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ELECTROCHEMICAL CELLS• Writing of equations representing oxidation and reduction half reactions and redox reactions
• Electrolytic and galvanic cells;
Writing Equations
• Relation of current and potential to rate and equilibrium;
• Understanding of the processes and redox reactions taking place in cells;• Standard electrode potentials;
Calculating Ecell
ElectrolysisRust
Industrial applications
________
Metal Electrodes. Metal atoms can leave their
electrons (weakly held) and go into solution.
_M__
+ +++
+++
+
+
+++
++
+++
OH
H
OH
H
OH
H
M(s) M+(aq) + e-
e-
e-
e-
e-
e-
e-
e-
M+
M+
An electric potential develops between the solution(….) and electrode(……). Move the red charges to show the electric potential.
O-
H+
H+
M+
M+
-
+
________
Direct Contact.
_Zn__
+ +++
+++
+
+
++
+
++
+O
H
H
OH
H
OH
H
Zn(s) Zn2+(aq) + 2e-
e-
e-
e-
e-
e-
e-
e-
Cu2+
Cu2+
The copper ions are reduced to copper metal and the zinc metal is oxidised to zinc ions.
Cu2+
Cu2+
(SO42-)
Cu2+
OH H
OH
H
OH
H
Cu2+(aq) + 2e- Cu(s)
• The zinc rod would become layered with copper
• The blue solution would become colourless
Copper has a STRONGER attraction for electrons than Zinc.
State which ions would be OXIDISED AND WHICH WOULD BE REDUCED
Electrochemical Cell. Two metals can react indirectly.
_____
Zn_
_
Cu
V
-
KNO3(aq)
+
> e-
Zn Zn2+ + 2e-Cu2+ + 2e- Cu
CuSO4ZnSO4
Salt bridgeanode cathode
• Electrons flow through the external circuit from Zn to Cu.
• Ions flow through the salt bridge (barrier).
• Zinc metal is being .................
• Copper ions are being …………...
• Observations: The zinc electrode would be ………….- the copper electrode would ……………….
Decide which electrode would be negative and which positive and put the charges over the correct electrode.
Electrochemical Cell. Two metals can react indirectly.
_____
Zn_
_
Cu
V
_
KNO3(aq)
+
>
Zn Zn2+ + 2e-Cu2+ + 2e- Cu
Zn2++ 2e- --> Zn
Cu2+ + 2e- --> CuAttract elelctons strongly
Attract elelctons weakly
CuSO4ZnSO4
Salt bridge
anodecathode Zn + Cu2+ Zn2+ + Cuoxidation
reduction REDUCING AGENTOXIDISING AGENT
OXIDISING AGENT
REDUCING AGENT
Top right reacts with bottom left.
e-
Move the labels until they show the correct locations of oxidizing & reducing agents.
ELECTROCHEMICAL CELLS GENERAL A METAL IN CONTACT with a SOLUTION OF ITS IONS is
known as A HALF-CELL.
M M+ + e- N+(aq) + e N(s)
When TWO HALF – CELLS are COMBINED an ELECTROCHEMICAL CELL IS OBTAINED.
OXIDATION takes place at the POSITIVE/NEGATIVE (delete incorrect one) electrode.
PLACE THE TWO SYMBOLS AT THE APPROPRIATE PLACE ON THE
DIAGRAM
M+(aq)
M+(s)
N+(aq)
N+(s)
V
- e-
CELL DIAGRAMS The cell undergoing reduction (more +ve Eθ value) is assigned as the
R.H.S. electrode.
M1(s) M1n+(aq) M2
n+(aq) M2(s)
The solid line represents the phase boundary solid metal & the aqueous ions.
The double dashed line represents the salt bridge. The cell potential is calculated by:
Eθcell = Eθ
RHS EθLHS
(do not change the signs of the Eθ values )
Phase boundary
salt bridge Phase boundary
+
-
ELECTROCHEMICAL CELLS
Electrons flow from –ve to +ve in the external circuit.
The salt bridge allows the flow of ions and completes the circuit.
…….. ……….. // …………. …………. Zinc is ……………& copper is …………….
The Daniell cell :
Galvanic cell converts chemical energy to electrical energy
E0
0.34
-0.76
?
Reactions written as …………………..
Positive potentials ………….. electrons are good ………………...... AGENTS.
