Review Lecture 2

1
Review Lecture 2
2
This week

• Lectures Monday, Wednesday
• Two Tutorials on Friday (on demand)

3
Review Lecture 2

• Electrochemistry
• Solutions
• Acids and Bases

4
Electrolysis

• Eocell lt 0
• must apply EltEocell
• What gets reduced or oxidized?
• Remember that H2O is present!
• Choose half reactions with the HIGHER reduction
  potentials

5
for example,

• electrolysis of CuCl2(aq)
• oxidizable Cl-(aq) and H2O(l)
• 2 Cl-(aq) ? Cl2(g) 2 e- E
  -1.36 V
• 2 H2O(l) ? O2(g) 4 H(aq) 4 e- E -1.23 V

6
for example,

• electrolysis of CuCl2(aq)
• reducible Cu2(aq) and H2O(l)
• Cu2(aq) 2 e- ? Cu(s) E 0.34 V
• 2 H2O(l) 2 e- ? H2(g) 2 OH-(aq) E -0.83 V

7
Quantitative Electrochemistry

• charge required q nF
• charge current x time
• (q it)
• (C A x s)
• nF it

moles electrons required
8
Batteries, etc.

• Primary, Secondary

rechargeable i.e. reversible reaction
need to apply E gt Ecell in the opposite
direction
9
Batteries, etc.

• Fuel Cells
• usual overall reaction
• 2 H2(g) O2(g) ? 2 H2O(g)
• is actually
• 2 H2(g) ? 4 H 4 e-
• O2(g) 4 H 4 e- ? 2 H2O(g)

10
ElectroAnalytical Chemistry
Pt(s) H2(g, 1 atm)
H(aq)
H(aq)
Pt(s) H2(g, 1 atm)
unknown concentration
1 M
0.0592
log10(Q)
but, Ecell Eo -
n
Q
H(aq, unknown)2 / 12
11
Corrosion

• unwanted oxidation of metals
• - generally requires H2O, O2
• - oxidation and reduction may
• occur in separate locations
• Prevent by
• - excluding H2O and O2 use of sacrificial anode
  electric potential

12
Solutions
solute solvent ? solution
DHsoln lt 0
DHsoln gt 0
strong solute-solvent interactions or weak
solute-solute interactions
weak solute-solvent interactions or strong
solute-solute interactions
13
Intermolecular Interactions

• ion-ion
• ion-dipole
• ion-induced dipole
• dipole - dipole
• (including H-bond)
• dipole - induced dipole
• dispersion (induced-induced)

weaker
14
Solubility

• for a solid solute
• solubility µ T
• for a gaseous solute
• solubilityi KH pi

Henrys law constant
15
Colligative Properties

• Adding a solute
• lowers the vapor pressure of the solvent
  (Raoults Law, psolv ptotXsolv)
• Tm is lowered (by Kfm)
• Tb is raised (by Kbm)
• Solution exerts osmotic pressure,
• P MRT

16
Acids and Bases

• Arrhenius acids donate H
• bases donate OH-
• Bronsted, Lowry acids donate H
• bases accept H
• Lewis acids accept e-, bases donate e-

17

• Strength degree of dissociation
• Strong acid Weak conjugate base
• (e.g. HNO3 NO3-)
• pH -log10H3O(aq)

18
Acids and Bases

• Ka Kw / Kb

Kb of conjugate base of a
10-14
19
Acid Strength

• determined by two things
• 1. H-A bond strength
• 2. Electronegativity of terminal atoms

20
Acids and Bases

• How to calculate pH of weak acids
• Use the tabular method!
• USUALLY, only the first dissociation is
  significant for calculating the pH

21
for example,

• H2SO3 H2O ¾ H3O HSO3-
• Ka1 .015
• HSO3- H2O ¾ H3O SO3-2
• Ka2 6.3 x 10-8

H3O and HSO3- from 1st dissociation
SO3-2 from 2nd
22
Buffered Solutions

• Use the Henderson Hasselbalch equation

23
Salt Solutions

• Salt Water ? Solution

pH may not be neutral
review the rules make sure you understand the
concept of conjugate acids and bases