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General Chemistry IIGeneral Chemistry IICHEM 152 Unit 4CHEM 152 Unit 4
Week 13
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Week 13 Reading Assignment
Chapter 18 – Sections 18.1 (intro), 18.3 (voltaic cells), 18.4 (cell
potential)
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“Free Energy”
• All spontaneous (product favored) reactions can be harnessed in some way to do work!
OR Burn fuel to create steamTo turn the turbines
How can you store“free energy?”
Power Generation
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ELECTROCHEMISTRYELECTROCHEMISTRYUnit 4Unit 4
Study of chemical reactions that
produce electrical
currents or voltages and of
the chemical reactions that are
caused by the action of currents
or voltages.
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A 3.41 x 10-6 g sample of a compound is known to
contain 4.67 x 1016 molecules. This compound
is
1. CO2
2. CH4
3. NH3
4. H2O
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What is different in the second reaction?
Ag+ + Cl- AgCl(s)
Ag+ + Cu(s) Cu2++ Ag(s)
Cu
Ag
Ag+
The charge on Ag and Cu change. That’s anOxidation- reduction reaction (REDOX)
Cu2+
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Ag+ + Cu(s) Cu2+ + Ag(s)
Copper loses two electrons
Cu was OXIDIZED
Ag was REDUCED Ag+ was the OXIDIZING AGENT
Cu was the REDUCING AGENT
Charge balance vs. mass balance
2 2
Ag+ gains electrons
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Identifying REDOXOXIDATION NUMBERS
1)Any element in any neutral form-- standing alone
OXIDATION NUMBER = 0
Bookkeeping method only---not real CHARGES
S8 O.N. = 0 O2, O3, O O.N. = 0
I2(s) , I2(g) , I O.N. = 02) Any Element as a single ion – standing alone OXIDATION NUMBER = Charge on the ION
S2- O.N. = -2 O2- O.N. = -2
I - Ox no = -1
Cu2+ O.N. = +2 Al3+ O.N. = +3
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In a neutral COMPOUND some elements often have a fixed OXIDATION NUMBER
H almost always is +1O almost always is -2F always is -1
3) In a NEUTRAL COMPOUND the SUM of all the atomic oxidation numbers is ZERO
CH4
CH3COOH
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4) In a COMPLEX ION the SUM of all the atomic oxidation numbers is equal to the TOTAL CHARGE on the ion.
ClO3−
MnO4−
PO33-
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What is the oxidation number of P in H3PO4?
What is the oxidation number of Cr in Cr2O7
2-?
What is the oxidation number of C in (NH4)2CO3?
What is the oxidation number of C in C3H8(propane)?
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Another Way
C OO
Use the structure – the oxidation number of an atom is equal to
# of valence electrons − # of lone pair e- − # of bond e- IF the atom is the most
electronegative in the bond
C = 4 – 0 – 0 = +4(oxygen is more electronegative)
This method treats atoms like ions – it assumes all the bonded electrons go to the more electronegative atom in the bond
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Propane again…
Using the structure method, what is the oxidation number of each carbon in propane (C3H8)?
C
C
C
H H
H H
HHH H
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What is the oxidation number of the oxygen-bonded C in
CH3COOH (acetic acid)?
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OXIDATION IS THE LOSS OF ELECTRONS
REDUCTION IS THE GAIN OF ELECTRONS(Oxidation Number is reduced)
Oil RigOILRIG
LEO the LION goes GERLEO
GER
Oxidation and Reduction Reactions
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Cu + AgNO3 Ag + Cu(NO3)2
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At the atomic level
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2Al + Fe2O3 2Fe + Al2O3 THERMITE REACTION
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Identifying REDOX
S8 + 12O2 8SO3 Ionic or covalent? O.N.? REDOX? Ox. Agent; Red. Agent?
2AgNO3 + Na2S Ag2S + 2NaNO3
REDOX?
