ElectrochemistryRedox reactions

neMMMneM

examples

treducoxidant

OxddOx

naanaa )()(

2121

:

tan

Re Re

electrochemical cellsgalvanic:

spontaneous chemical rxns to produce electrical energy (G = -nFE, negative)

electrolytic:utilisation of energy (ex: applied V) to force a chemical rxn to take place (G +)

Cellsreduction at the cathode oxidation at the anode

galvanic cells

- +

CathodeAnode

Cd(s) --> Cd 2+(aq) +2e -

Cd

e-

AgSalt bridge

Cl - K+

Cd2+

AgNO3(aq)

NO3-

V

CdCl2

2Ag +(aq) + 2e - -->2Ag(s)

 half-rxns

(aq)Cd+2Ag(s)Cd(s)+(aq)2Ag :net

_____________________________

2e+(aq)Cd(s) Cd :

2Ag(s)2e+(aq)2Ag :

+2+

-+2

-+

anode

cathode

line notation|interface between two phases.||salt bridge

Cd(s) | CdCl2(aq, M) || AgNO3(aq, M) | Ag(s)

Standard reduction potentialsTo predict the reactivity of oxidants or reductants we need to measure the

potential of each half-rxn.

 impossible!!....for every oxidation rxn we have a reduction reaction

Define a standard half-cell of potential = 0V

against which all other half-cell reduction potentials are measured. Each component in these standard cells having unit concentration.

 By convention: Standard (or Normal)

Hydrogen Electrode is usedPt(s) | H2(g) | H+(a) || Ag+(a) | Ag(s) |_______________|

NHEH+(aq) + e-<=>H2(g)E0=0V

- +e-

AgSalt bridge

AAg+=1

V=+0.799V

AH+=1

AH2=1

Electrochemical series  

half-rxns oxidant reductant E0 (V)

 

  stronger oxidant F2(g) + 2e- <=> 2F- 2.890 Ce4+ + e- <=> Ce3+ 1.720 Ag+ + e- <=> Ag(s) 0.799 Fe3+ + e- <=> Fe2+ 0.771 O2 + 2H+ + 2e- <=> H2O2 0.695 Cu2+ + 2e- <=> Cu(s) 0.339 2H+ + 2e- <=> H2(g) 0.000 Cd2+ + 2e- <=> Cd(s) -0.402 Zn2+ + 2e- <=> Zn(s) -0.762 K+ + e- <=> K(s) -2.936 Li+ +e- <=> Li(s) -3.040 stronger reducer

When Concentrations are not unity?

Nernst equation 

for a half-rxnaOx + ne- <=> bRed

 E = E0 – RT/nF ln{[Red]b/[Ox]a}

R= gas constant T= temperature in Kelvinn= no. of electrons in half-rxn F= Faraday constant

Converting ln to log10 (x 2,303) and at 25oC (298.15K) 

E = E0 – 0.05912/n log{[Red]b/[Ox]a}The cell potential

 

E = E+ - E-(E+ and E- calculated using the Nernst equation)

Applications of Galvanic CellsPotentiometry and Ion Selective Electrodes

the measure of the cell potential to yield chemical information (conc., activity, charge ....)

A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potential difference being created across that membrane

C a 2 + C a 2 + 0 . 0 1 M C a 2 +

0 . 0 2 M C l -

0 . 1 M C a 2 +

0 . 2 M C l -

( 0 . 0 1 + ) M C a 2 + ( 0 . 1 - ) M C a 2 +

0 . 0 2 M C l - 0 . 2 M C l -

+

+

+

+

-

-

-

-

Calcium selective molecular recognition ligand

G RT lnA1A2

nFE

E RT

nFlnA1A2

0. 05916

nlog

A1A2

(à 25C)

The glass pH electrode

int ref

int+H

analyte

ext ref

Ag(s)|AgCl(s)|(aq)-Clint),(aq,Hext)(aq,H||(aq)-Cl|AgCl(s)|Ag(s)

Ag

Soln. aq. satdin KCl + AgCl

AgCl(s) + KCl(s)

AgCl porousglass

+ -

0.1M HCl inAgCl sat.

