Electrochemistry

1
Electrochemistry
Chapter 19
2
Electron Transfer Reactions

• Electron transfer reactions are
  oxidation-reduction or redox reactions.
• Results in the generation of an electric current
  (electricity) or be caused by imposing an
  electric current.
• Therefore, this field of chemistry is often
  called ELECTROCHEMISTRY.

3

• Electrochemical processes are oxidation-reduction
  reactions in which
• the energy released by a spontaneous reaction is
  converted to electricity or
• electrical energy is used to cause a
  nonspontaneous reaction to occur

0
0
2
2-
Oxidation half-reaction (lose e-)
Reduction half-reaction (gain e-)
19.1
4
Terminology for Redox Reactions

• OXIDATIONloss of electron(s) by a species
  increase in oxidation number increase in oxygen.
• REDUCTIONgain of electron(s) decrease in
  oxidation number decrease in oxygen increase in
  hydrogen.
• OXIDIZING AGENTelectron acceptor species is
  reduced. (an agent facilitates something ex.
  Travel agents dont travel, they facilitate
  travel)
• REDUCING AGENTelectron donor species is
  oxidized.

5
You cant have one without the other!

• Reduction (gaining electrons) cant happen
  without an oxidation to provide the electrons.
• You cant have 2 oxidations or 2 reductions in
  the same equation. Reduction has to occur at the
  cost of oxidation

LEO the lion says GER!
ose
lectrons
xidation
ain
lectrons
eduction
GER!
6
Another way to remember

• OIL RIG

s
s
xidation
ose
eduction
ain
7
Review of Oxidation numbers
The charge the atom would have in a molecule (or
an ionic compound) if electrons were completely
transferred.

1. Free elements (uncombined state) have an
   oxidation number of zero.

Na, Be, K, Pb, H2, O2, P4 0

1. In monatomic ions, the oxidation number is equal
   to the charge on the ion.

Li, Li 1 Fe3, Fe 3 O2-, O -2

1. The oxidation number of oxygen is usually 2. In
   H2O2 and O22- it is 1.

4.4
8

1. The oxidation number of hydrogen is 1 except
   when it is bonded to metals in binary compounds.
   In these cases, its oxidation number is 1.

1. Group IA metals are 1, IIA metals are 2 and
   fluorine is always 1.

6. The sum of the oxidation numbers of all the
atoms in a molecule or ion is equal to the charge
on the molecule or ion.
HCO3-
O -2
H 1
3x(-2) 1 ? -1
C 4
4.4
9
Balancing Redox Equations
The oxidation of Fe2 to Fe3 by Cr2O72- in acid
solution?

1. Write the unbalanced equation for the reaction
   ion ionic form.

1. Separate the equation into two half-reactions.

Oxidation
Reduction

1. Balance the atoms other than O and H in each
   half-reaction.

19.1
10
Balancing Redox Equations

1. For reactions in acid, add H2O to balance O atoms
   and H to balance H atoms.

1. Add electrons to one side of each half-reaction
   to balance the charges on the half-reaction.

1. If necessary, equalize the number of electrons in
   the two half-reactions by multiplying the
   half-reactions by appropriate coefficients.

19.1
11
Balancing Redox Equations

1. Add the two half-reactions together and balance
   the final equation by inspection. The number of
   electrons on both sides must cancel. You should
   also cancel like species.

Oxidation
Reduction

1. Verify that the number of atoms and the charges
   are balanced.

14x1 2 6x2 24 6x3 2x3

1. For reactions in basic solutions, add OH- to both
   sides of the equation for every H that appears
   in the final equation. You should combine H and
   OH- to make H2O.

19.1
12
CHEMICAL CHANGE ---gtELECTRIC CURRENT

• To obtain a useful current, we separate the
  oxidizing and reducing agents so that electron
  transfer occurs thru an external wire.

