Why do some solids dissolve in water but others do not? Why are some substances gases at room temperature, but others are liquid or solid? The answers have to do with … Intermolecular forces Questions

There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces Intramolecular bonds are BONDS - ionic, polar covalent or non-polar covalent Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule) Intermolecular forces

Intermolecular vs Intramolecular 16 kJ to vaporize 1 mole of HCl (inter) 431 kJ to break H-Cl bonds in 1 mole of HCl (intra)

Electronegativity & IMFs  EN essentially defines the type of Bonding and IMF. Ionic bonds form if the  EN is 1.7 or greater and contains a metal or positive ion. eg.  EN (NaCl) = 3.0 – 0.9 = 2.1 Covalent - there are 3 Van Der Waals or Intermolecular Forces (IMF). They depend on the  EN and symmetry of the molecule.

Ionic Bonding Ionic bonds form as previously described. The force is an intraparticle 3D force. They have very high melting and boiling points and high solubility. – + + – eg. mp(LiCl) is 605°C and solubility is 84 g/100 g

lon – Dipole Attractions Ionic compounds are generally water soluble due to the formation of attractions between the dipole of the water molecules and the charges of the ions.

London Dispersion Forces exist in all molecules, (and noble gases), and in non-polar covalent molecules (where  EN is 0) or they are symmetrical. -weakest of the 3 IMFs. -strength depends on the # of electrons and how close the molecular packing is. -lowest melting, boiling points and water solubility. eg. mp (Cl 2 is -161°C and solubility is 0.7g/100 g)

Dipole - Dipole attractions Molecules are polar if any 1 of the bonds has a  EN > 0 and if it is non-symmetrical in shape  EN (HCl) = 3.0 – 2.1 = 0.9 Much stronger than London Dispersion Forces. Higher melting and boiling points and more soluble than non-polar molecules. HCl ++ –– ++ –– ++ –– ++ –– ++ –– eg. mp(HCl) is -114°C and solubility is 66 g/100 mL

Hydrogen Bonding Hydrogen bonding is a type of dipole-dipole. - The H is covalently bonded to N, O or F in the compound. -this forms a H Bonding IMF with ANOTHER molecule. -strongest IMF, about 5 times stronger than D-D -melting, boiling and water solubility is higher than to D-D. -eg. CH 3 OH mp is -97 ° C and solubility is complete. eg.  EN (OH) = =1.4  polar bond non-symmetrical  polar molecule