chapter 17 electrochemistry – part 2. led-acid storage batteries consists of six cells wired in...

Electrochemistry – part 2

Led-acid storage batteriesConsists 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

Lead 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 BatteriesZinc acts as the anode and a graphite rod

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

Dry-Cell Batteries

Alkaline cell

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

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

Cathode:

Dry-Cell Batteries

Leclanché cell

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

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

Cathode:

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:

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 cellsLike 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

Corrosion: The oxidative deterioration of a metal.

CorrosionMoisture must be present for rusting to occurAdditional 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.

Preventing CorrosionKeep dryCoat the iron with a substance that is

impervious to waterPainting

Placing a sacrificial electrode in electrical contact with the iron.

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

Prevention of Corrosion

1. Galvanization: The coating of iron with zinc.

Prevention 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

Prevention 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:

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

Electrolysis and Electrolytic Cells• 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

Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 17/18

Electrolysis and Electrolytic CellsAnode: where oxidation takes place

Anions are oxidized at this electrodelabeled positive to reflect anions attraction to

anodeCathode: where reduction takes places

Cations are reduced at this electrodeLabeled negative to reflect the cations

attraction to cathode

Molten salt- mixture of cations and anionsIn 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

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:

In aqueous solutionsThe 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 of Aqueous Sodium Chloride

Cl2(g) + H2(g) + 2OH(aq)2Cl(l) + 2H2O(l)

H2(g) + 2OH(aq)2H2O(l) + 2e

Cl2(g) + 2e2Cl(aq)

Overall:

Anode:

Cathode:

ExamplesPredict the half-reaction occurring at the

anode and the cathode for electrolysis of the following:a mixture of molten AlBr3 and MgBr2An aqueous solution of LiI

Down’s Cell for the Production of Sodium Metal

A Membrane Cell for Electrolytic Production of Cl2 and NaOH

© 2012 Pearson Education, Inc.Chapter

17/27

Hall-Heroult Process for the Production of Aluminum

© 2012 Pearson Education, Inc.Chapter

17/28

Electrorefining of Copper Metal

Moles of e = Charge(C) ×

Charge(C) = Current(A) × Time(s)

96,500 C

1 mol e

Faraday constant

ExampleGod 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?

ExampleSilver 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?