4 Subjects in Term 2

1. Inter Molecular Forces Chap 13
2. Equilibria - Solution Chap 14,17,18
3. Kinetics - Rates Chap 15
4. Organic Chemistry Notes to Purchase from McMaster

No more handouts for problems and tutorials - use the Web

http://www.chemistry.mcmaster.ca/courses/1aa3

Course notes: (I HOPE)

http://www.chemistry.mcmaster.ca/courses/1aa3/1aa3blue.htm

If you can't find them, go to

McMaster (www.mcmaster.ca), then follow the path to: i) Academic ii) Faculty of Science iii) Chemistry iv) Current Courses v) Chem 1AA3

Tutorials and labs start next week.

Purpose of the course

Intermolecular Forces

Bonds

Covalent 

Ionic Li-Cl 

Metallic Mn-Mn-Mn etc.

These bonds represent very strong forces 100 - 450 kJ/mol

By contrast, intermolecular forces are weak. Always < 100 kJ/mol. Typically 1-25 kJ/mol.

Thus, as one heats up a molecule, one overcomes weaker intermolecular forces first, only at high T, are strong intramolecular forces broken.

Table: Comparison of the Fahrenheit and Celsius temperature scales.

States of Matter Gas Liquid Solid (Polymer)

Gas - no boundary

-essentially no intermolecular forces even in Li⁺Cl^- vapour (there are strong ion-ion intramolecular forces, but weak LiCl - LiCl intermolecular forces)

fast molecular movement

rotate (ii) translate (iii) vibrate

Liquids - defined boundary

- molecules close together

-rapid molecular movement

(i) rotate (ii) translate (iii) vibrate

- weak intermolecular forces

Solids - defined boundary & shape

- ordered

(i) sometime rotate (ii) vibrate

- NO translation

- forces depend on crystal type

Ionic solid Very strong intermolecular forces

(may actually be considered intramolecular forces)

Molecular solid Very weak intermolecular forces

Forces between Substances

a) ion - ion

b) dipole - dipole

c) induced dipole - induced dipole

(London forces)

d) H bonding

a) Ion- Ion

F q²/r² (charge / distance)

Ionic Solids A 3 D network of positive (+) and negative (-) charges

close together high mp

Ionic Liquids conduct electricity close together, very high bp

Ionic Gases separation of ion pairs, NaCl molecules are far apart from each other

Dipole - Dipole

Polar Molecules

= (d+) (d-) / r

In a liquid

c) Induced Dipole - Induced Dipole (London) Dispersion Forces

i) (otherwise gases wouldn't ever condense)

ii) consider attraction of electrons of 1 atom to nucleus of another. These forces are additive: more points of contact, more attraction

CH₃CH₂CH₂CH₂CH₃   CH₃CH(CH₃)CH₂CH₃        CH₃C(CH₃)₂CH₃

b.p. 36 ^oC                           b.p. 28 ^oC                           b.p. 9^o C

CH₄ mp - 183^oC               C₂₀H₄₂ 80 ^oC             C₁₀₀H₂₀₂ 135 ^oC

If molecules far apart, little interaction

if molecules close , attraction > repulsion

if molecules too close, repulsion >> attraction

Factors affecting the magnitude of induced dipole interactions

Polarizability

As ¯ Periodic table size ­ polarizability ­

Why?

Outer electrons shielded from nucleus more affected by external stimuli

i.e., big, soft blobby I- is much more polarizable than hard small F-

d) H- Bonding

- because H so small can approach close to atoms, so small migration easy

- stronger than London or Dipole forces (see Fig. 13.9 p. 605)

Magnitude of Forces London ~1 - 5 kJ/mol

Dipole ~1 - 10 kJ/mol

H-bond ~5 - 25 kJ/mol

NB: Polymers have glass transition

Demo: Latex rubber

Liquid N₂ - Bunsen Burner

NaCl + Bunsen burner

Ice

CO₂ - Balloon

Freezing / Melting - Break up intermolecular forces.

Cost is proportional to the Strength of the intermolecular force

	Hfus kJ/mol	m.p. C	Hv kJ/mol	b.p. C
H2	6.12	-259
CH4	6.94	-183	10.4	164
Hg	2.3	-40
Ccl4	2.51	-23	30	76.7
EtOH	5.01	-115
H2O	6.01	0	40.7	100
NaCl	27.2	801

NB: These effects aren't linear with molecular weight - why not? - other forces!

Evaporation / Condensation

Molar enthalpy of vaporization , Hv

Escape velocity higher, vaporization Hv lower

Molecules in liquids have energy distribution

Demo: pail test tubes and corks

Rubber duck

Tennis ball

What happens with a liquid in:

1. a closed container
2. at a given T
3. over a period of time water vapour exerts a pressure

= as Temperature increases, so does the vapour pressure

The effect of elevation on the boiling point of water

Solids do not have large vapour pressure

CO₂(s) H₂O @ 0 ^oC

very large Pv = 4.6 mm

(dry ice unusual) i.e. Frozen underwear sublimes

Some examples

1. bp of EtOH at 600 mm Hg from graph? (answer, look at graph p. 616, Pv ~ 74 ^oC)

2. If 1L H₂O in 10L sealed flask at 60 C (Pv from graph of @ 60^o = 180 mm Hg). How much H₂O will evaporate?

PV = nRT

180 mmHg x 9L = n R 60 ^oC

(180mm Hg) / (760 mm Hg / atm) x 9L = n 0.082 L atm x 333 K / K mol

n = 0.078 moles of water will fill space in the 10 L vessel (1L of which is air). You can figure out how many grams that is.

Solids

Liq. N₂

Molecular Arrangement in Solids

Network solid (covalent network crystal)

e.g.

Molecular solid

ABA = Molecule

e.g. CO₂ A=O, B=C

Combination - Graphite

Diamond Eq 2 D network solid

In D molecular solid

Crystals

3 common types

i) atoms at vertices, Simple Cubic

=8 apices x 1/8 atom

= 1 atom/unit cell

74% space consumed
 

ii) Body - centered cubic

(Fe, Na, Ba)

- 1 atom each apex

+ 1 in centre

= 1 atom centre

+ 8 x 1/8 (= 1)

Total 2 atoms /unit cells

68% space consumed

iii) Face centered Cubic

(Cu, Al, Ca, Ne, K)

1 atom each apex (o) and 1 atom at each face (o)

6 faces x ½ atom ( = 3 atoms)

+ 8 x ½ ( = 1 atom)

= 4 atoms/unit cell

Stuff you can determine

eg. Bragg Reflection

- can back calculate A. Using cell dimension, can calculate metal density

e.g. Density of Vanadium

- crystal body-centred cubic

- edge length 305 pm

- density = g/cm³

- volume of unit cell = Edge³ = 305 pm³

Mass (body centered cubic 2 atoms)

Density = Mass / volume

Mass = 2 atoms (50.94 g / mol) x 1 / 6.02 x 10²³ atoms/mol = 1.692 x 10^-22 g

Volume = (305 pm (1 cm / 10¹⁰ pm))³

Density = 5.96 g / cm³

If you know density and unit cell can calculate Avagadro's number (see tutorial)

What about non-crystalline materials, amorphous materials don't have a narrow melting point

e.g. glass