CH 104: Voltaic Cells. Oxidation is a loss of an electron or electrons by an atom or group of atoms. Zn (s) → Zn 2+ (aq) + 2e – Zn (s) is oxidized to Zn 2+ (aq) in this half-reaction . Reduction is a gain of an electron or electrons by an atom or group of atoms. - PowerPoint PPT Presentation
• Oxidation Oxidation is a loss of an electron or electrons by an is a loss of an electron or electrons by an atom or group of atoms. atom or group of atoms. Zn Zn (s) (s) → Zn → Zn 2+ 2+ (aq) (aq) + 2e + 2e – • Zn Zn (s) (s) is oxidized to Zn is oxidized to Zn 2+ 2+ (aq) (aq) in this in this half-reaction half-reaction . . • Reduction Reduction is a gain of an electron or electrons by an is a gain of an electron or electrons by an atom or group of atoms. atom or group of atoms. Cu Cu 2+ 2+ (aq) (aq) + 2e + 2e – → Cu → Cu (s) (s) • Cu Cu 2+ 2+ (aq) (aq) is reduced to Cu is reduced to Cu (s) (s) in this half-reaction. in this half-reaction. • These 2 half-reactions make an These 2 half-reactions make an oxidation-reduction oxidation-reduction reaction reaction . That is, electrons are transferred from 1 . That is, electrons are transferred from 1 reactant to another reactant. reactant to another reactant. Oxidation: Oxidation: Zn Zn (s) (s) → Zn → Zn 2+ 2+ (aq) (aq) + 2e + 2e – Reduction: Reduction: Cu Cu 2+ 2+ (aq) (aq) + 2e + 2e – – → Cu → Cu (s) (s) ___________ ___________ Oxidation-reduction: Oxidation-reduction: Zn Zn (s) (s) + Cu + Cu 2+ 2+ (aq) (aq) → Zn → Zn 2+ 2+ (aq) (aq) + Cu + Cu (s) (s) CH 104: Voltaic Cells CH 104: Voltaic Cells
Transcript
• OxidationOxidation is a loss of an electron or electrons by an atom or group of is a loss of an electron or electrons by an atom or group of atoms.atoms.
ZnZn(s)(s) → Zn → Zn2+2+(aq)(aq) + 2e + 2e––
• ZnZn(s)(s) is oxidized to Zn is oxidized to Zn2+2+(aq)(aq) in this in this half-reactionhalf-reaction..
• ReductionReduction is a gain of an electron or electrons by an atom or group of is a gain of an electron or electrons by an atom or group of atoms.atoms.
CuCu2+2+(aq)(aq) + 2e + 2e–– → Cu → Cu(s)(s)
• CuCu2+2+(aq)(aq) is reduced to Cu is reduced to Cu(s)(s) in this half-reaction. in this half-reaction.
• These 2 half-reactions make an These 2 half-reactions make an oxidation-reduction reactionoxidation-reduction reaction. That is, . That is, electrons are transferred from 1 reactant to another reactant.electrons are transferred from 1 reactant to another reactant.
• An An electrochemical cellelectrochemical cell (or (or batterybattery) is made of 2 half-cells ) is made of 2 half-cells with electrodes joined by a wire and solutions joined by a with electrodes joined by a wire and solutions joined by a salt bridge.salt bridge.
• The The salt bridgesalt bridge lets ions move from 1 half-cell to the other lets ions move from 1 half-cell to the other half-cell; however, it does not let the bulk solutions move.half-cell; however, it does not let the bulk solutions move.
• A A potentiometerpotentiometer (or (or voltmetervoltmeter) measures the difference in ) measures the difference in electric potential (or voltage) between these 2 electrodes. electric potential (or voltage) between these 2 electrodes. In this battery the difference is 1.097 volts.In this battery the difference is 1.097 volts.