CorrosionFe2+ +2e Fe E0=-0.44V

2H+ + 2e H2 E0=0V

2H2O + O2 =4e4OH- E0=1.23V

Iron is oxidized in water or humid conditions to give rust.

Inhibit this by coating with another material (Zn for example that forms a protective oxide on the iron), or by providing a sacrificial anode (b).

Batteries-providing electricity from chemistry

The Lead Acid Storage Battery was developed in the late 1800's and has remained the most common and durable of the battery technologies (in vehicles).

Lead-acid batteriesWhen the battery is used as a voltage supply, electrons flow from the Pb metal to the Pb(IV)oxide. The reactions aren't quite the reverse of the formation reactions, because now the sulfate ions in the solution begin to play a role. The two reactions are:

PbO2 + 4H+ + 2e + SO4-2 PbSO4 + 2H2O

Pb + SO4-2 PbSO4 + 2e

The overall reaction if we combine the hydrogen ions and the sulfate: PbO2 + Pb + 2H2SO4 2 PbSO4 + 2 H2O Lead sulfate is fairly insoluble so that as soon as Pb(II) ions are formed by either reaction, the ions immediately precipitate as lead sulfate. The beauty is that this lead sulfate stays attached to the grids so that it is there for recharging of the battery.

Other batteriesPrimary battery-non rechargeable

Longer shelf-life

Rechargeable

Offer higher efficiencies compared to burning fuels

Images of batteries

Leclanche Alkaline Fuel Cell

ElectrolysisUse Faraday’s Laws to evaluate the number of moles of a substance oxidised or reduced by passage of charge (current over a given period of time = I.t) through an electrode

Faraday: Q (charge) = nF

N=number of moles of electrons

F=constant of 96500 Coulomb/mole

Example (try it): What current is needed to deposit 0.500g of chromium from a solution containing Cr3+ over a one hour period (MW for Cr=52)? (Ans=0.77A)

Applications of Electrolytic Cells

Aluminium refining: The major ore of aluminium is bauxite, Al2O3.

Anhydrous Al2O3 melts at over 2000°C. This is too high to permit its use as a molten medium for electrolytic formation of free aluminium. The electrolytic process commercially used to produce aluminium is known as the Hall process, named after its inventor, Charles M. Hall. Al2O3 is dissolved in molten cryolite, Na3AlF6, which has a melting point of 1012oC and is an effective conductor of electric current. Graphite rods are employed as anodes and are consumed in the electrolysis process. The cell electrolytic reaction is:

2Al2O3 + 3C 4Al(l) + 3CO2(g)

Electrolysis of brineChlorine and sodium hydroxide are both manufactured by electrolysis of brine (aqueous sodium chloride) using inert electrodes. Chlorine is evolved at the anode, Cl- 1/2Cl2 + e

Hydrogen is evolved at the cathode: H+ + e 1/2H2

The removal of chloride ions and hydrogen ions leaves sodium ions and hydroxide ions in solution.

Chlorine is used to disinfect municipal water supplies and water in swimming pools. It is used to manufacture household bleaches and disinfectants. It is used to manufacture plastics (e.g. PVC), pesticides, anaesthetics, CFCs etc.

Sodium hydroxide is used in the manufacture of synthetic fibres, soaps and detergents.

ElectroplatingIn all aspects of our lives we are surrounded by products with electroplated surfaces. Whether we are looking at a silver-plated watch through gold-plated glasses, watching television, using the washing machine, getting into a car or boarding a plane: electroplating plays an important part in all of these situations. The objective is to prevent corrosion and wear,produce hardness and conductivity, and give products an attractive appearance.

Silver electroplating was the first large scale use of electrolysis for coating base metal objects with a higher value decorative finish.

The principle: thin metallic layers with specific properties are deposited on base materials including steel, brass, aluminium, plastic and die-cast parts.