chemistry 1011
DESCRIPTION
Chemistry 1011. Introductory Chemistry II http://www.mi.mun.ca/~pfisher/chemistry.html Password for final exams Midgley. Chemistry 1011. TOPIC Electrochemistry TEXT REFERENCE Masterton and Hurley Chapter 18. 18.2 Standard Voltages. YOU ARE EXPECTED TO BE ABLE TO: - PowerPoint PPT PresentationTRANSCRIPT
Chemistry 1011 Slot 5 1
Chemistry 1011
Introductory Chemistry II
http://www.mi.mun.ca/~pfisher/chemistry.html
Password for final exams
Midgley
Chemistry 1011 Slot 5 2
Chemistry 1011
TOPICElectrochemistry
TEXT REFERENCEMasterton and Hurley Chapter 18
Chemistry 1011 Slot 5 3
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
Chemistry 1011 Slot 5 4
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)
Chemistry 1011 Slot 5 5
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
Chemistry 1011 Slot 5 6
The Zinc – Hydrogen Voltaic Cell
Chemistry 1011 Slot 5 7
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
Chemistry 1011 Slot 5 8
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
Chemistry 1011 Slot 5 9
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
Chemistry 1011 Slot 5 10
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
Chemistry 1011 Slot 5 11
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
Chemistry 1011 Slot 5 12
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
Chemistry 1011 Slot 5 13
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
Chemistry 1011 Slot 5 14
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
Chemistry 1011 Slot 5 15
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
Chemistry 1011 Slot 5 16
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
Chemistry 1011 Slot 5 17
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
Chemistry 1011 Slot 5 18
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
Chemistry 1011 Slot 5 19
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?
Chemistry 1011 Slot 5 20
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
Chemistry 1011 Slot 5 21
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
Chemistry 1011 Slot 5 22
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
Chemistry 1011 Slot 5 23
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 Slot 5 24