Properties of Molecular and Ionic Compounds

SCH3U1

Winter 2015

Recall the Bonding Continuum…

EN = EN1 – EN2

COVALENT POLAR IONIC COVALENT

0.0 0.5 1.7 3.3

Form Molecular CompoundsForm Ionic Compounds

POLA

R

NON-P

OLA

R

Molecules

Molecules are groups of two or more atoms bonded together by C or PC bonds. The formulas are not simplified:

e.g. hydrogen peroxide H2O2 ≠ HO

e.g. dinitrogen tetrahydride N2H4 ≠ NH2

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:

N2 (s) N2 (l) N2 (g)

Note the very strong :N≡N: triple bond is not broken!

mp = -210 oC bp = -196 oC

As the bond polarity (ΔEN) of the covalent bonds increases, the intermolecular bonds can become stronger. This increases the mp and bp:

H2O (s) H2O (l) H2O

(g)

mp = 0 oC bp = 100 oC

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. Na1Cl1 Mg1Cl2 Mg1O1

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 oC bp = 1413 oC

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 Molecules Crystal Lattice

Low Moderate Very 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

N2 (-210oC)CH4 (-184oC)

H2O (0 oC)C6H12O6(148o

C)

NaCl (801oC)

ZnS (1185oC)insoluble soluble soluble

No No Yes

No No Yes

No No 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