Properties of Molecular and Ionic Compounds SCH3U1 Winter 2015

Recall the Bonding Continuum… Form Molecular CompoundsForm Ionic Compounds

Molecules Molecules are groups of two or more atoms bonded together by C or PC bonds. The formulas are not simplified: e.g. hydrogen peroxideH 2 O 2 ≠ HO e.g. dinitrogen tetrahydride N 2 H 4 ≠ NH 2

Bonding in Molecular Compounds Molecular compounds have two types of attractions (bonds): Intramolecular bonds: strong bonds within the molecules (covalent/polar-covalent). Intermolecular bonds: weaker bonds between molecules.

Properties of Molecular Compounds  Molecular compounds have LOW boiling points (bp) and melting points (mp) since the weak intermolecular bonds are easily broken.  Non-polar molecules have the lowest bp/mp: N 2 (s) N 2 (l) N 2 (g)  Note the very strong :N ≡ N: triple bond is not broken! mp = -210 o Cbp = -196 o C

 As the bond polarity ( ΔEN) of the covalent bonds increases, the intermolecular bonds can become stronger. This increases the mp and bp: H 2 O (s) H 2 O (l) H 2 O (g) mp = 0 o Cbp = 100 o C Only the weak intermolecular bonds are broken. The very strong covalent bonds remain.

Polarity and Shape  Although the bond between carbon and oxygen is polar covalent (  EN= 0.9), it is actually a non-polar molecule. Compare the shape of carbon dioxide and water:  In carbon dioxide, the linear shape cancels out the effect of each polar bond. NON-POLAR MOLECULE (gas)POLAR MOLECULE (liquid)

Ionic Compounds  These substances only contain ionic bonds.  The chemical formula represents the ratio of cations (+) to anions (-) in the compound. e.g. Na 1 Cl 1 Mg 1 Cl 2 Mg 1 O 1 1:1 1:2 1:1

 In ionic compounds, there are no molecules.  Since each ion can bond to several others, the structure is called a crystal lattice:

 Although ionic bonds are weaker than covalent bonds, the lattice of ionic bonding requires more energy to break apart than weak intermolecular bonds. NaCl (s) NaCl (l) NaCl (g)  This results in very high mp and bp. mp = 801 o C bp = 1413 o C

Comparing Molecular and Ionic Compounds: Summary Covalent Polar Covalent Ionic State at Room Temperature ∆EN Structure Melting Point and Boiling Point gas gas - liquid - solid solid – 1.7> 1.7 Molecules Crystal Lattice Low ModerateVery High Molecular Compounds

Comparing Molecular and Ionic Compounds: Summary Covalent Polar Covalent Ionic Examples (with mp) Solubility in Water Conductivity in Solid State Conductivity in Liquid State Conductivity in Aqueous N 2 (-210 o C) CH 4 (-184 o C) H 2 O (0 o C) C 6 H 12 O 6 (148 o C) NaCl (801 o C) ZnS (1185 o C) insoluble soluble No Yes No Yes No

Homework:  1. Read textbook chapter.  2. Do Q. 13, 14, 15, 16 on “Learning Check” p. 79  3. Do Q. 1 – 6 on back of the handout.  Next: Hydrocarbon Structure and Names (W, T)  Compound Research Assignment  UNIT 2 TEST Wednesday March 25