Download Electrode Potentials Knockhard

AN INTRODUCTION TO
ELECTRODE
POTENTIALS
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
KNOCKHARDY PUBLISHING
ELECTRODE POTENTIALS
INTRODUCTION
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ELECTRODE POTENTIALS
CONTENTS
• Types of half cells
• Cell potential
• The standard hydrogen electrode
• Measuring electrode potentials
• The electrochemical series
• Combining half cells
• Cell diagrams
• Uses of E° values
ELECTRODE POTENTIALS
Before you start it would be helpful to…
• Recall the definitions of oxidation and reduction
• Be able to balance simple ionic equations
• Have a knowledge of simple circuitry
TYPES OF HALF CELL
These are systems involving oxidation or reduction
METALS IN CONTACT WITH SOLUTIONS OF THEIR IONS
Reaction
Cu2+(aq) + 2e¯
Electrode
copper
Solution
Cu2+(aq) (1M)
Potential
+ 0.34V
Reaction
Zn2+(aq) + 2e¯
Electrode
zinc
Solution
Zn2+(aq) (1M)
Potential
- 0.76V
Cu(s)
- 1M copper sulphate solution
Zn(s)
- 1M zinc sulphate solution
TYPES OF HALF CELL
These are systems involving oxidation or reduction
GASES IN CONTACT WITH SOLUTIONS OF THEIR IONS
Reaction
Electrode
2H+(aq) + 2e¯
platinum
Solution
H+(aq) (1M)
Gas
hydrogen ( 1 atm pressure )
Potential
0.00V
Reaction
Cl2(aq) + 2e¯
Electrode
platinum
Solution
Cl¯(aq) (1M)
Gas
chorine ( 1 atm pressure )
Potential
+ 1.36V
H2(g)
- 1M HCl or 0.5M H2SO4
2Cl¯(g)
- 1M sodium chloride
TYPES OF HALF CELL
These are systems involving oxidation or reduction
SOLUTIONS OF IONS IN TWO DIFFERENT OXIDATION STATES
Reaction
Electrode
Fe3+(aq) + e¯
platinum
Solution
Fe3+(aq) (1M) and Fe2+(aq) (1M)
Potential
+ 0.77 V
Fe2+(aq)
SOLUTIONS OF OXIDISING AGENTS IN ACID SOLUTION
Reaction
MnO4¯ + 8H+(aq) + 5e¯
Electrode
platinum
Solution
MnO4¯(aq) (1M) and
Potential
+ 1.52 V
Mn2+(aq) + 4H2O(l)
Mn2+(aq) (1M) and H+(aq)
CELL POTENTIAL
Each electrode / electrolyte combination has its own
half-reaction which sets up a potential difference
The value is
affected by ...
Measurement
BUT...
TEMPERATURE
PRESSURE OF ANY GASES
SOLUTION CONCENTRATION
it is impossible to measure the potential of a single electrode…
you can measure the potential difference between two electrodes
it is measured relative to a reference cell under standard conditions
The ultimate reference is the STANDARD HYDROGEN ELECTRODE.
However, as it is difficult to set up, secondary standards are used.
THE STANDARD HYDROGEN ELECTRODE
The ultimate reference is the STANDARD HYDROGEN ELECTRODE. However as it is
difficult to set up secondary standards are used.
298K (25°C)
SOLUTION OF 1M H+(aq)
e.g. 1M HCl or 0.5M H2SO4
HYDROGEN GAS AT 1
ATMOSPHERE
PRESSURE
PLATINUM
ELECTRODE
The standard hydrogen electrode is assigned an E° value of 0.00V.
