Intermolecular Forces

Question 1

– Holds ionic compounds together
– These are very strong
– Ionic compounds have high m.p.
– Example: NaCl (m.p. 801°C)

Answer 1

Ion-Ion

Question 2

– Permanent dipole moments
– These forces hold most organic compounds together
– These are forces between polar compounds
– Weak force, easy to break
– Example: acetone, CH3COCH3 (b.p. 56°C)

Answer 2

Dipole-Dipole

Question 3

– Very strong dipole-dipole interaction
– Occurs between O, N, or F (strong electronegative atoms) and a hydrogen attached to a O, N, or F
– Weaker than ordinary covalent bonds but much stronger than dipole-dipole forces
– hold DNA together
– Takes more energy to break

Answer 3

Hydrogen bonds

Question 4

– Induced dipole or van der Waals or London forces
– Temporary dipole results from the electrons in atoms moving within the atom
– Molecules have an induced (+) & (–) end
– Attractive force between nonpolar molecules

Answer 4

Dispersion/London/Vanderwaals forces

Question 5

Rank the IM forces from strongest to weakest:

Answer 5

1. Ion-ion
2. Ion-dipole
3. H-bonds
4. Dipole-dipole
5. Dispersion London vanderwaals forces

Question 6

What two factors determine the magnitude of dispersion forces?

Answer 6

1. Polarizability of electrons – how easily the electrons
   respond to a changing electric field
   • The larger the atom, the farther away its electrons are from the nucleus, the more polarizability (Cl < Br < I)
   • Atoms with unshared electrons are easily polarized compared to atoms with all bonding electrons
2. Surface area – the larger the surface area, the larger the overall attraction between molecules