putting redox reactions to work. electrons are transferred lose electrons oxidation gain...
TRANSCRIPT
Putting redox reactions to work
Electrons are transferred
Lose Electrons Oxidation
Gain Electrons Reduction
Made of two half-cells
Based upon two half-reactions
Electrons travel between the two half-cells
Also called voltaic cells
Convert chemical energy into electrical energy
Spontaneous
Convert electrical energy into chemical energy
Non-spontaneous
Write the reaction for solid magnesium placed in a copper (II) sulfate solution.
Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s)
Balance the reaction using the half-reaction method
Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s) 0 +2+6 -2 +2+6 -2 0
Balance the reaction using the half-reaction method
Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s) 0 +2+6 -2 +2+6 -2 0
Mg (s) Mg2+ (aq) + 2e-
Cu2+ (aq) + 2e- Cu (s)
Balance the reaction using the half-reaction method
Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s) 0 +2+6 -2 +2+6 -2 0
1(Mg (s) Mg2+ (aq) + 2e-)
1(Cu2+ (aq) + 2e- Cu (s))
Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-
Anode
Cathode
Salt Bridge
Flow of electrons
Potential (either half-cell or cell)◦ Pull on the electrons◦ Electromotive force (emf)◦ Volt (V)◦ Joule/Coulomb (J/C)
Voltmeter◦ Analog◦ Digital
Potentiometer Positive potential…spontaneous Negative potential…nonspontaneous
Standard Reduction Potentials Chart◦ Only reduction reactions◦ Must look up the reverse of the oxidation and flip
the sign of the potential
Add standard half-cell potentials to get standard cell potential
1(Mg (s) Mg2+ (aq) + 2e-) E˚ox= +2.37 V
1(Cu2+ (aq) + 2e- Cu (s)) E˚red= +0.342 V
Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-
E˚cell = +2.71 V
Oxidation||Reduction
X(s)|X+(aq)||Y+(aq)|Y(s)
Mg(s)|Mg2+(aq)||Cu2+(aq)|Cu(s)
1(Mg (s) Mg2+ (aq) + 2e-)
1(Cu2+ (aq) + 2e- Cu (s))
Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-
Cu(s)|Cu2+(aq)||Ag1+(aq)|Ag(s)
1(Cu (s) Cu2+ (aq) + 2e-) E˚ox= -0.342V
2(Ag1+ (aq) + 1e- Ag (s)) E˚red= +0.800V
Cu(s)+2Ag1+(aq)+2e- Cu2+(aq)+2Ag (s)+2e-
E˚cell= +0.458V
Series of electrochemical cells connected to each other
Completes the circuit
Dry cell◦ Flashlight battery◦ Watch battery
Wet Cell◦ Car battery
Carbon-Zinc Battery◦ Zinc casing…anode◦ Carbon rod…cathode◦ MnO2 is actually reduced
◦ Alkaline battery…has KOH rather than NH4Cl
Carbon-Zinc Battery◦ Zn(s) Zn2+(aq) + 2e-
◦ 2NH41+(aq) + 2MnO2(s) + 2e- Mn2O3(s) + 2NH3(g) + H2O(l)
Lead-Acid Storage Battery Pb(s) + PbO2(s) + H2SO4(aq) PbSO4(s) +
H2O(l) Spontaneous & nonspontaneous
Not 1M
Require additional calculations
Can manipulate potential to a particular V
Nernst Equation◦ Ecell = E˚cell – {(0.0592/n)(logQ)}
n Q
A 0.500M solution of copper (II) sulfate is reacted with magnesium metal. A 0.750M solution of magnesium sulfate is one of the products. What is the cell potential?◦ Write two half reactions◦ Write balanced equation◦ Determine n◦ Determine E˚cell
◦ Use Nernst to solve for Ecell
Mg(s)|Mg2+(aq)||Cu2+(aq)|Cu(s)
1(Mg (s) Mg2+ (aq) + 2e-) E˚ox= +2.37 V
1(Cu2+ (aq) + 2e- Cu (s)) E˚red= +0.342 V
Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-
E˚cell = +2.71 V
Ecell = 2.71 V – {(0.0592/2)(log([0.75]/[0.5]))}
Ecell = 2.71 V – {(0.0296)(0.176)}
Ecell = 2.71 V – 0.00521
Ecell = 2.70 V
Cu(s)|Cu2+(0.0100M)||Ag1+(0.0250M)|Ag(s)