδ+) (F-Hδ+) on one molecule
- a lone pair of e- on a highly electronegative atom (H-Oδ-) (H-Nδ-) on a different molecule

H^δ+ — O^….δ- - - - - - - - H^δ+ — O^….δ- (…. is 2 lone pairs)

Question 8

hydrogen bonding effects on water properties

Answer 8

due to strong hydrogen bonds:
ice is less dense than water (ice has open lattice w/ hydrogen bonds holding three water molecules APART. when melts the rigid hydrogen bonds collapse so H2O molecules can move closer together (denser)

hydrogen bonds are much stronger than other IMF, so the extra strength has to be overcome to melt/boil H2O, therefore has higher melting and boiling points