MAKING A BATTERYMAKING A BATTERY
ZnZn(s)(s) + Cu + Cu2+2+(aq)(aq) → Zn → Zn2+2+
(aq)(aq) + Cu + Cu(s)(s)
• OxidationOxidation occurs at the occurs at the anodeanode. Which electrode is the anode?. Which electrode is the anode?• Hint: oxidation and anode both start with vowels.Hint: oxidation and anode both start with vowels.• ReductionReduction occurs at the occurs at the cathodecathode. Which electrode is the cathode?. Which electrode is the cathode?• Hint: reduction and cathode both start with consonants.Hint: reduction and cathode both start with consonants.
MAKING A BATTERYMAKING A BATTERY
ZnZn(s)(s) + Cu + Cu2+2+(aq)(aq) → Zn → Zn2+2+
(aq)(aq) + Cu + Cu(s)(s)
• What is the oxidation half-reaction?What is the oxidation half-reaction?• What is the reduction half-reaction?What is the reduction half-reaction?• Therefore, electrons flow from theTherefore, electrons flow from the to theto the ..• Electrons are produced at the anode and are consumed at the cathode.Electrons are produced at the anode and are consumed at the cathode.• The mass of ZnThe mass of Zn(s)(s) at the anode will increase, decrease, or stay the at the anode will increase, decrease, or stay the
same?same?• The mass of CuThe mass of Cu(s)(s) at the cathode will increase, decrease, or stay the at the cathode will increase, decrease, or stay the
same?same?
MAKING A BATTERYMAKING A BATTERY
ZnZn(s)(s) → Zn → Zn2+2+(aq)(aq) + 2e + 2e––
CuCu2+2+(aq)(aq) + 2e + 2e–– → Cu → Cu(s)(s)
Decrease, ZnDecrease, Zn(s)(s) is converted into Zn is converted into Zn2+2+(aq)(aq)..
Increase, CuIncrease, Cu2+2+(aq)(aq) is converted into Cu is converted into Cu(s)(s)..
anodeanode cathodecathode
ZnZn(s)(s) + Cu + Cu2+2+(aq)(aq) → Zn → Zn2+2+
(aq)(aq) + Cu + Cu(s)(s)
• Therefore, anions (NOTherefore, anions (NO33––) flow from the salt bridge to the) flow from the salt bridge to the . .
• That is, anions from the salt bridge are attracted to the ZnThat is, anions from the salt bridge are attracted to the Zn2+2+(aq)(aq) produced produced
at the anode.at the anode.• Therefore, cations (KTherefore, cations (K++) flow from the salt bridge to the ) flow from the salt bridge to the . .• That is, cations from the salt bridge are attracted to the deficit of CuThat is, cations from the salt bridge are attracted to the deficit of Cu2+2+
(aq)(aq)
at the cathode.at the cathode.• What is the voltage of this battery?What is the voltage of this battery?• 1.097 volts (V).1.097 volts (V).
MAKING A BATTERYMAKING A BATTERY
anodeanode
cathodecathode
Now let’s examine a different oxidation-reduction reaction:Now let’s examine a different oxidation-reduction reaction:CuCu(s)(s) + 2Ag + 2Ag++
(aq)(aq) → Cu → Cu2+2+(aq)(aq) + 2Ag + 2Ag(s)(s)
______ is being oxidized to _______.______ is being oxidized to _______.
______ is being reduced to _______.______ is being reduced to _______.
MAKING A SECONDMAKING A SECOND BATTERYBATTERY
ReactantsReactants ProductsProducts
CuCu(s)(s) CuCu2+2+(aq)(aq)
AgAg(s)(s)AgAg++(aq)(aq)
CuCu(s)(s) + 2Ag + 2Ag++(aq)(aq) → Cu → Cu2+2+
(aq)(aq) + 2Ag + 2Ag(s)(s)
• Notice that replacing ZnNotice that replacing Zn(s)(s) and 1.00 M Zn(NO and 1.00 M Zn(NO33))2(aq)2(aq) with Ag with Ag(s)(s) and 1.00 M and 1.00 M
AgNOAgNO3(aq)3(aq) decreases the voltage from 1.097 V to 0.463 V. decreases the voltage from 1.097 V to 0.463 V.• Which electrode is the anode?Which electrode is the anode?• Hint: oxidation and anode both start with vowels.Hint: oxidation and anode both start with vowels.• Which electrode is the cathode?Which electrode is the cathode?• Hint: reduction and cathode both start with consonants.Hint: reduction and cathode both start with consonants. • Notice the first battery had a CuNotice the first battery had a Cu(s)(s)|Cu|Cu2+2+
(aq)(aq) cathodecathode. This second battery . This second battery
has a Cuhas a Cu(s)(s)|Cu|Cu2+2+(aq)(aq) anodeanode..
