Chemistry 1011

Chemistry 1011 Slot 5 1

Chemistry 1011

Introductory Chemistry II

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Midgley

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Chemistry 1011

TOPICElectrochemistry

TEXT REFERENCEMasterton and Hurley Chapter 18


18.2 Standard Voltages

YOU ARE EXPECTED TO BE ABLE TO:

• Define the standard electrode potential of a half cell• Order species according to their ease of oxidation or

reduction based on a table of standard reduction potentials• Calculate the net cell voltage, Eo, of a combination of half

cells from standard electrode potential data• Determine whether a given redox reaction will be

spontaneous or non-spontaneous


Cell Voltage• The force that pushes the electrons through

the external circuit of a cell is known as the– Potential difference, or– Electromotive force (emf), or– Voltage

• It is measured in volts• The magnitude of the voltage depends on

– The nature of the redox reaction– The concentrations of the ions in solution, (or

pressures of any gases)


Standard Voltage• In order to compare the voltages of different

cells, or to calculate the expected voltage of a given cell, measurements are taken under standard conditions:– Current flow is almost zero– All ions and molecules in solution are at a

concentration of 1.0 mol/L– All gases are at a pressure of 1.0 atm


The Zinc – Hydrogen Voltaic Cell


The Standard Voltage of the Zinc – Hydrogen Voltaic Cell

Zn(s) + 2H+(aq) Zn2+

(aq) + H2(g)

1.0 mol/L 1.0 mol/L 1.0 atm

Zn | Zn2+ || H+

| H2 | Pt• Cell voltage with no current flowing is +0.762V• This is the standard voltage for this cell

Zn(s) + 2H+(aq, 1.0M) Zn2+

(aq, 1.0M) + H2(g, 1.0atm)

Eo = +0.762V


Standard Half Reaction Voltages

• Each half reaction has a standard voltage

• Eoox (standard oxidation voltage)

• Eored (standard reduction voltage)

Eo = Eoox + Eo

red

• Only Eo can be measured - the standard voltage of a half reaction cannot be measured directly


Obtaining Values for Standard Half Reaction Voltages

• Standard half reaction voltages are determined by arbitrarily assigning the value of zero to the standard reduction half reaction for hydrogen ions to give hydrogen gas

2H+(aq,1.0M) + 2e H2(g,1.0atm)

Eored (H+ H2) = 0.000V

Since Eo = Eoox + Eo

red

Eoox (Zn Zn2+) = +0.762V


Obtaining Values for Standard Half Reaction Voltages

• Once one half reaction standard voltage is established, others can be deduced:

• For: Zn(s) + Cu2+(aq) Zn2+

(aq) + Cu(s) the standard cell voltage is +1.101V

Zn(s) Zn2+(aq,1.0M) + 2eEo

ox (Zn Zn2+) = +0.762V

Cu2+(aq,1.0M) + 2e Cu(s) Eo

red (Cu2+ Cu) = ??V

Since Eo = Eoox + Eo

red

+1.101V = +0.762V + Eored

Eored = +0.339V


Standard Reduction Potentials

• Standard half cell voltages are found in tables of standard potentials

• These are the values for reduction half reactions based upon the convention that

2H+(aq,1.0M) + 2e H2(g,1.0atm)

Eored (H+ H2) = 0.000V

• Standard reduction potential = Eored


Standard Reduction PotentialsOxidizing Agent Reducing Agent Eo

red (V)

Li+(aq) + e Li(s) -3.040

Na+(aq) + e Na(s) -2.714

Zn2+(aq) + 2e Zn(s) -0.762

Ni2+(aq) + 2e Ni(s) -0.236

2H+(aq) + 2e H2(g) 0.000

Cu2+(aq) + 2e Cu(s) +0.339

Ag+(aq) + e Ag(s) +0.799

NO3(aq) + 4H+

(aq) + 3e NO(g) + 2H2O +0.964

MnO4(aq) + 8H+

(aq) + 5e Mn2+(aq) + 4H2O +1.512

F2(g) + 2e 2F(aq) +2.889


Standard Reduction Potentials• Elements above hydrogen in the table of standard

reduction potentials will react with a solution of hydrogen ions to produce hydrogen gas

M(s) + 2H+(aq) M2+

(aq) + H2(g)

M2+(aq) + 2e M(s) Eo

red = negative

M(s) M2+(aq) + 2e Eo

ox = positive

2H+(aq) + 2e H2(g) Eo

red = 0.000V

M | M2+ || H+

| H2 | Pt Eocell

= positive

• Elements below hydrogen in the table of standard reduction potentials will NOT react with a solution of hydrogen ions to produce hydrogen gas