Negative potentials …………… electrons are good …………………… AGENTS.
ELECTROCHEMICAL SERIESElectrochemical half-cell potentials (Eθ) are listed from +ve to –ve values.
Zn2+ + 2e- Zn
Cu2+ + 2e- Cu
ElectricPotentials
(….)
Zn ………………. so is MORE ……………….
(….)
Reactions written as reductions.
Positive potentials accept electrons are good OXIDISING AGENTS.
Negative potentials donate electrons are good REDUCING AGENTS.
ELECTROCHEMICAL SERIESequilibrium E° (volts)
1.5
0.8
0.34
0
-0.13
-0.44
-0.76
-1.66
-2.37
-2.71
-2.87
-2.92
-3.03
Electrons flow in external circuit
Ele
ctro
ns
flow
in e
xter
nal
cir
cuit
.
Electrochemical half-cell potentials are listed from +ve to –ve Eθ values.
Oxidizing Agents
REDUCING AGENTS
Reactions take placeTop LEFT to bottom RIGHT
STANDARD HYDROGEN ELECTRODE
……….. electrode H2 gas pressure of ……….
(………. kPa) Solution of …..M H+ ions
(HCl) …………K Eθ = ……………v S.H.E. is ………. to set up. ……………….. standard
electrodes used.
DETERMINATION OF CELL POTENTIALS
Eθ values are determined using a ……………………………… electrode. The S.H.E is always the …………………. electrode. Standard Conditions : …………………………………………………. A secondary standard electrode the ………………. electrode can be used.
Eθcell= …………………………………………
Eθcell= ………………. – ……….
N+(aq)
N+(s)
Unknown electrodeStandard electrodeHigh resistance voltmeter
Salt bridge
If a cell potential of 1.45v is obtained
with metal N. Then the electrode
potential of N+ /N is .................................. .................................. .................................. ........................
..........
DETERMINATION OF CELL POTENTIALS
If the calomel electrode (+0.241 V ) is used as a standard with another electrode the cell potential is -1.901V.
By means of a calculation, identify the other metal.
M+(aq)
M+(s)
Unknown electrodeCalomel electrode
Salt bridge
High resistance voltmeter
SPONTANEOUS REACTIONS
N+(aq)
N+(s)
Positive electrode
M+(aq)
M+(s)
Negative electrode
Salt bridge
High resistance voltmeter E0 M - ve
E 0.15 VE -0.25 V
NON SPONTANEOUS REACTIONS
N+(aq)
N(s)
M+(aq)
M(s)
Salt bridge
High resistance voltmeter
N+(aq)
E 0.15 VE 0.25 V
WORKED EXAMPLES1. The redox couples Mn2+(aq) Mn(s) and U3+(aq) U(s) have
Eθ values of –1.18 and –1.79 respectively. Determine Eθcell
when they are combined and write an equation for the reaction that takes place.
2. The redox couples Co2+(aq) Co(s) and Ni2+(aq) Ni(s) have Eθ values of –0.28v and –0.23v respectively. Write an equation for the spontaneous reaction and use the Eθ
values to justify your answer.
EXAMINATION QUESTIONSUse the standard Eθ values for the half-equations shown below to answer the questions that follow.
MnO4-(aq)+8H+
(aq)+5e- Mn2+(aq)+ 4H2O(l) Eθ = +1.70v
Cl2(g) + 2e- 2Cl-(aq) Eθ = +1.36v
Tl3+(aq) + 2e- Tl+(aq) Eθ = +1.25v
AgCl(s) + e- Ag (s) + Cl-(aq) Eθ = + 0.22v Determine Eθ
cell for the reaction of MnO4-(aq) with Tl+(aq) in acidic
solution and write an equation for the reaction. Why is it not possible to use HCl acid in the reaction above. Use
the Eθ values to justify your answer. Construct an equation for the reaction of Cl2(g) with Ag(s) and
calculate the Eθ value for the reaction.
ELECTROCHEMICAL CELLS• Writing of equations representing oxidation and reduction half reactions and redox reactions
• Electrolytic and galvanic cells;
Writing Equations
• Relation of current and potential to rate and equilibrium;
• Understanding of the processes and redox reactions taking place in cells;• Standard electrode potentials;
Calculating Ecell
ElectrolysisRust
Industrial applications