MAKING A SECONDMAKING A SECOND BATTERYBATTERY
CuCu(s)(s) + 2Ag + 2Ag++(aq)(aq) → Cu → Cu2+2+
(aq)(aq) + 2Ag + 2Ag(s)(s)
• What is the oxidation half-reaction?What is the oxidation half-reaction?• What is the reduction half-reaction?What is the reduction half-reaction?• Therefore, electrons flow from theTherefore, electrons flow from the to theto the ..• Electrons are produced at the anode and are consumed at the cathode.Electrons are produced at the anode and are consumed at the cathode.• The mass of CuThe mass of Cu(s)(s) at the anode will increase, decrease, or stay the at the anode will increase, decrease, or stay the
same?same?• The mass of AgThe mass of Ag(s)(s) at the cathode will increase, decrease, or stay the at the cathode will increase, decrease, or stay the
same?same?
MAKING A SECONDMAKING A SECOND BATTERYBATTERY
CuCu(s)(s) → Cu → Cu2+2+(aq)(aq) + 2e + 2e––
AgAg++(aq)(aq) + e + e–– → Ag → Ag(s)(s)
Decrease, CuDecrease, Cu(s)(s) is converted into Cu is converted into Cu2+2+(aq)(aq)..
Increase, AgIncrease, Ag++(aq)(aq) is converted into Ag is converted into Ag(s)(s)..
anodeanode cathodecathode
CuCu(s)(s) + 2Ag + 2Ag++(aq)(aq) → Cu → Cu2+2+
(aq)(aq) + 2Ag + 2Ag(s)(s)
• Therefore, anions (NOTherefore, anions (NO33––) flow from the salt bridge to the) flow from the salt bridge to the . .
• That is, anions from the salt bridge are attracted to the CuThat is, anions from the salt bridge are attracted to the Cu2+2+(aq)(aq)
produced at the anode.produced at the anode.• Therefore, cations (KTherefore, cations (K++) flow from the salt bridge to the ) flow from the salt bridge to the . .• That is, cations from the salt bridge are attracted to the deficit of AgThat is, cations from the salt bridge are attracted to the deficit of Ag++
(aq)(aq)
at the cathode.at the cathode.
MAKING A SECONDMAKING A SECOND BATTERYBATTERY
anodeanode
cathodecathode
• Standard reduction potentialsStandard reduction potentials (E° (E°redred) are measured with all ) are measured with all
ions at 1.0 M concentration, all gases at 1.0 atmosphere ions at 1.0 M concentration, all gases at 1.0 atmosphere pressure, all solids in their most thermodynamically stable pressure, all solids in their most thermodynamically stable forms, and the temperature at 298 K.forms, and the temperature at 298 K.
• Therefore, the Therefore, the standard hydrogen electrodestandard hydrogen electrode has pure H has pure H2(g)2(g)
at 1.0 atm bubbling into a solution of 1.0 M Hat 1.0 atm bubbling into a solution of 1.0 M H++(aq)(aq) at 298 K. at 298 K.
This electrode is assigned a standard reduction potential of This electrode is assigned a standard reduction potential of zero volts.zero volts.
2 H2 H++(aq)(aq) + 2e + 2e–– → H → H2(g)2(g), E°, E°redred = 0 volts = 0 volts
• All standard reduction potentials are measured relative to All standard reduction potentials are measured relative to the standard hydrogen electrode.the standard hydrogen electrode.