This is accomplished in a GALVANIC or VOLTAIC
cell. A group of such cells is called a battery.
http//www.mhhe.com/physsci/chemistry/essentialche
mistry/flash/galvan5.swf
13
Galvanic Cells
anode oxidation
cathode reduction
-

spontaneous redox reaction
19.2
14
Galvanic Cells

• The difference in electrical potential between
  the anode and cathode is called
• cell voltage
• electromotive force (emf)
• cell potential

Cell Diagram
Cu2 1 M Zn2 1 M
Zn (s) Zn2 (1 M) Cu2 (1 M) Cu (s)
anode
cathode
19.2
15
Standard Electrode Potentials
Zn (s) Zn2 (1 M) H (1 M) H2 (1 atm) Pt
(s)
Anode (oxidation)
Cathode (reduction)
19.3
16
Standard Electrode Potentials
Standard reduction potential (E0) is the voltage
associated with a reduction reaction at an
electrode when all solutes are 1 M and all gases
are at 1 atm.
Reduction Reaction
E0 0 V
Standard hydrogen electrode (SHE)
19.3
17

• E0 is for the reaction as written
• The more positive E0 the greater the tendency for
  the substance to be reduced
• The half-cell reactions are reversible
• The sign of E0 changes when the reaction is
  reversed
• Changing the stoichiometric coefficients of a
  half-cell reaction does not change the value of E0

19.3
18
Standard Electrode Potentials
If the reaction is backwards, be sure to flip the
sign!
Zn (s) Zn2 (1 M) H (1 M) H2 (1 atm) Pt
(s)
2
2
So Eo Zn/Zn 0.76 V
2
19.3
19
Standard Electrode Potentials
0
Ecell ECu /Cu E H /H
0
0
2
2
Pt (s) H2 (1 atm) H (1 M) Cu2 (1 M) Cu
(s)
Anode (oxidation)
Cathode (reduction)
19.3
20
What is the standard emf of an electrochemical
cell made of a Cd electrode in a 1.0 M Cd(NO3)2
solution and a Cr electrode in a 1.0 M Cr(NO3)3
solution?
Cd is the stronger oxidizer Cd will oxidize Cr
x 2
Anode (oxidation)
Cathode (reduction)
x 3
19.3
21
Spontaneity of Redox Reactions
DG -nFEcell
n number of moles of electrons in reaction
96,500 C/mol
DG0 -RT ln K
19.4
22
Spontaneity of Redox Reactions
19.4
23
Oxidation
n 2
Reduction
0
0
2

K 1.23 x 10-42
19.4
24
The Effect of Concentration on Cell Emf
DG DG0 RT ln Q
DG -nFE
-nFE -nFE0 RT ln Q
Nernst equation
At 298
19.5
25
Oxidation
n 2
Reduction
E0 EFe /Fe ECd /Cd
0
0
2
2
E 0.013
E gt 0
Spontaneous
19.5
26
Charging a Battery
When you charge a battery, you are forcing the
electrons backwards (from the to the -). To do
this, you will need a higher voltage backwards
than forwards. This is why the ammeter in your
car often goes slightly higher while your battery
is charging, and then returns to normal.
In your car, the battery charger is called an
alternator. If you have a dead battery, it could
be the battery needs to be replaced OR the
alternator is not charging the battery properly.
27
Batteries
Dry cell
Leclanché cell
Anode
Cathode
19.6
28
Batteries
Mercury Battery
Anode
Cathode
19.6
29
Batteries
Lead storage battery
Anode
Cathode
19.6
30
Batteries
Solid State Lithium Battery
19.6
31
Batteries
A fuel cell is an electrochemical cell that
requires a continuous supply of reactants to keep
functioning
Anode
Cathode
19.6
32
Corrosion
19.7
33
Cathodic Protection of an Iron Storage Tank
19.7
34
Electrolysis is the process in which electrical
energy is used to cause a nonspontaneous chemical
reaction to occur.
19.8
35
Electrolysis of Water
19.8
36
Chemistry In Action Dental Filling Discomfort
37
Electrolysis and Mass Changes
charge (Coulombs) current (Amperes) x time (sec)
1 mole e- 96,500 C 1 Faraday
19.8
38
Anode
Cathode
0.0126 mol Ca
0.50 g Ca
19.8