Standard Voltages for Voltaic Cells

• The table of standard reduction potentials gives standard voltages for reduction half reactions

• Standard voltages for oxidation half reactions are obtained by reversing these reactions and changing the sign of the Eo

red value

• If: Zn2+(aq) + 2e Zn(s) Eo

red = -0.762

• Then: Zn(s) Zn2+(aq) + 2e Eo

ox = +0.762


Computing Standard Cell Potential• The standard voltage of a cell is the sum of the

standard potentials for the two half reactions• For the cell:

Zn | Zn2+ || Cu2+

| Cu

Zn(s) Zn2+(aq) + 2e Eo

ox = +0.762V

Cu2+(aq) + 2e Cu(s) Eo

red = +0.339V

Zn(s) + Cu2+(aq) Zn2+

(aq) + Cu(s)

• Eocell

= Eoox + Eo

red = + 0.762 + 0.339 = 1.101V


Oxidizing Agents• An oxidizing agent is a species that can

gain electrons– The strongest oxidizing agents are the species

that gain electrons most readily

– They have the largest positive Eored values

– Oxidizing strength increases moving down the left column of the table of standard reduction potentials

– Oxidizing agents in the table of standard reduction potentials can oxidize any species above


Reducing Agents• A reducing agent is a species that readily

loses electrons– The strongest reducing agents are the species that

lose electrons most readily

– They have the largest negative Eored values (The

largest positive Eoox values)

– Reducing strength increases moving up the right column of the table of standard reduction potentials

– Reducing agents in the table of standard reduction potentials can reduce any species below


Strong Reducing and Oxidizing Agents

Reducing agent causes another species to be reduced - it is oxidized Li(s) Li+

(aq) + e Eoox = +3.040V

Oxidizing agent causes another species to be oxidized - it is reduced

F2(g) + 2e 2F(aq) Eo

red = +2.889V

R

O

Table of Standard Reduction Potentials

R = strongest reducing agent

O = strongest oxidizing agent


Spontaneity of Redox Reactions

• In order for a redox reaction to occur spontaneously, the calculated cell potential MUST BE POSITIVE

• Questions:– Will copper metal be oxidized to Cu2+ ions by

dilute hydrochloric acid?– Will copper metal be oxidized to Cu2+ ions by

dilute nitric acid?


Reaction of Copper with Dilute Hydrochloric Acid??

• Possible oxidation half reaction:

Cu(s) Cu2+(aq) + 2e Eo

ox = -0.339V• Possible reduction half reaction (H+ and Cl ions are

present - Cl ions cannot be reduced):

2H+(aq) + 2e H2(g) Eo

red = 0.000• Net possible reaction:

Cu(s) + 2H+(aq) Cu2+

(aq) + H2(g)

• Net calculated cell voltage

Eocell

= Eoox + Eo

red = - 0.339 + 0.000 = -0.339 V• Reaction will not be spontaneous i.e no reaction


Reaction of Copper with Dilute Nitric Acid??

• Possible oxidation half reaction:

Cu(s) Cu2+(aq) + 2e Eo

ox = -0.339V

• Possible reduction half reactions (H+ and NO3 ions are present):

2H+(aq) + 2e H2(g) Eo

red = 0.000V

NO3(aq) + 4H+

(aq) + 3e NO(g) + 2H2O Eored = +0.964V

• Net spontaneous reaction (Add multiples of the two half reactions so that same #electrons (6) in each half):

3Cu(s) + 2NO3(aq) + 8H+

(aq) 3Cu2+(aq) + 2NO(g) + 4H2O

• Net calculated cell voltage:

Eocell

= Eoox + Eo

red = - 0.339 + 0.964 = +0.629 V• Reaction will be spontaneous i.e reaction takes place


Voltaic Cells with Inert Electrodes

• Half cells will frequently be constructed with inert electrodes (often carbon or platinum)

• The Hydrogen half cell is one example:H+

| H2 | Pt

• A cell with two inert electrodes might be:

Pt | Fe2+(aq) | Fe

(aq) || Cl (aq) | Cl2(g) | Pt


The Leclanché Cell• The Leclanché cell is the ordinary commercial

flashlight battery

Zn | Zn2+ ||MnO2 | Mn2O3 | C

• Anode half reaction:Zn(s) Zn2+

(aq) + 2e Eoox = +0.762V

• Cathode half reaction (complex):2MnO2(s) + 2NH4

+(aq) + 2e Mn2O3(s) + 2NH3(aq) + H2O Eo

red = +0.7 V

• Net cell voltage Eocell = 1.5V

Chemistry 1011

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