STANDARD REDUCTION POTENTIALS, E°STANDARD REDUCTION POTENTIALS, E°redred
• What is the voltage of the anode?What is the voltage of the anode?• The anode is a standard hydrogen electrode. By definition it has 0 The anode is a standard hydrogen electrode. By definition it has 0
volts.volts.• What is the voltage of the cathode?What is the voltage of the cathode?• The cathode is a standard CuThe cathode is a standard Cu2+2+
(aq)(aq)|Cu|Cu(s)(s) electrode. It has 0.34 volts, electrode. It has 0.34 volts,
according to the meter. This voltage agrees with the following table of according to the meter. This voltage agrees with the following table of standard reduction potentials (E°standard reduction potentials (E°redred).).
STANDARD REDUCTION POTENTIALS, E°STANDARD REDUCTION POTENTIALS, E°redred
• This table of standard reduction potentials (E°This table of standard reduction potentials (E°redred) is used to calculate ) is used to calculate
standard cell potentialsstandard cell potentials (E° (E°cellcell).).
E°E°cellcell = E° = E°oxox + E° + E°redred
STANDARD REDUCTION POTENTIALS, E°STANDARD REDUCTION POTENTIALS, E°redred
• For example, use this table of standard reduction potentials (E°For example, use this table of standard reduction potentials (E°redred) to ) to
calculate the standard cell potential (E°calculate the standard cell potential (E°cellcell) of the following battery.) of the following battery.• Oxidation:Oxidation: CuCu(s)(s) → Cu → Cu2+2+
• Important:Important: Oxidation is the opposite of reduction; therefore, the sign of Oxidation is the opposite of reduction; therefore, the sign of E°E°oxox for a half-reaction is always the opposite of E° for a half-reaction is always the opposite of E°redred. For example, the . For example, the
+0.34 volts for the standard reduction potential of this oxidation half-+0.34 volts for the standard reduction potential of this oxidation half-reaction was changed to –0.34 volts.reaction was changed to –0.34 volts.
• Important:Important: Standard reduction potential is an intensive property; Standard reduction potential is an intensive property; therefore, the values of E°therefore, the values of E°oxox and E° and E°redred are not changed if the are not changed if the
stoichiometric coefficients are multiplied by a constant. For example, stoichiometric coefficients are multiplied by a constant. For example, the 0.80 volts for Agthe 0.80 volts for Ag++
(aq)(aq) + e + e–– → Ag → Ag(s)(s) was not changed when these was not changed when these
stoichiometric coefficients were multiplied by 2 to balance the net stoichiometric coefficients were multiplied by 2 to balance the net reaction.reaction.
• Important:Important: If E° If E°cellcell is +, then the cell reaction is spontaneous as written. is +, then the cell reaction is spontaneous as written.• Important:Important: If E° If E°cellcell is –, then the cell reaction is non-spontaneous as is –, then the cell reaction is non-spontaneous as
• Is this reaction spontaneous?Is this reaction spontaneous?• Yes, E°Yes, E°cellcell is +. is +.
STANDARD CELL POTENTIALS, E°STANDARD CELL POTENTIALS, E°cellcell
• The The Nernst equationNernst equation is used to calculate is used to calculate cell potentialscell potentials (E(Ecellcell) at non-standard conditions.) at non-standard conditions.
• For the cell reaction: aA + bB = cC + dDFor the cell reaction: aA + bB = cC + dD
• WhereWhere• EEcellcell = the cell potential at non-standard conditions = the cell potential at non-standard conditions• E°E°cellcell = the standard cell potential = the standard cell potential• R = the gas constant = 8.314 JKR = the gas constant = 8.314 JK–1–1molmol–1–1
• T = the temperature in KelvinT = the temperature in Kelvin• n = the total number of electrons transferred in the n = the total number of electrons transferred in the
equation as writtenequation as written• F = the Faraday constant = 96,500 JvoltF = the Faraday constant = 96,500 Jvolt–1–1molmol–1–1
THE NERNST EQUATIONTHE NERNST EQUATION
• Use the Nernst equation to calculate EUse the Nernst equation to calculate Ecellcell for this battery at 25° C. for this battery at 25° C.
• Oxidation:Oxidation: FeFe2+2+(aq)(aq) → Fe → Fe3+3+
• Oxidation-reduction causes the corrosion of iron and steel. Steel is Oxidation-reduction causes the corrosion of iron and steel. Steel is refined Fe containing less than 1.7% C.refined Fe containing less than 1.7% C.
