CHAPTER 17CHAPTER 17

Electrochemistry – part 2

Electrolysis and Electrolytic Electrolysis and Electrolytic CellsCells Anode: where oxidation takes place

◦ Anions are oxidized at this electrode◦ labeled positive to reflect anions attraction to

anode Cathode: where reduction takes places

◦ Cations are reduced at this electrode◦ Labeled negative to reflect the cations attraction to

cathode

Electrolysis and Electrolytic Electrolysis and Electrolytic CellsCells

• Electrolysis: The process of using an electric current to bring about chemical change.

• Process occurring in galvanic cell and electrolytic cells are the reverse of each other

• In an electrolytic cell, two inert electrodes are dipped into an aqueous solution

Electrolysis and Electrolytic Electrolysis and Electrolytic CellsCells

Electrolysis: The process of using an electric current to bring about chemical change.

Quantitative Aspects of Quantitative Aspects of ElectrolysisElectrolysis

Moles of e = Charge(C) ×

Charge(C) = Current(A) × Time(s)

96,500 C

1 mol e

Faraday constant

ExampleExample God can be plated out of a solution containing Au3+

according to the following half-reaction:

Au3+(aq) + 3e- Au(s)

What mass of gold (in grams) will be plated by the follow of 5.5A of current for 25 minutes?

ExampleExample Silver can be plated out of a solution containing Ag+

according to the following half-reaction:

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

How much time (in minutes) would it takes to plate 12.0 g of silver using a current of 3.0A?

Led-acid storage batteriesLed-acid storage batteries Consists of six cells wired in series. Each cell contains a porous lead anode and a lead

oxide cathode, both immersed in sulfuric acid. An electric current is drawn from the battery, both the

anode and cathode become coated with PbSO4(s) Can be recharged by running electric current through

it in reverse direction

BatteriesBatteriesLead Storage Battery

2PbSO4(s) + 2H2O(l)Pb(s) + PbO2(s) + 2H+(aq) + 2HSO41(aq)

PbSO4(s) + 2H2O(l)PbO2(s) + 3H+(aq) + HSO4(aq) + 2e

PbSO4(s) + H+(aq) + 2ePb(s) + HSO4(aq)

Overall:

Anode:

Cathode:

Dry-Cell BatteriesDry-Cell Batteries Zinc acts as the anode and a graphite rod immersed

in a moist, slightly acidic pasted of MnO2 and NH4Cl acts a cathode.

Mn2O3(s) + 2OH(aq)2MnO2(s) + H2O(l) + 2e

ZnO(s) + H2O(l) + 2eZn(s) + 2OH(aq)Anode:

Cathode:

BatteriesBatteriesDry-Cell Batteries

Leclanché cell

Mn2O3(s) + 2NH3(aq)+ H2O(l)2MnO2(s) + 2NH4+(aq) + 2e

Zn2+(aq) + 2eZn(s)Anode:

Cathode:

BatteriesBatteries

Nickel-Metal Hydride (“NiMH”) Batteries

Nickel-Cadmium (“ni-cad”) Batteries

Ni(OH)2(s) + OH(aq)NiO(OH)(s) + H2O(l) + e

Cd(OH)2(s) + 2eCd(s) + 2OH(aq)Anode:

Cathode:

M(s) + Ni(OH)2(s) MHab(s) + NiO(OH)(s)

Ni(OH)2(s) + OH(aq)NiO(OH)(s) + H2O(l) + e

M(s) + H2O(l) + eMHab(s) + OH(aq)

Overall:

Anode:

Cathode:

BatteriesBatteries

Lithium and Lithium Ion Batteries

LiCoO2(s)Li1-xCoO2(s) + xLi+(soln) + xe

xLi+(soln) + 6C(s) + xeLixC6(s)Anode:

Cathode:

LixMnO2(s)MnO2(s) + xLi+(soln) + xe

xLi+(soln) + xexLi(s)Anode:

Cathode:

Lithium Ion

Lithium

Fuel cellsFuel cells

Like batteries, but the reactants must be constanly replenished.

Normal batteries los their ability to generate voltage with use because the reactants become depleted as electric current is drawn from the battery.

In fuel cell, the reactant – the fuel-constanly flow through the battery, generating electric current as they undergo redox reaction.

Hydrogen-Oxygen Fuel Cell

CorrosionCorrosion

Moisture must be present for rusting to occur

Additional electrolytes promote more rusting◦ Such as NaCl, on the

surface of iron because it enhances current flow

The presence of acid promotes rusting. (H+ ions are involved in the reduction of oxygen, lower pH enhances the cathodic reaction and leads to faster rusting.

Corrosion: The oxidative deterioration of a metal.

Preventing CorrosionPreventing Corrosion Keep dry Coat the iron with a substance that is impervious to

water◦ Painting

Placing a sacrificial electrode in electrical contact with the iron.

For some metals, oxidation protects the metal (aluminum, chromium, magnesium, titanium, zinc, and others).

CorrosionCorrosionPrevention of Corrosion

1. Galvanization: The coating of iron with zinc.

When some of the iron is oxidized (rust), the process is reversed since zinc will reduce Fe2+ to Fe:

Fe(s)Fe2+(aq) + 2e

Zn(s)Zn2+(aq) + 2e E° = 0.76 V

E° = 0.45 V

CorrosionCorrosionPrevention of Corrosion

Attaching a magnesium stake to iron will corrode the magnesium instead of the iron. Magnesium acts as a sacrificial anode.

Mg2+(aq) + 2eMg(s)Anode:

Cathode: 2H2O(l)O2(g) + 4H+(aq) + 4e E° = 1.23 V

E° = 2.37 V

2. Cathodic Protection: Instead of coating the entire surface of the first metal with a second metal, the second metal is placed in electrical contact with the first metal:

Molten salt- mixture of Molten salt- mixture of cations and anionscations and anions In general:

◦ The cation that is most easily reduced (the one with least negative, or most positive, reduction-half cell potential) is reduced first

◦ The anion is most easily oxidize ( the one has the least negative, or most positive, oxidation half-cell potential) is oxidized first

The cations of active metals-those that are not easily reduced, such as Li+, K+, Na+, Mg2+, Ca2+, and Al3+

- Cannot be reduced from aqueous solution by electrolysis because water is reduced at lower voltage.

Electrolysis and Electrolytic Electrolysis and Electrolytic CellsCells

Electrolysis of Molten Sodium Chloride

2Na(l) + Cl2(g)2Na+(l) + 2Cl(l)

2Na(l)2Na+(l) + 2e

Cl2(g) + 2e2Cl(l)

Overall:

Anode:

Cathode: