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+ + Cus 2Ags + Cu2+

A “silver” xmas tree

Reaction: spontaneous

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

SHEanode

E0cell = ?

What is E0 for5Fe3+ + 5e- = 5Fe2+

givenFe3+ + e- = Fe2+ E0 = 0.771 V

a. 0.771b. 3.86c. 0.154d. 0

MnO4- + NO2

- Mn2+ + NO3-

What is oxidizing agent in the following equation

a. MnO4-

b. NO2-

c. Mn

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

Given the following cell, calculate E0cell

Ag l Ag+ (a = 0.02 M) ll Cu2+ (a = 0.02 M) l Cu

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