5As4O6 + 8MnO4- + 18H2O
20 AsO4 3- + 8 Mn2+ + 36H+
REDOX?
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S8 + 12O2 8SO3
Is it redox?
If so,What is oxidized?What is the reducing agent?
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2 AgNO3 + Na2S Ag2S + 2 NaNO3
Is it redox?
5 As4O6 + 8 MnO4- + 18 H2O
20 AsO43- + 8 Mn2+ + 36 H+
What is oxidized and reduced?What is the oxidizing agent (OA) and reducing agent (RA)?
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.Uncontrolled spontaneous reaction
How can this be harnessed to do work?
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.
Controlled redox reaction
Zn Zn2+ + 2e− Cu2+ + 2e− Cu
Zn + Cu 2+ Cu + Zn2++
overall reaction but electrons are forced outside of the cells
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Voltaic Cell ΔG = negGalvanicBattery
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Electrochemical CellsElectrochemical Cells
Devices to harness a chemical reaction which produces an electric
current
Zinc anode
Copper cathode
Cu+2Cu+2
Cu+2Cu+2
1.10 v
Zn+2
Zn+2
Zn+2
Zn+2
Daniel's Cell
Zn is oxidized Zn is oxidized and is the and is the reducing agent reducing agent
Zn(s) Zn(s) Zn Zn2+2+(aq) + (aq) + 2e-2e-
ANODE (oxidation)ANODE (oxidation)
CuCu2+2+ is reduced is reduced and is and is the oxidizing agentthe oxidizing agent
CuCu2+2+(aq) + (aq) + 2e-2e- Cu(s) Cu(s)
CATHODE (reduction)CATHODE (reduction)
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Simple Electrochemical Cell
Also called voltaic or galvanic cells
UA GenChemElectrochemical CellElectrochemical Cell
Explain what Explain what happens in this happens in this
system.system.
Follow the Follow the circulation of circulation of one negative one negative charge in the charge in the
cell.cell.
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CELL POTENTIAL, ECELL POTENTIAL, E
• Electrons are “driven” from anode to Electrons are “driven” from anode to cathode by an electromotive force or cathode by an electromotive force or emfemf..
• For Zn/Cu cell, this is indicated by a For Zn/Cu cell, this is indicated by a voltage of 1.10 V at voltage of 1.10 V at 25 ˚C and when 25 ˚C and when [Zn[Zn2+2+] and [Cu] and [Cu2+2+] = 1.0 M] = 1.0 M..
Zn and ZnZn and Zn2+2+,,anodeanode
Cu and CuCu and Cu2+2+,,cathodecathode
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons1.10 V1.10 V
1.0 M1.0 M 1.0 M1.0 M
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CELL POTENTIAL, CELL POTENTIAL, EE
• For Zn/Cu cell, For Zn/Cu cell, potentialpotential is is +1.10 V+1.10 V at at 25 ˚C and when [Zn25 ˚C and when [Zn2+2+] and [Cu] and [Cu2+2+] = 1.0 ] = 1.0 M.M.
• is the is the STANDARD CELL POTENTIAL, ESTANDARD CELL POTENTIAL, Eoo
• a a quantitativequantitative measure of the measure of the tendency of reactants to proceed to tendency of reactants to proceed to products when all are in their products when all are in their standard statesstandard states at 25 ˚C. at 25 ˚C.
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
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Calculating Cell VoltageCalculating Cell Voltage• Balanced half-reactions can be Balanced half-reactions can be
added together to get overall, added together to get overall, balanced equation. balanced equation.
Zn(s) Zn(s) Zn Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- (aq) + 2e- Cu(s) Cu(s)--------------------------------------------------------------CuCu2+2+(aq) + Zn(s) (aq) + Zn(s) Zn Zn2+2+(aq) + (aq) + Cu(s)Cu(s)
Zn(s) Zn(s) Zn Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- (aq) + 2e- Cu(s) Cu(s)--------------------------------------------------------------CuCu2+2+(aq) + Zn(s) (aq) + Zn(s) Zn Zn2+2+(aq) + (aq) + Cu(s)Cu(s)
If we know EIf we know Eoo for each half-reaction, for each half-reaction, we could get Ewe could get Eoo for net reaction. for net reaction.
