Post on 20-Aug-2015
Need to KnowWhat is oxidation/reductionWhat is oxidizing or reducing agentOxidation NumbersBalancing redox reactions
Focus: Electrochemistry as a Chemical Analysis Tool
MnO4- + NO2
- Mn2+ + NO3-
What is reduced in the following equation
a. Mnb. Nc. O
Split into two reactionsCe4+ + e- = Ce3+ (Reduction)Fe2+ = Fe3+ + e- (Oxidation)
Electrochemical CellsGalvanic cell - cell in which the reactions occur
spontaneously to produce electrical energy
Electrolytic cell – electrical energy used to force a non-spontaneous reaction to occur
Cathode: where reduction takes placeAnode: where oxidation takes place
memorize
Two ways to make an electrochemical cell
1- all in same container
2- separate containers : half cellsconnect with bridge
Copper – oxidizedsoln blue
Silver – reducedCu turns grey
Piece of Cu ina soln of AgNO3
2Ag+ + 2e- 2AgsCds Cd2+ + 2e-anode cathode
Balanced rxn = ?
G0 = - nFE0 E0 = E0cath – E0
anode
E0 = ?
Reaction proceeds until “chemical equilibrium”
Cu + 2Ag+ Cu2+ + 2Ag
E0 = ?
From skoog, west, holler etal
Can not measure the potential of any half-cell rxn
Can measure Potential Difference
Establish a reference electrodetabulate wrt that half cell
Half cell Ref: SHE or NHE
1. Make SHE the anode
2. Make the other cell the cathode
3. Measure potential difference
2H+ + 2e- H2(g) E0 = 0.00 Vmemorize
E0 cell = E0 cathode - E0 anode
If E0 anode is 0then E 0
cell = E 0cathode
All reactions must be written as reduction rxns
Important Points:
Simplified Cell Schematic
Anode ll Cathode
E0 cell = E0 cathode – E0 anode
All rxns written as reduction
ΔG0 = - nFE0
memorize
memorize
ΔG0 is negative: spontaneous
ΔG0 is positive: non-spontaneous
Which one of the following would be the best oxidizing agent
a. Ag+
b. H+
c. Cd2+
d. Zn2+
Ag+ + e- = Ags E0 = 0.799 V2H+ + 2e- = H2(g) E0 = 0.00VCd2+ + 2e- = Cds E0 = -0.403Zn2+ + 2e- = Zns E0 = -0.763
Write a simplified cell schematic for the followingunbalanced redox reaction (std conditions)
Ni2+ + Crs Cr3+ + Nis
If E0 (Ni2+/Ni) is -0.25 V and E0 (Cr3+/Cr) is -0.74, what is E0
cell?
Ni2+ + Crs Cr3+ + Nis
Cell Potential is Concentration Dependent
Nernst Equation
ba
dc
BA
DC
nF
RTEE
][][
][][ln0
Nernst Equation
Nernst Equation:
at 250C
ba
dc
BA
DC
nEE
][][
][][log
0592.00
At Equilibrium
0592.0
0
10cellnE
K E0
cell: units of Vn: total #e-
memorize
memorize
What is the total number of electrons transferred in the following unbalanced redox rxn
MnO4- + NO2
- = Mn2+ + NO3-
a. 2b. 4c. 5d. 7e. 10
Calculate the equilibrium constant for the followingredox reaction given E0 (MnO4
-/Mn2+) is 1.51 V and E0
(Fe3+/Fe2+) is 0.771 V.
MnO4- + 5Fe2+ + 8H+ Mn2+ + 5Fe3+ + 4H2O
What is the concentration of aluminum ions if the cell potentialIs 0.47 V and the Mn2+ concentration is 0.49 M.
E0 (Mn2+/Mn) is -1.18 V and E0 (Al3+/Al) is -1.66 V.
2Als + 3Mn2+ 2Al3+ + 3Mns