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AN INTRODUCTION TOAN INTRODUCTION TO
ELECTRODEELECTRODEPOTENTIALSPOTENTIALS
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2008 2008
SPECIFICATIONSSPECIFICATIONS
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
Navigation is achieved by...
either clicking on the grey arrows at the foot of each page
or using the left and right arrow keys on the keyboard
ELECTRODE POTENTIALSELECTRODE 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 POTENTIALSELECTRODE 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
ELECTRODE POTENTIALSELECTRODE POTENTIALS
TYPES OF HALF CELLTYPES OF HALF CELL
These are systems involving oxidation or reduction
METALS IN CONTACT WITH SOLUTIONS OF THEIR IONS
Reaction Cu2+(aq) + 2e¯ Cu(s)
Electrode copper
Solution Cu2+(aq) (1M) - 1M copper sulphate solution
Potential + 0.34V
Reaction Zn2+(aq) + 2e¯ Zn(s)
Electrode zinc
Solution Zn2+(aq) (1M) - 1M zinc sulphate solution
Potential - 0.76V
TYPES OF HALF CELLTYPES OF HALF CELL
These are systems involving oxidation or reduction
GASES IN CONTACT WITH SOLUTIONS OF THEIR IONS
Reaction 2H+(aq) + 2e¯ H2(g)
Electrode platinum
Solution H+(aq) (1M) - 1M HCl or 0.5M H2SO4
Gas hydrogen at 100 kPa (1atm) pressure
Potential 0.00V
Reaction Cl2(aq) + 2e¯ 2Cl¯(g)
Electrode platinum
Solution Cl¯(aq) (1M) - 1M sodium chloride
Gas chorine at 100 kPa (1atm) pressure
Potential + 1.36V
TYPES OF HALF CELLTYPES OF HALF CELL
These are systems involving oxidation or reduction
SOLUTIONS OF IONS IN TWO DIFFERENT OXIDATION STATES
Reaction Fe3+(aq) + e¯ Fe2+(aq) Electrode platinum
Solution Fe3+(aq) (1M) and Fe2+(aq) (1M)
Potential + 0.77 V
TYPES OF HALF CELLTYPES OF HALF CELL
These are systems involving oxidation or reduction
SOLUTIONS OF IONS IN TWO DIFFERENT OXIDATION STATES
Reaction Fe3+(aq) + e¯ Fe2+(aq) Electrode platinum
Solution Fe3+(aq) (1M) and Fe2+(aq) (1M)
Potential + 0.77 V
SOLUTIONS OF OXIDISING AGENTS IN ACID SOLUTION
Reaction MnO4¯ + 8H+(aq) + 5e¯ Mn2+(aq) + 4H2O(l)
Electrode platinum
Solution MnO4¯(aq) (1M) and Mn2+(aq) (1M) and H+(aq)
Potential + 1.52 V
CELL POTENTIALCELL POTENTIAL
Each electrode / electrolyte combination has its ownhalf-reaction which sets up a potential difference
The value isaffected by ... TEMPERATURE
PRESSURE OF ANY GASESSOLUTION CONCENTRATION
Measurement it is impossible to measure the potential of a single electrode…
BUT... you can measure the potential difference between two electrodesit 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 ultimate reference is the STANDARD HYDROGEN ELECTRODE. However as it is difficult to set up secondary standards are used.
THE STANDARD HYDROGEN ELECTRODETHE STANDARD HYDROGEN ELECTRODE
The standard hydrogen electrode is assigned an E° value of 0.00V.
HYDROGEN GAS AT 100 kPa PRESSURE
PLATINUM ELECTRODE
SOLUTION OF 1M H+(aq)
e.g. 1M HCl or 0.5M H2SO4
298K (25°C)
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 298Ksolution conc. 1 Molar (1 mol dm-3) with respect to ionsgases 100 kPa pressure
salt bridge filled with saturated potassium chloride solutionit enables the circuit to be completed
MEASUREMENT OF EMEASUREMENT OF E°° VALUES VALUES
SALT BRIDGE
ZINC
ZINC SULPHATE (1M)
HYDROGEN (100 kPa)
PLATINUM ELECTRODE
HYDROCHLORICACID (1M)
E° / V
F2(g) + 2e¯ 2F¯(aq) +2.87
MnO4¯(aq) + 8H+(aq) + 5e¯ Mn2+(aq) + 4H2O(l) +1.52
Cl2(g) + 2e¯ 2Cl¯(aq) +1.36
Cr2O72-(aq) + I4H+(aq) + 6e¯ 2Cr3+(aq) + 7H2O(l) +1.33
Br2(l) + 2e¯ 2Br¯(aq) +1.07
Ag+(aq) + e¯Ag(s) +0.80
Fe3+(aq) + e¯ Fe2+(aq) +0.77
O2(g) + 2H+(aq) + 2e¯ H2O2(aq) +0.68
I2(s) + 2e¯2I¯(aq) +0.54
Cu+(aq) + e¯Cu(s) +0.52
Cu2+(aq) + 2e¯Cu(s) +0.34
Cu2+(aq) + e¯Cu+(aq) +0.15
2H+(aq) + 2e¯H2(g) 0.00
Sn2+(aq) + 2e¯Sn(s) -0.14
Fe2+(aq) + 2e¯ Fe(s) -0.44
Zn2+ (aq) + 2e¯Zn(s) -0.76
Layout If species are arranged in order of their standard electrode potentials youget 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 SERIESTHE ELECTROCHEMICAL SERIES
REACTION MORELIKELY TO WORK
SPECIES ON LEFTARE MORE POWERFUL
OXIDATION AGENTS
E° / V
F2(g) + 2e¯ 2F¯(aq) +2.87
MnO4¯(aq) + 8H+(aq) + 5e¯ Mn2+(aq) + 4H2O(l) +1.52
Cl2(g) + 2e¯ 2Cl¯(aq) +1.36
Cr2O72-(aq) + I4H+(aq) + 6e¯ 2Cr3+(aq) + 7H2O(l) +1.33
Br2(l) + 2e¯ 2Br¯(aq) +1.07
Ag+(aq) + e¯Ag(s) +0.80
Fe3+(aq) + e¯ Fe2+(aq) +0.77
O2(g) + 2H+(aq) + 2e¯ H2O2(aq) +0.68
I2(s) + 2e¯2I¯(aq) +0.54
Cu+(aq) + e¯Cu(s) +0.52
