SAHAR ADHAM
LECTUERE OF PHYSICAL CHEMISTRY

galvanic electrolytic produces electrical current anode (-) cathode (+) two electrodes conductive medium salt bridge vessel
E ° cell
> 0.
need power source anode (+) cathode (-)
E ° cell
< 0.

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.

YOU CAN’T HAVE ONE… WITHOUT THE OTHER!
 oxidation: loss of electrons
 reduction: gain of electrons
LEO the lion says GER !
GER!

WHY STUDY ELECTROCHEMISTRY?
 Batteries
 Corrosion
 Industrial production of chemicals such as Cl
2 and Al
, NaOH, F
2
 Biological redox reactions
The heme group

GALVANIC CELLS



One ½ cell rxn. occurs in each compartment.
Zn  Zn 2+ + 2e – in the anode.
Cu 2+ + 2e –  Cu in cathode.

But not without a connection.
Zn
SO
4
2 –
Zn 2+
SO
4
2 –
Cu 2+
Cu
Zn + Cu 2+

Zn 2+ + Cu

ION (“SALT”) BRIDGE

But even with a connection of the electrodes, no current flows.

We need to allow neutrality in the solutions with a salt bridge to shift counterions.
Zn
2e
–
SO
4
2 –
Zn 2+
SO
4
2 –
Cu 2+
2e
–
Cu
Zn + Cu 2+

Zn 2+ + Cu

STANDARD REDUCTION POTENTIALS, E °

The voltage generated by the Zn/Cu galvanic cell is
+1.1V under standard conditions .

Standard conditions are:

T = 25°C and P = 1 bar for gases.

Solids and liquids are pure.

Solutions are 1 M in all species.

E ° cell is sum of ½ cell E ° values.

CELL POTENTIALS AND REDUCTION POTENTIALS
E °cell = E °reduced - E°oxidized
E °cell = E °cathode - E°anode

½ CELL REDUCTION POTENTIALS

All ½ cells are catalogued as reduction reactions & assigned reduction potentials, E °.


The lower reduction potential ½ rxn is reversed to become the oxidation. E ° oxidation
= – E ° reduction

That makes spontaneous E ° cell
> 0.
But E ° red can’t be found w/o E ° ox
!

ORIGIN FOR REDUCTION POTENTIALS


We need a standard electrode to make measurements against!

The Standard Hydrogen Electrode (SHE)
2H + ( aq ) + 2e –  H
2
(1 bar) E °


 0 V
1 bar H
2
E ° cell
E °
SHE flows over a Pt electrode, and the full is assigned to the other electrode.
= 0 V.
E.g., standard calomel electrode:


Hg
2
Cl
2
( s ) + 2e –  2 Hg( l ) + Cl – E °
SCE
= +0.27V
a more physically convenient reference.

GALVANIC LINE NOTATION

Shorthand for a complete redox cell is of the form:

Anode | anodic soln. || cathodic soln. | Cathode

So making a cell of Cu corrosion,

Cu | Cu 2+ || NO
3
– , NO( g ), H + |Pt
 where all ions should be suffixed ( aq ) and both metals should have ( s ).

TYPES OF GALVANIC CELLS
Primary Battery : can not be recharged e.g.
Mercury Battery
Secondary Battery : rechargeable (storage batteries) e.g.
Ni-Cad Battery
Fuel Cell : reactants supplied from an external source e.g.
H2/O2 fuel cells.

MERCURY BATTERY
Anode:
Zn is reducing agent under basic conditions
Cathode
:
HgO + H
2
O + 2e- ---> Hg + 2 OH can not be recharged

Anode (-)
Cd + 2 OH ---> Cd(OH)
2
Cathode (+)
+ 2e-
NiO(OH) + H
2
O + e- ---> Ni(OH)
2 rechargeable
+ OH -

WHY RECHARGEABLE?.
It is because the products of the reaction are solids that the Ni-Cd battery can be recharged
The solid hydroxides are sticky, and remain in place.
If current is applied, the reaction can be driven backwards
!

HOW TO CHARGE?
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
.

WHY NOT RECHARGEABLE ?
But in mercury battery the ZnO is not sticky, and doesn’t remain attached to the electrode. This battery is not rechargeable

2
Cars can use electricity generated by H
2
/O
2 fuel cells.
H
2 carried in tanks or generated from hydrocarbons

Galvanic cells for which the reactants are continuously supplied.
anode:
2H
2
+ 4OH
 
4H
2
O + 4e
 cathode :
4e

+ O
2
+ 2H
2
O

4OH

2H
2
( g ) + O
2
( g )

2H
2
O( l )
17-44

Mercury batteries take advantage of the high density of Hg to be quite small: used in watches, hearing aids, calculators, etc.
Lithium-iodine batteries are particularly small and lightweight, but also very long-lived
Often used in pacemakers, where they can last for 10 years

V1.06

For a galvanic cell, the electrode at which reduction occurs is called the:
A: Anode B: Cathode

ANSWER
For a galvanic cell, the electrode at which reduction occurs is called the:
B: Cathode

For a galvanic cell, the electrode with negative polarity is called the:
A: Anode B: Cathode

ANSWER
For a galvanic cell, the electrode with negative polarity is called the:
A: Anode

32
Which of the following statements is incorrect a.
b.
In a galvanic cell, reduction occurs at the anode.
The cathode is labeled "+" in a voltaic cell.
c.
d.
Oxidation occurs at the anode in a voltaic cell.
Electrons flow from the anode to the cathode in all electrochemical cells.

ANSWER a.
In a galvanic cell, reduction occurs at the anode.

34
Consider the following notation for an electrochemical cell
Zn|Zn 2+ (1M)||Fe 3+ (1M), Fe 2+ (1M)|Pt
What is the balanced equation for the cell reaction?
a.
Zn(s) + 2Fe 3+ (aq) → 2Fe 2+ (aq) + Zn 2+ (aq) b.
Zn 2+ (aq) + 2Fe 2+ (aq) → Zn(s) + 2Fe 3+ (aq) c.
Zn(s) + 2Fe 2+ (aq) → 2Fe 3+ (aq) + Zn 2+ (aq) d.
Zn(s) + Fe 3+ (aq) → Fe 2+ (aq) + Zn 2+ (aq) e.
Zn(s) + Fe 2+ (aq) → Fe(s) + Zn 2+ (aq)

ANSWER
 Zn(s) + 2Fe 3+ (aq ) → 2Fe 2+ (aq) + Zn 2+ (aq)

What is the oxidation state of nitrogen in
HNO
3
?
A: +3 B: +4
C: +5 D: -5

ANSWER
What is the oxidation state of nitrogen in
HNO
3
?
C: +5

38
Consider the following electrode potentials:
Mg 2+ + 2e
– 
Mg E ° = –2.37 V
V 2+
Cu 2+
+ 2e
+ e
–
–

V E ° = –1.18 V

Cu + E ° = 0.15 V
Which one of the reactions below will proceed spontaneously from left to right?
a. Mg 2+ + V

V 2+ + Mg b. Mg 2+ + 2Cu +

2Cu 2+ + Mg c. V 2+ + 2Cu +

V +2 + Cu 2+ d. V + 2Cu 2+

V 2+ + 2Cu + e. none of these

ANSWER
 d. V + 2Cu 2+

V 2+ + 2Cu +

What is the oxidative state of iodine in IO
3
?
A: +7 B: +6
C: +5 D: +4

ANSWER
What is the oxidative state of iodine in IO
3
?
C: +5