MEASUREMENT OF E° VALUES
SALT BRIDGE
HYDROGEN (1 ATM)
ZINC
PLATINUM ELECTRODE
HYDROCHLORIC
ACID (1M)
ZINC SULPHATE (1M)
In the diagram the standard hydrogen electrode is shown coupled up to a zinc half
cell. The voltmeter reading gives the standard electrode potential of the zinc cell.
conditions
temperature
solution conc.
gases
298K
1 Molar (1 mol dm-3) with respect to ions
1 atmosphere pressure
salt bridge
filled with saturated potassium chloride solution
it enables the circuit to be completed
THE ELECTROCHEMICAL SERIES
E° / V
F2(g) + 2e¯
MnO4¯(aq) + 8H+(aq) + 5e¯
Cl2(g) + 2e¯
Cr2O72-(aq) + I4H+(aq) + 6e¯
Br2(l) + 2e¯
Ag+(aq) + e¯
2F¯(aq)
+2.87
Mn2+(aq) + 4H2O(l)
+1.52
2Cl¯(aq)
+1.36
2Cr3+(aq) + 7H2O(l)
+1.33
2Br¯(aq)
+1.07
Ag(s)
+0.80
REACTION MORE
LIKELY TO WORK
+0.68
SPECIES ON LEFT
ARE MORE POWERFUL
OXIDATION AGENTS
Fe3+(aq) + e¯Fe2+(aq)
O2(g) +
2H+(aq)
+ 2e¯
+0.77
H2O2(aq)
I2(s) + 2e¯2I¯(aq)
Cu+(aq) + e¯
Cu(s)
+0.52
Cu2+(aq) + 2e¯
Cu(s)
+0.34
Cu2+(aq) + e¯
2H+(aq) + 2e¯H2(g)
Cu+(aq)
+0.15
Sn2+(aq) + 2e¯
Fe2+(aq) + 2e¯Fe(s)
Sn(s)
(aq) + 2e¯
Zn(s)
Zn2+
Layout
+0.54
0.00
-0.14
-0.44
-0.76
If species are arranged in order of their standard electrode potentials you
get a series that shows how good each substance is at gaining electrons.
All equations are written as reduction processes ... i.e. gaining electrons
A species with a higher E° value oxidise (reverses) one with a lower value
THE ELECTROCHEMICAL SERIES
E° / V
F2(g) + 2e¯
MnO4¯(aq) + 8H+(aq) + 5e¯
Cl2(g) + 2e¯
Cr2O72-(aq) + I4H+(aq) + 6e¯
Br2(l) + 2e¯
Ag+(aq) + e¯
2F¯(aq)
+2.87
Mn2+(aq) + 4H2O(l)
+1.52
2Cl¯(aq)
+1.36
2Cr3+(aq) + 7H2O(l)
+1.33
2Br¯(aq)
+1.07
Ag(s)
+0.80
Fe3+(aq) + e¯Fe2+(aq)
O2(g) +
2H+(aq)
+ 2e¯
+0.77
H2O2(aq)
I2(s) + 2e¯2I¯(aq)
+0.68
+0.54
Cu+(aq) + e¯
Cu(s)
+0.52
Cu2+(aq) + 2e¯
Cu(s)
+0.34
Cu2+(aq) + e¯
2H+(aq) + 2e¯H2(g)
Cu+(aq)
+0.15
Sn2+(aq) + 2e¯
Fe2+(aq) + 2e¯Fe(s)
Sn(s)
(aq) + 2e¯
Zn(s)
Zn2+
AN EQUATION
WITH A
HIGHER E°
VALUE WILL
REVERSE AN
EQUATION
WITH A LOWER
VALUE
0.00
-0.14
-0.44
-0.76
Application Chlorine is a more powerful oxidising agent - it has a higher E°
Chlorine will get its electrons by reversing the iodine equation
Cl2(g) + 2e¯ ——>
2Cl¯(aq)
and
Overall equation is
Cl2(g) +
2I¯(aq)
2I¯(aq) ——> I2(s) + 2e¯
——> I2(s)
+
2Cl¯(aq)
SECONDARY STANDARDS
Why?