Cu2+(aq) + 2e¯Cu(s) +0.34
Cu2+(aq) + e¯Cu+(aq) +0.15
2H+(aq) + 2e¯H2(g) 0.00
Sn2+(aq) + 2e¯Sn(s) -0.14
Fe2+(aq) + 2e¯ Fe(s) -0.44
Zn2+ (aq) + 2e¯Zn(s) -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 2I¯(aq) ——> I2(s) + 2e¯
Overall equation is Cl2(g) + 2I¯(aq) ——> I2(s) + 2Cl¯(aq)
THE ELECTROCHEMICAL SERIESTHE ELECTROCHEMICAL SERIES
AN EQUATION WITH A HIGHER E° VALUE WILL REVERSE AN EQUATION WITH A LOWER VALUE
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
SECONDARY STANDARDSSECONDARY STANDARDS
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 cellpotential which depends on the difference between the two potentials
It is affected by ... • current
• temperature
• pressure of any gases
• solution concentrations
ELECTROCHEMICAL CELLSELECTROCHEMICAL CELLS
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 Cu2+(aq) + 2e¯ ——> Cu(s)As a result, copper is deposited
overall reaction Zn(s) + Cu2+(aq) ——> Zn2+(aq) + Cu(s)
A TYPICAL COMBINATION OF HALF CELLSA TYPICAL COMBINATION OF HALF CELLS
ZINC
ZINC SULPHATE (1M)
COPPER
COPPERSULPHATE (1M)
E° = - 0.76V E° = + 0.34V
1.10V
+_
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
CELL DIAGRAMSCELL DIAGRAMS
Zn Zn2+ Cu2+ Cu
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
ZINC IS IN CONTACT THE SOLUTIONS A SOLUTION OFWITH A SOLUTION ARE JOINED VIA A COPPER IONS IN OF ZINC IONS SALT BRIDGE TO CONTACT WITH COPPER
CELL DIAGRAMSCELL DIAGRAMS
Zn Zn2+ Cu2+ Cu+
_
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
• 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 DIAGRAMSCELL DIAGRAMS
Zn Zn2+ Cu2+ Cu+
_
CELL DIAGRAMSCELL 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
• 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
Zn Zn2+ Cu2+ Cu
V
+_
CELL DIAGRAMSCELL 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
• Draw as shown… the cell voltage is E°(RHS) - E°(LHS) - it must be positive
cell voltage = +0.34V - (-0.76V) = +1.10V
Zn Zn2+ Cu2+ Cu
V
+_
USE OF EUSE OF Eoo VALUES - WILL IT WORK? 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 EUSE OF Eoo VALUES - WILL IT WORK? VALUES - WILL IT WORK?
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
An equation with a more positive E° value reverse a less positive one
USE OF EUSE OF Eoo VALUES - WILL IT WORK? VALUES - WILL IT WORK?
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 workit gets the electrons by reversing the half reaction with the lower E° value
therefore Cu2+(aq) ——> Cu(s) and
Sn(s) ——> Sn2+(aq)
An equation with a more positive E° value reverse a less positive one
USE OF EUSE OF Eoo VALUES - WILL IT WORK? VALUES - WILL IT WORK?
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 workit gets the electrons by reversing the half reaction with the lower E° value
therefore Cu2+(aq) ——> Cu(s) and
Sn(s) ——> Sn2+(aq)
the overall reaction is Cu2+(aq) + Sn(s) ——> Sn2+(aq) + Cu(s)
the cell voltage is the difference in E° values... (+0.34) - (-0.14) = + 0.48V
An equation with a more positive E° value reverse a less positive one
USE OF EUSE OF Eoo VALUES - WILL IT WORK? 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 Sn(s) ——> Sn2+(aq)
The cell voltage will be the differencein E° values and will be positive... (+0.34) - (- 0.14) = + 0.48V
If this is the equation you want then it will be spontaneousIf it is the opposite equation (going the other way) it will not be spontaneous
USE OF EUSE OF Eoo VALUES - WILL IT WORK? 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)
Split equation into two half equations Cu2+(aq) + 2e¯ ——> Cu(s)
Sn(s) ——> Sn2+(aq) + 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) + 2e¯ ; E° = +0.14V
Combine the two half equations Sn(s) + Cu2+(aq) ——> Sn2+(aq) + Cu(s)
Add the two numerical values (+0.34V) + (+ 0.14V) = +0.48V
If the value is positive the reaction will be spontaneous
REVISION CHECKREVISION 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? CAN YOU DO ALL OF THESE? YES YES NONO
You need to go over the You need to go over the relevant topic(s) againrelevant topic(s) again
Click on the button toClick on the button toreturn to the menureturn to the menu
WELL DONE!WELL DONE!Try some past paper questionsTry some past paper questions
© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
THE ENDTHE END
AN INTRODUCTION TOAN INTRODUCTION TO
ELECTRODEELECTRODEPOTENTIALSPOTENTIALS