knockhardy publishing an introduction to electrodepotentials

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AN INTRODUCTION TOAN INTRODUCTION TO

ELECTRODEELECTRODEPOTENTIALSPOTENTIALS

INTRODUCTION

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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


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 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



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 ( 1 atm pressure )

Potential 0.00V

Reaction Cl2(aq) + 2e¯ 2Cl¯(g)

Electrode platinum

Solution Cl¯(aq) (1M) - 1M sodium chloride

Gas chorine ( 1 atm pressure )

Potential + 1.36V



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 1 ATMOSPHERE 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 1 atmosphere 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 (1 ATM)

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





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


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


Zn Zn2+ Cu2+ Cu+

_






• 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

+_






• 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


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




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



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

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© 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

THE ENDTHE END

AN INTRODUCTION TOAN INTRODUCTION TO

ELECTRODEELECTRODEPOTENTIALSPOTENTIALS

knockhardy publishing an introduction to electrodepotentials

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