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CELL POTENTIALS, ECELL POTENTIALS, Eoo
We can not measure 1/2 reaction Eo directly. Therefore, measure it
relative to a STANDARD HYDROGEN CELL.STANDARD HYDROGEN CELL.
2 H2 H++(aq, 1 M) + 2e- (aq, 1 M) + 2e- H H22(g, 1 atm)(g, 1 atm)2 H2 H++(aq, 1 M) + 2e- (aq, 1 M) + 2e- H H22(g, 1 atm)(g, 1 atm)
EEoo = 0.00 V = 0.00 V
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How do we do it?
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Volts
ZnH2
Salt Bridge
Zn2+ H+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
Volts
ZnH2
Salt Bridge
Zn2+ H+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
Overall reaction is reduction of HOverall reaction is reduction of H++ by Zn by Zn metalmetal
Zn(s) + 2HZn(s) + 2H++ (aq) (aq) Zn Zn2+2+ + H + H22(g)(g)EEoo=+0.76 =+0.76 VV
EEoo for for Zn Zn ZnZn2+2+(aq) + 2e(aq) + 2e-- is is +0.76 V+0.76 V
EEoo for for ZnZn2+2+ + 2e + 2e-- Zn Zn is is –0.76 V–0.76 V Zn Zn is a is a better better reducing agent than Hreducing agent than H22..
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Volts
CuH2
Salt Bridge
Cu2+ H+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
Volts
CuH2
Salt Bridge
Cu2+ H+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
Cu/CuCu/Cu2+2+ and H and H22/H/H++ Cell Cell
EEoo = +0.34 V = +0.34 V
AcceptoAcceptor of r of
electronelectronss
SupplieSupplier of r of
electronelectronssCuCu2+2+ + 2e- + 2e- Cu Cu
ReductionReductionCathodeCathode
HH22 2 H 2 H++ + 2e- + 2e-
OxidationOxidationAnodeAnode
PositivePositive NegativeNegative
What is happening here?
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TABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS
TABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS
2
Eo (V)
Cu2+ + 2e- Cu +0.34
2 H+ + 2e- H 0.00
Zn2+ + 2e- Zn -0.76
oxidizingability of ion
reducing abilityof element
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Standard Redox Potentials, EStandard Redox Potentials, Eoo
Any substance on the Any substance on the right will reduce any right will reduce any substance higher substance higher than it on the left.than it on the left.
• Zn can reduce HZn can reduce H++ and and CuCu2+2+..
• HH22 can reduce Cu can reduce Cu2+2+ but not Znbut not Zn2+2+
• Cu cannot reduce HCu cannot reduce H++ or Znor Zn2+2+..
Eo (V)
Cu2+ + 2e- Cu +0.34
2 H+ + 2e- H2 0.00
Zn2+ + 2e- Zn -0.76
oxidizingability of ion
reducing abilityof element
Eo (V)
Cu2+ + 2e- Cu +0.34
2 H+ + 2e- H2 0.00
Zn2+ + 2e- Zn -0.76
oxidizingability of ion
reducing abilityof element
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Predict the outcome of the following reaction. Classify each of the reactions as processes as product-favored or reactant-favored.
Fe(s) + Sn2+(aq)
O2(g) + Au(s)
Na(s) + H2O(l)
Al3+ (aq) + Hg(l)
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Summary Activity
A lead acid battery uses the following two reactions to provide a voltage:
Pb + SO42- PbSO4 + 2e- +0.36V
PbSO4 + 2H2O PbO2 + 4H+ + SO42- + 2e- -
1.70V
What is the overall reaction?
What is the overall standard cell potential?
Which reaction is the oxidation and which reaction is the reduction?
