Transcript Electrochemical Cell

Electrochemical Cells & The
Electrochemical Series
1.
A simple cell from an exothermic redox reaction Zn (s) + Cu2+
(aq)
2.
Redox Equilibrium – zinc in water
3.
Standard Hydrogen Reference Half Cell
4.
Standard electrode potential – Zn/Zn2+
5.
Standard electrode potential – Cu/Cu2+
6.
Standard electrode potential – Fe2+/Fe3+
7.
The Electrochemical Series
8.
Calculation of Cell e.m.f.
9.
Questions
10. Useful Internet Link
Electrochemical Cell
V
SALT BRIDGE
Zn
H2O (l)
2+(aq) + 2eZn(s)
 Zn
HALF
EQUATION
Cu2+(aq)
Cu2+HALF
(aq) +EQUATION
2e-  Cu(s)
Salt Bridge
• Maintains electrical neutrality.
• Solution of ionic compound, usually
potassium bromide KBr.
• Positive (K+) ions move into negative half
cell
• Negative (Br-)ions move into positive half
cell
+
Br
K
Very High Resistance Voltmeter
• No current drawn.
• Measures the electromotive force [e.m.f.]
of the cell.
• The potential for the cell to provide energy
Zn(s) Zn2+(aq) + 2e• OXIDATION
• ANODE
OXID
ATION
An anode is a place of oxidation
Cu2+(aq) + 2e-  Cu(s)
• REDUCTION
• CATHODE
CTION
REDU
A cathode is a place of reduction
Redox Equilibrium
[PRESS ZINC]
Zn(s) Ý
2+
Zn (aq)
Zn2+
Zn2+
Zn2+
+
2e
Zn2+
- e- e- e- eeZn
e- e- eZn
Zinc
e- ee- e-
Zn2+
Add stronger oxidising
agent
Add
stronger reducing agent
ADDING
REDOX
AGENTS
Redox Equilibrium 2
MAGNESIUM
Mg(s) Ý
2+
Mg (aq)
Mg2+
Mg2+
Mg2+
+
2e
Mg2+
- e- e- e- eeMg
e- e- eMg
e- ee- e- Magnesium
Mg2+
Equilibrium for magnesium lies
further to right than for zinc
• magnesium sheds electrons and forms
ions more readily than zinc does.
• can't measure the absolute voltage
between the metal and the solution
• don't need to be able to measure the
absolute voltage between the metal and
the solution.
• enough to compare the voltage with a
standardised system [standard hydrogen
reference electrode].
Oxidising agent added, e.g. Cu2+:
Cu2+
Zn2+
Zn2+
Cu
e- e-
Zn2+
e- e-
Zinc
e- e-
Zn2+
e- e-
EQUATION
Zn(s) + Cu2+(aq)
 Zn2+(aq) + Cu(s)
Reducing agent added, e.g. Mg:
Zn2+
- e-2+
eMg
Zn2+
e- e-
Zn2+
e- e-
Zinc
e- e-
Zn2+
e- e-
EQUATION
Mg(s) + Zn2+(aq)
 Mg2+(aq) + Zn(s)
Standard Hydrogen ReferenceElectrode
HYDROGEN GAS
1 atmosphere
298K
Pt
Platinum Electrode
1 mol dm-3 H+(aq)
+(aq) + 2e-  H (g)
2HHALF
EQUATION 2
Zn2+
(aq) +
2e-  Zn(s)
HALF
EQUATION