• The oxidation of FeThe oxidation of Fe(s)(s) to Fe to Fe2+2+(aq)(aq) occurs at the strained regions of these occurs at the strained regions of these
iron nails: the heads, tips, and bend. This generation of Feiron nails: the heads, tips, and bend. This generation of Fe2+2+(aq)(aq) is is
marked by the formation of a blue precipitate in the presence of marked by the formation of a blue precipitate in the presence of KK33[Fe(CN)[Fe(CN)66].].
• The reduction of OThe reduction of O2(aq)2(aq) to OH to OH––(aq)(aq) occurs at the other regions of these occurs at the other regions of these
nails. This generation of OHnails. This generation of OH––(aq)(aq) is marked by the pink color of is marked by the pink color of
phenolphthalein.phenolphthalein.• Use these observations to write a balanced equation for the corrosion Use these observations to write a balanced equation for the corrosion
of iron or steel.of iron or steel.2Fe2Fe(s)(s) + O + O2(aq)2(aq) + 2H + 2H22OO(l)(l) → 2Fe → 2Fe2+2+
(aq)(aq) + 4OH + 4OH––(aq)(aq)
• Notice that exposure to air and water causes the corrosion of iron and Notice that exposure to air and water causes the corrosion of iron and steel.steel.
ELECTROCHEMISTRY AND CORROSIONELECTROCHEMISTRY AND CORROSION
• Coatings, such as paints or ZnCoatings, such as paints or Zn(s)(s), are used to retard the , are used to retard the
corrosion of iron and steel by preventing this exposure to corrosion of iron and steel by preventing this exposure to air and water.air and water.
• GalvanizedGalvanized iron or steel is coated with Zn iron or steel is coated with Zn(s)(s). This retards . This retards
the corrosion of Fethe corrosion of Fe(s)(s)..• Coating iron or steel with CuCoating iron or steel with Cu(s)(s) accelerates the corrosion of accelerates the corrosion of
FeFe(s)(s)..
ELECTROCHEMISTRY AND CORROSIONELECTROCHEMISTRY AND CORROSION
• Why is stainless steel stainless?Why is stainless steel stainless?
• Stainless steel contains at least 10.5% Cr. This Cr reacts Stainless steel contains at least 10.5% Cr. This Cr reacts with the Owith the O2(g)2(g) in air to form a complex chrome-oxide surface in air to form a complex chrome-oxide surface
layer that prevents further corrosion. Unlike the rusting layer that prevents further corrosion. Unlike the rusting surface of iron, this chrome-oxide layer does not flake surface of iron, this chrome-oxide layer does not flake away and expose additional atoms to air and water.away and expose additional atoms to air and water. Higher Higher levels of Cr and the addition of other alloying elements levels of Cr and the addition of other alloying elements such as Ni and Mo further improve this surface layer and such as Ni and Mo further improve this surface layer and the corrosion resistance of stainless steels.the corrosion resistance of stainless steels.
ELECTROCHEMISTRY AND CORROSIONELECTROCHEMISTRY AND CORROSION
• Barnes, D.S., J.A. Chandler. 1982. Chemistry 111-112 Workbook and Barnes, D.S., J.A. Chandler. 1982. Chemistry 111-112 Workbook and Laboratory Manual. Amherst, MA: University of Massachusetts.Laboratory Manual. Amherst, MA: University of Massachusetts.
• Merriam-Webster, Inc. 1987. Webster’s 9th New Collegiate Dictionary. Merriam-Webster, Inc. 1987. Webster’s 9th New Collegiate Dictionary. Springfield, MA: Merriam-Webster, Inc.Springfield, MA: Merriam-Webster, Inc.
• Petrucci, R.H. 1985. General Chemistry Principles and Modern Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.Applications, 4th ed. New York, NY: Macmillan Publishing Company.
• Specialty Steel Industry of North America. 2006. SSINA: Stainless Specialty Steel Industry of North America. 2006. SSINA: Stainless Steel: About. Available: Steel: About. Available: http://www.ssina.com/index2.html [accessed 12 [accessed 12 October 2006].October 2006].
