1. Properties of Molecular and Ionic Compounds SCH3U1 Winter 2015

2. Recall the Bonding Continuum… Form Molecular CompoundsForm Ionic Compounds

3. 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

4. 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.

5. 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

6.  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.

7. 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)

8. 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

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

10.  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

11. 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 0 - 0.5 0.5 – 1.7> 1.7 Molecules Crystal Lattice Low ModerateVery High Molecular Compounds

12. 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

13. 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