Eθ Zn2+ / Zn
V
H2 (g)
SALT BRIDGE
Zn
Pt
1 mol
dm-3 H+(aq)
θ
Eθ= E
-0.76V
1 mol dm-3 Zn2+(aq)
+(aq)Zn
CELL
NOTATION 2+(aq)Zn(s)
Pt (s) H2(g), 2H
Cu2+
(aq) EQUATION
+ 2e-  Cu(s)
HALF
Eθ Cu2+ / Cu
V
H2 (g)
SALT BRIDGE
Cu
Pt
1 mol
dm-3 H+(aq)
Eθ= +0.34V
Eθ
1 mol dm-3 Cu2+(aq)
CELL
NOTATION
+(aq)Cu
2+(aq)Cu(s)
Pt (s) H2(g), 2H
HALF
EQUATION
Fe3+
(aq) +
e-  Fe2+(aq)
Eθ Fe3+ / Fe2+
V
H2 (g)
SALT BRIDGE
Pt
1 mol dm-3 H+(aq)
Pt
θ
Eθ= E
+0.77V
1 mol dm-3 Fe2+(aq)
1 mol dm-3 Fe3+(aq)
CELL NOTATION
3+(aq), Fe2+(aq)Pt(s)
Pt (s) H2(g), 2H+(aq)Fe
+
−
Ý Li(s)
Electrochemical Li (aq) + e
K+(aq) + e−
Ý K(s)
Series
Ca2+(aq) + 2e−
Ý Ca(s)
−2.76V
Na+(aq) + e−
Ý Na(s)
−2.71V
Mg2+(aq) + 2e−
Ý Mg(s)
−2.38V
Al3+(aq) + 3e−
Ý Al(s)
Zn2+(aq) + 2e−
Ý Zn(s)
Fe2+(aq) + 2e−
Ý Fe(s)
2H+(aq) + 2e−
Ý H2(g)
Cu2+(aq) + e−
Ý Cu+(aq)
Cu2+(aq) + 2e−
Ý Cu(s)
+0.34V
O2(g) + 2H2O(l) + 4eÝ 4OH-(aq)
Cu+(aq) + e−
Ý Cu(s)
+0.52V
I2(s) + 2e−
Ý 2I−(aq)
Fe3+(aq) + e−
Ý Fe2+(aq)
Ag+(aq) + e−
Ý Ag(s)
+0.80V
Br2(aq) + 2e−
Ý 2Br−(aq)
Cl2(g) + 2e−
Ý 2Cl−(aq)
Cr2O72−(aq) + 14H+ + 6e− Ý 2Cr3+(aq) + 7H2O(l)
MnO4−(aq) + 8H+ + 5e− Ý Mn2+(aq) + 4H2O(l)
F2(g) + 2e−
Ý 2F−(aq)
−3.05V
−2.93V
−1.68V
−0.76V
−0.44V
0.00V
+0.16V
+0.40V
+0.54V
+0.77V
+1.09V
+1.36V
+1.38V
+1.51V
+2.87V
θ
EθCELL= +0.34
(-0.76) = +1.10V
E –CELL
Cell e.m.f.
Questions
V
SALT BRIDGE
Zn
1 mol dm-3 Zn2+(aq)
Cu
1 mol dm-3Cu2+(aq)
2+
Zn (s) Zn2+CELL
(aq)Cu
NOTATION (aq)Cu(s)
opposite
θ
EθCELL= -0.76
(+0.34) = -1.10V
E –CELL
Cell e.m.f.
Questions
V
SALT BRIDGE
Cu
1 mol dm-3 Cu2+(aq)
Zn
1 mol dm-3 Zn2+(aq)
2+
Cu (s) Cu2+CELL
(aq)Zn
NOTATION (aq)Zn(s)
QUESTIONS
Feasibility Of Reactions
Electrochemical Cells
QUESTIONS
Feasibility of Reactions
•
For each of the following mixtures predict
whether a reaction is or isn’t feasible at
s.t.p. If a reaction is feasible then write
the balanced ionic equation.
1. Li(s) and Cu2+(aq)
2. Zn(s) and Al3+(aq)
3. O2(g), H2O(l) and Mg(s)
4. Br2(aq) and Fe(s)
5. Acidified Cr O 2- and I-(aq)
•
QUESTIONS
electrochemical cells
For each of the following redox reactions
write the cell notation and calculate the
e.m.f. of the cell at s.t.p.
1.
2.
3.
4.
5.
Li(s) + Ag+(aq)  Li+(aq) + Ag(s)
3Na(s) + Al3+(aq)  3Na+(aq) + Al(s)
Cr2+(aq) + Fe3+(aq)  Fe2+(aq) + Cr3+(aq)
O2(g) + 2H2O(l) + 2Cu(s)  4OH-(aq) + 2Cu2+(aq)
Ca(s) + 2K+(aq)  Ca2+(aq) + 2K(s)