The standard hydrogen electrode (SHE) is difficult to set up
it is easier to choose a more convenient secondary standard
the secondary standard has been calibrated against the SHE
Calomel • the calomel electrode contains Hg2Cl2
• it has a standard electrode potential of +0.27V
• is used as the LH electrode to determine the potential of an unknown
• to get the value of the other cell ADD 0.27V to the measured cell potential
ELECTROCHEMICAL CELLS
CELLS
• electrochemical cells contain two electrodes
• each electrode / electrolyte combination has its own half-reaction
• the electrons produced by one half reaction are available for the other
• oxidation occurs at the anode
• reduction occurs at the cathode.
ANODE
Zn(s) ——> Zn2+(aq) + 2e¯
OXIDATION
CATHODE
Cu2+(aq) + 2e¯ ——> Cu(s)
REDUCTION
The resulting cell has a potential difference (voltage) called the cell
potential which depends on the difference between the two potentials
It is affected by ... • current
• temperature
• pressure of any gases
• solution concentrations
A TYPICAL COMBINATION OF HALF CELLS
1.10V
ZINC
COPPER
ZINC
SULPHATE (1M)
COPPER
SULPHATE (1M)
E° = - 0.76V
E° = + 0.34V
zinc is more reactive - it dissolves to give ions
Zn(s) ——> Zn2+(aq) + 2e¯
the electrons produced go round the external circuit to the copper electrode
electrons are picked up by copper ions
As a result, copper is deposited
overall reaction
Cu2+(aq) + 2e¯ ——> Cu(s)
Zn(s) + Cu2+(aq) ——> Zn2+(aq) + Cu(s)
CELL DIAGRAMS
These give a diagrammatic representation of what is happening in a cell.
• Place the cell with the more positive E° value on the RHS of the diagram.
Cu2+(aq) + 2e¯
Cu(s)
E° = + 0.34V
put on the RHS
Zn2+(aq) + 2e¯
Zn(s)
E° = - 0.76V
put on the LHS
Zn
Zn2+
Cu2+
Cu
CELL DIAGRAMS
These give a diagrammatic representation of what is happening in a cell.
• Place the cell with the more positive E° value on the RHS of the diagram.
Cu2+(aq) + 2e¯
Cu(s)
E° = + 0.34V
put on the RHS
Zn2+(aq) + 2e¯
Zn(s)
E° = - 0.76V
put on the LHS
Zn
ZINC IS IN CONTACT
WITH A SOLUTION
OF ZINC IONS
Zn2+
Cu2+
THE SOLUTIONS
ARE JOINED VIA A
SALT BRIDGE TO
Cu
A SOLUTION OF
COPPER IONS IN
CONTACT WITH COPPER
CELL DIAGRAMS
These give a diagrammatic representation of what is happening in a cell.
• Place the cell with the more positive E° value on the RHS of the diagram.
Cu2+(aq) + 2e¯
Cu(s)
E° = + 0.34V
put on the RHS
Zn2+(aq) + 2e¯
Zn(s)
E° = - 0.76V
put on the LHS
_
Zn
Zn2+
Cu2+
Cu
+
• Draw as shown… the cell reaction goes from left to right
• the zinc metal dissolves
Zn(s) ——> Zn2+(aq) + 2e¯
OXIDATION
• copper is deposited
Cu2+(aq) + 2e¯ ——> Cu(s)
REDUCTION
• oxidation takes place at the anode
• reduction at the cathode
CELL DIAGRAMS
These give a diagrammatic representation of what is happening in a cell.
• Place the cell with the more positive E° value on the RHS of the diagram.
Cu2+(aq) + 2e¯
Cu(s)
E° = + 0.34V
put on the RHS
Zn2+(aq) + 2e¯
Zn(s)
E° = - 0.76V
put on the LHS
_
Zn
Zn2+
Cu2+
Cu
+
V
• Draw as shown… the electrons go round the external circuit from left to right
• electrons are released when zinc turns into zinc ions
• the electrons produced go round the external circuit to the copper
• electrons are picked up by copper ions and copper is deposited
CELL DIAGRAMS
These give a diagrammatic representation of what is happening in a cell.
• Place the cell with the more positive E° value on the RHS of the diagram.
