Forces in Solids and Liquids

Types Strong forces – ionic bonds between ions

(metal and non-metal atoms), covalent bonds (C and Si structures), metallic bonds (metal atoms only)

Weak forces – between MOLECULES (non-metal atoms only)

The nature of intermolecular forces Intermolecular forces are the forces that

hold MOLECULES together in a MOLECULAR SOLID or LIQUID.

The forces are WEAK in comparison to covalent, metallic and ionic bonds – this explains why molecular solids have LOW melting and boiling points

The three types Hydrogen bondingOccurs only between molecules that contain a F, O

or N bonded to a H

Permanent dipole-permanent dipoleOccurs between molecules which are polar

Temporary dipole-temporary dipoleOccurs between all molecules. Is significant for

non-polar molecules and large molecules

Questions:For each compound state the type of force holding the

particles together. If molecular, draw its structure and state the most significant type of WIF

Cobalt Methanol Tetrafluoromethane

Hydrogen sulfide Magnesium fluoride Phosphine (PH3)

Dichloromethane Hydrogen peroxide Hydrogen chloride

Nickel Graphite Ammonia

Helium Carbon dioxide Copper sulfate

Answers:Cobalt

Metallic bond

Methanol

Hydrogen bonding

Tetrafluoromethane

Temporary dipole

Hydrogen sulfide

Permanent dipole

Magnesium fluoride

Ionic bond

Phosphine (PH3)

Permanent dipole

Dichloromethane

Permanent dipole

Hydrogen peroxide

Hydrogen bonding

Hydrogen chloride

Permanent dipole

Nickel

Metallic bond

Graphite

Covalent bonds

Ammonia

Hydrogen bonding

Helium

Temporary dipole

Carbon dioxide

Temporary dipole

Copper sulfate

Ionic bond

Question:

Bromine melts at -7°C, and sodium bromide melts at 755°C. Explain why there is a big difference in their melting points.

Answer:Bromine is a molecular solid with molecules composed of

2 covalently bonded bromine atoms. The molecules are held together by temporary dipoles as Br2 is a non-polar molecule. It is non-polar because the both Br atoms have identical electronegativity. Temporary dipoles are relatively weak, therefore little energy is required to break them and the melting point is low.

NaBr is an ionic solid composed of Na+ and Br- ions in a 3D lattice. The ionic bonds holding the ions together are strong, requiring a lot of energy to break and therefore the melting point is high.

Question: (complete and hand in for marking)

Rank the following compounds from highest to lowest melting point. Explain your reasoning:

Water diamond carbon dioxide sulfur dioxide sulfur trioxide iron

Answer:Highest to lowest:Diamond - strong covalent bonds between C atomsIron – strong metallic bonds between Fe atomsWater – WIF – hydrogen bonds between H2O molecules

(contains O bonded to H)Sulfur dioxide – WIF – permanent dipoles between SO2

molecules (polar)Sulfur trioxide – WIF – temporary dipoles between SO3

molecules (non-polar)Carbon dioxide - WIF – temporary dipoles between CO2

molecules (non-polar and fewer electrons than SO3)