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Electric energy Chemical energy
Electrolysis
Galvanic cell
Chapter 8 Electrochemistry
Electrochemistry• Study of chemical reactions that can produce
electricity or use electricity to produce desired product.
• Study of interchange of chemical and electrical energy
• Electrochemical reaction always involves oxidation-reduction reactions– Electron transfer reactions– Electrons transferred from one substance to another
• Also called redox reactions
2
Galvanic Cells
The Daniell Cell
Flow of Zn2+
Flow of SO42-
Half-cell
Half-cell reaction
5
• Needed to complete circuit• Tube filled with solution of an electrolyte– Salt composed of ions not involved in cell reaction– KNO3 and KCl often used
• Porous plugs at each end of tube – Prevent solution from pouring out
– Enable ions from salt bridge to migrate between half-cells to neutralize charges in cell compartments• Anions always migrate toward anode • Cations always migrate toward cathode
Salt Bridge Salt bridge
Anode compartment
Half-cell
Half-reaction:
Oxidation
Cathode compartment
Half-cell
Half-reaction:
Reduction
Zn(s) → Zn2+(aq) +2e- Cu2+
(aq) + 2e- → Cu(s)
Cu2+(aq) + 2e- → Cu(s)
Cu2+(aq) + Zn(s) → Cu(s) + Zn2+
(aq) total cell reaction
2e-
7
Cell Notation
Cu(s)|Cu2+(aq)||Ag+(aq)|Ag(s)
• Single slash = boundary between phases (solid electrode and aqueous solution of ions)
• Double slash represents salt bridge– Separates cell reactions
• In each half (half-cell)– Electrodes appear at outsides– Reaction electrolytes in inner section– Species in same state separated with ;– Concentrations shown in ( )
anode cathode
anodeelectrode
anodeelectrolyte
cathodeelectrolyte
cathodeelectrode
Salt Bridge
8
Learning Check• Write the standard cell notation for the
following electrochemical cells:• Fe (s) + Cd2+ (aq) Cd(s) + Fe2+(aq) Anode = ox = Fe(s) Cathode = red = Cd2+(aq) Fe(s)|Fe2+(aq)||Cd2+(aq)|Cd(s) • Al(s) + Au3+(aq) Al3+(aq) + Au(s) Anode = ox = Al(s) Cathode = red = Au3+(aq) Al(s)|Al3+(aq)||Au3+(aq)|Au(s)
Your Turn!Write the standard cell notation (Pt electrodes) for the following reaction:
2Mn3+(aq) + 2I-(aq) → Mn2+(aq) + I2(s)
A. Pt(s)|Mn3+(aq); Mn2+(aq)||I-(aq)|I2(s)|Pt(s)
B. Pt(s)|I-(aq)|I2(s)||Mn3+(aq); Mn2+(aq)|Pt(s)
C. Mn3+(aq)|Pt(s); Mn2+(aq)||I-(aq)|I2(s)|Pt(s)
D. Pt(s)|Mn3+(aq); I-(aq)||Mn2+(aq)|I2(s)|Pt(s)
Oxidation reaction is on the right and reduction reaction is on the left of
the salt bridge (||).
9
Reaction can be performed without
harnessing electricity!
DG of reaction: maximum work over and above volume work (electricity) that can be harnessed from the chemical reaction.
Cu wire is dipped into Zn2+ solution: nothing happens.
Cu wire is dipped into Ag+ solution:
Ag+ has higher tendency to be reduced than Cu2+.Cu2+ has higher tendency to be reduced than Zn2+.
2Ag+(aq) + Cu(s) → 2Ag(s) + Cu2+
(aq)
Work harnessed!!
Ag+ has higher tendency to be reduced than Cu2+.Cu2+ has higher tendency to be reduced than Zn2+.
How can we know?
Electrode PotentialReflects tendency towards reduction
Problem: Only potential difference can be measured between two half-cells.
Hydrogen Standard Electrode
This electrode used as standard.EMF of all other electrodes measured with reference to this electrode.
H2(g) → 2 H+(aq) +2e-
Standard Reduction PotentialSHE as anode (Oxidation).The other electrode cathode (Reduction).
H2(g) → 2 H+(aq) +2e- Cu2+
(aq) + 2e- → Cu(s)
Voltage: Potential difference
Measured voltage =
Potential of reduction electrode
- Potential of anode electrode
H2(g) → 2 H+(aq) +2e-
Cu2+(aq) + 2e- → Cu(s)
Cu2+(aq) + H2(g) → Cu(s) + 2H+
(aq)
Spontaneousat
Standard conditions
ReductionPotential
Cu2+(aq) + 2e- → Cu(s) Eo
red =0.34 V
H2(g) → 2 H+(aq) +2e- Zn2+
(aq) + 2e- → Zn(s)
Zn2+(aq) + H2(g) → Zn(s) + 2H+
(aq)
Zn(s) → Zn2+(aq) +2e-
2 H+(aq) +2e- → H2(g)
Zn(s) + 2H+(aq) → Zn2+
(aq) + H2(g)
ReductionPotential
Zn2+(aq) + H2(g) → Zn(s) + 2H+
(aq) nonspontaneous
Electricity must be applied to force this process to take place!
Zn2+(aq) + 2e- → Zn(s) Eo
red = -0.76 V
Cu2+(aq) + 2e- → Cu(s)
Zn(s) → Zn2+(aq) +2e-
Cu2+(aq) + Zn(s) → Cu(s) + Zn2+
(aq)
Electrochemical thermodynamics
DG of reaction: maximum work over and above volume work (available work) (electricity) that can be harnessed from the chemical reaction.
maxmax
electricalVp
non wworkG
Electrical heater Heating elementsresistive
EFzwG
EV
FzVw
FzQ
CueCu
reactionsmicalelectrochefor
QVtIVw
el
el
el
..
..
.
2
...
2
Work done
c
o Kz
E log0591.0
0591.0log
o
c
EzK
Concentration Cells
• Consider the cell presented on the left.
• The 1/2 cell reactions are the same, it is just the concentrations that differ.
• Will there be electron flow?
Concentration Cells (cont.)
Ag Ag+ + e- -E1/2 Anode:
Ag+ + e- Ag E1/2 Cathode:
Q Ag
anode
Ag cathode
0.1
10.1
Ecell = E°cell - (0.0591/n)log(Q)0 V
Ecell = - (0.0591)log(0.1) = 0.0591 V
1
Concentration Cells (cont.)
Another Example:
What is Ecell?
Concentration Cells (cont.)
Ecell = E°cell - (0.0591/n)log(Q)0
Fe2+ + 2e- Fe2 e- transferred…n = 2
2
Q Fe2
anode
Fe2 cathode
0.01
.10.1
Ecell = -(0.0296)log(.1) = 0.0296 V
anode cathode
e-