Cu2+(aq) + 2e¯
Cu(s)
E° = + 0.34V
put on the RHS
Zn2+(aq) + 2e¯
Zn(s)
E° = - 0.76V
put on the LHS
_
Zn
Zn2+
Cu2+
Cu
+
V
• Draw as shown… the cell voltage is E°(RHS) - E°(LHS) - it must be positive
cell voltage = +0.34V - (-0.76V) = +1.10V
USE OF Eo VALUES - WILL IT WORK?
E° values
Can be used to predict the feasibility of redox and cell reactions
In theory ANY REDOX REACTION WITH A POSITIVE E° VALUE WILL WORK
In practice, it proceeds if the E° value of the reaction is greater than + 0.40V
An equation with a more positive E° value reverse a less positive one
USE OF Eo VALUES - WILL IT WORK?
An equation with a more positive E° value reverse a less positive one
What happens if an Sn(s) / Sn2+(aq) and a Cu(s) / Cu2+(aq) cell are connected?
Write out the equations
Cu2+(aq) + 2e¯
Cu(s)
; E° = +0.34V
Sn2+(aq) + 2e¯
Sn(s)
; E° = -0.14V
the half reaction with the more positive E° value is more likely to work
it gets the electrons by reversing the half reaction with the lower E° value
Cu2+(aq) ——> Cu(s)
therefore
Sn(s)
the overall reaction is
Cu2+(aq)
+ Sn(s)
——>
and
Sn2+(aq)
——> Sn2+(aq) + Cu(s)
the cell voltage is the difference in E° values... (+0.34) - (-0.14) = + 0.48V
USE OF Eo VALUES - WILL IT WORK?
An equation with a more positive E° value reverse a less positive one
Will this reaction be spontaneous?
Sn(s) + Cu2+(aq) ——> Sn2+(aq) + Cu(s)
Write out the appropriate equations
Cu2+(aq) + 2e¯
Cu(s) ; E° = +0.34V
as reductions with their E° values
Sn2+(aq) + 2e¯
Sn(s) ; E° = - 0.14V
The reaction which takes place will involve the more positive one reversing the other
i.e.
Cu2+(aq) ——> Cu(s)
and
The cell voltage will be the difference
in E° values and will be positive...
Sn(s)
——>
Sn2+(aq)
(+0.34) - (- 0.14) = + 0.48V
If this is the equation you want then it will be spontaneous
If it is the opposite equation (going the other way) it will not be spontaneous
USE OF Eo VALUES - WILL IT WORK?
An equation with a more positive E° value reverse a less positive one
Will this reaction be spontaneous?
Sn(s) + Cu2+(aq) ——> Sn2+(aq) + Cu(s)
Cu2+(aq)
Split equation into two half equations
Sn(s)
+ 2e¯ ——>
——> Sn2+(aq)
Cu(s)
+ 2e¯
Find the electrode potentials
Cu2+(aq) + 2e¯
Cu(s) ;
E° = +0.34V
and the usual equations
Sn2+(aq) + 2e¯
Sn(s) ;
E° = - 0.14V
Reverse one equation and its sign
Sn(s) ——> Sn2+(aq)
Combine the two half equations
Sn(s) + Cu2+(aq)
Add the two numerical values
+ 2e¯ ; E° = +0.14V
——>
Sn2+(aq) + Cu(s)
(+0.34V) + (+ 0.14V) = +0.48V
If the value is positive the reaction will be spontaneous
REVISION CHECK
What should you be able to do?
Recall the different types of half cells
Recall the structure of the standard hydrogen electrode
Recall the methods used to calculate standard electrode electrode potentials
Write balanced full and half equations representing electrochemical processes
Know that a reaction can be spontaneous if it has a positive E value
Calculate if a reaction is feasible by finding its E value
CAN YOU DO ALL OF THESE?
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NO
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AN INTRODUCTION TO
ELECTRODE
POTENTIALS
THE END
© 2004, 2009 KNOCKHARDY PUBLISHING AND JONATHAN HOPTON