Oxidation & ReductionElectrochemistry

BLB 10th Chapters 4, 20

Lab Summary

Oxidation and Reduction (redox) Activity series with three metals (Zn, Mg, Cu) Writing equations for reactions Electrochemical cells – standard & non-standard

(Assemble for E° > 0.) Writing equations for cells (Reactant metal is the

most reactive; product metal the least.) E° calculation (cathode – anode)

21.1, 4.4 Oxidation-Reduction Reactions

Oxidation Loss of electrons Increase in oxidation number Gain of oxygen or loss of hydrogen

Reduction Gain of electrons Decrease in oxidation number Loss of oxygen or gain of hydrogen

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

Oxidizing agent or oxidant – reactant that contains the element being reduced; is itself reduced

Reducing agent or reductant – reactant that contains the element being oxidized; is itself oxidized

Redox Reactions

Combustion, corrosion, metal production, bleaching, digestion, electrolysis

Metal oxidation Activity Series (Table 4.5, p. 143) Some metals are more easily oxidized and

form compounds than other metals. Displacement reaction – metal or metal ion is

replaced through oxidationA + BX → AX + B

20.3 Voltaic Cells

A spontaneous redox reaction can perform electrical work.

The half-reactions must be placed in separate containers, but connected externally.

This creates a potential for electrons to flow.

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

Line notation:Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)

Cu2+(aq) + 2 e¯ → Cu(s)Zn(s) → Zn2+(aq) + 2 e¯

Movement of Electrons

e¯

Net reaction: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

20.4 Cell EMF

EMF – electromotive force – the potential energy difference between the two electrodes of a voltaic cell; Ecell; measured in volts

E°cell – standard cell potential (or standard emf) For the Zn/Cu cell, E°cell = 1.10 V electrical work = Coulombs x volts

J = C x V

CJV

Standard Reduction (Half-cell) Potentials

E° - potential of each half-cellE°cell = E°cell(cathode) - E°cell(anode) For a product-favored reaction:

ΔG° < 0E°cell > 0

Measured against standard hydrogen electrode (SHE); assigned E° = 0 V.

Problem 28 Sketch and calculate the E°.

Voltaic cell with: Al(s) in Al(NO3)3(aq) on one side and a SHE on the other.

20.5 Spontaneity of Redox Reactions

ΔG° < 0 E°cell > 0

ΔG° for problem 28

ΔG° = wmax = −nFE°

n = # moles of e¯ transferred

F = 96,485 C/mol (Faraday constant)

wmax = max. work

20.6 Effect of Concentration on Cell EMF

Concentrations change as a cell runs. When E = 0, the cell is dead and reaches equilibrium. Nernst equation allows us to calculate E under

nonstandard conditions:

Qn

EEorQn

EE

KFR

QnFRTEE

molCKmol

J

log0592.0ln0257.0298@

485,963145.8

ln

Cell EMF and Equilibrium

When E = 0, no net change in flow of electrons and cell reaches equilibrium.

K of problem 28

0592.0log

0257.0ln

log0592.0ln0257.0

nEKornEK

and

Kn

EorKn

E