Chemistry Lesson23 ( Electrochemistry3)

8/2/2019 Chemistry Lesson23 ( Electrochemistry3)

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FORM 4 Chemistry Lesson 23

Chapter 1 Introduction to Chemistry

Chapter 2 The Structure of Atom

Chapter 3 Chemical Formulae and Equations

Chapter 4 Periodic Table and Elements

Chapter 5 Chemical Bonds

Chapter 6 Electrochemistry

Chapter 7 Acids and Bases

Chapter 8 Salts

Chapter 9 Manufacture Substances in Industry


2/23

WHATISITALLABOUT?

Electrolytes

Electrolysis what, how?

- applications

Voltaic cells

types Electrochemical series

Electrons movement, voltage, metal displacement


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ELECTROLYTESVS NON-ELECTROLYTES

Electrolytes Non-electrolytes conduct electricity inmolten state or in aqueoussolution

chemical changes occur

cannot conduct electricityin any states

Electrolyte vs Conductor??

Conductors conduct electricity as well BUT are notchemically changed NOT electrolytes

An electrolyte is NOT NECESSARILYa conductor


4/23

EXAMPLESOF ELECTROLYTES Acids, alkalis & salt in their molten state or in solutions

Ionic compounds conduct electricity ONLY in their moltenstates or in solution

Covalent compounds (HCl, NH3) ONLY conduct electricity inwater

Solid state held in lattice Molten state free to move

In organic solvent in molecules In water molecules break up into free ions


5/23

ELECTROLYSIS

Decomposition of compounds into their constituentelements in presence of an electric current through

an electrolyte

Discovered by Michael Faraday in 1834

An electrolytic cell consists:

1. Battery drive chemical reactions

2. & 3. Electrodes one acts as positiveterminal & the other acts as negativeterminal

4. Electrolyte in molten state or insolution to allow free ions movement


6/23

HOWDOESANELECTROLYTICCELLWORK?

e-

e-

e-

e-

e-e-

e-

e-e-

e-

--

-+

++

+

+

-

-

Electric current drive the electron movementfrom anode to cathodeanode positively charged terminalcathode negatively charged terminal

Movement of ions: cations to cathode

anions to anode

What happen next?

Cations being discharged toatoms/molecules by accepting electrons fromcathode

anions being discharged to atoms/moleculesby donating electrons to anode

electrons continue to flow from anode tocathode via external circuit


7/23

ELECTRODES..

Plays important roles in electrolytic cells 2 types:

Active electrode

Participate in chemical reactions during electrolysis

E.g.: copper, silver, mercury

Inert electrode

DOES NOT participate in chemical reactions duringelectrolysis

E.g.: carbon (graphite), platinum


8/23

ELECTROLYSISIN MOLTEN COMPOUNDS

Free ions available: Pb2+ (l) and Br- (l) Half equations at anode: 2Br- (l) Br2 (g) + 2e-

cathode : Pb2+ (l) + 2e- Pb (s)__________

overall equation : Pb2+ (l) + 2Br- (l) Pb (s) + Br2 (g)

Carbonelectrode

Carbonelectrode

Molten Lead (II) bromide


9/23

ELECTROLYSISOF AQUEOUS SOLUTIONS

An aqueous solution solution of a compound that dissolvedin water

Aqueous solution that becomes an electrolyte also causespartial dissociation of water molecules

H2O (l) H+ (aq) + OH- (aq)

So electrolysis of an aqueous solution involve movements ofFOUR ions: 2 from compound constituents; 2 from watermolecule


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ELECTROLYSISIN AQUEOUS SOLUTION

Free ions available: Cu2+ (aq) and SO4

2- (aq) ; H+ (aq) and OH- (aq)

Half equations at anode: 4OH- (aq) O2 (g) + H2O (l) + 4e-

cathode : 2Cu2+ (aq) + 4e- 2Cu (s)__________

overall equation : 2Cu2+ (aq) + 4OH- (aq) 2Cu (s) + O2 (g) + H2O (l)


11/23

DOYOUNOTICETHAT?

only ONE type of ions are selected to be discharged Why?

Positions of ions in electrochemical series

Concentration of ions in electrolyte

Types of electrodes used in electrolysis Active vs inert electrode


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ELECTROCHEMICALSERIES List of ions arranged in ascending order of their

tendency to dischargeCations Anions

K+ F-

Na+ SO42-

Mg2+ NO3-

Al3+ Cl-

Fe2+ Br-

Sn2+ I-

Pb2+ OH-

H+

Cu2+

Ag+

Easier to

discharge


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CONCENTRATIONOFIONSINASOLUTION An ion is discharged in preference when the concentration

of that ion is high in electrolyte Pb2+ (aq) is discharged in a concentrated lead(II) nitrate solution

although H+ (aq) has lower position in electrochemical series

Cl- (aq) is discharged in a concentrated sodium chloride solutionalthough OH- (aq) has lower position in electrochemical series

HOWEVER, the selectivity is still biasedto electrochemicalseries, ESPECIALLY when the positions of ions involvedare far apart OH- (aq) is discharged in a concentrated lead(II) nitrate solution

instead of NO3- (aq) H+ (aq) is discharged in a concentrated sodium chloride solution

instead of Na+ (aq)


14/23

ELECTRODEUSEDIN ELECTROLYSIS

Case One: electrolysis of copper (II) sulphatesolution with copper electrodes Anode:

OH- (aq) & SO42- (aq): Copper electrode dissolves to form

Cu2+ (aq); no anions were discharged

Cathode:

Cu2+ (aq) & H+ (aq) : Cu2+ (aq) was discharged to formCu (s)

Observations:

Anode electrode becomes thinner while cathodeelectrode becomes thicker

Concentration of electrolyte is unchanged


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ELECTRODEUSEDIN ELECTROLYSIS

Case Two: electrolysis of saturated sodiumchloride solution with graphite anode & mercurycathode Anode:

OH-

(aq) & Cl-

(aq): Cl-

(aq) was discharged to formchlorine gas due to concentration factor

Cathode:

Na+ (aq) & H+ (aq) : Na+ (aq) was discharged to form Na(s) then formed Na/Hg (l) amalgam

Observation:Pungent greenish-yellow gas was formed on anode while

shiny liquid was formed on cathode


16/23

ELECTROLYSISIN INDUSTRY

Extraction of metals

Purification of metals

Electroplating of metals


17/23

EXTRACTIONOFMETALS

Extraction of metals from their ores

2 examples:

Extraction of aluminium from molten aluminium oxide

Extraction of sodium from molten sodium oxide


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EXTRACTIONOFALUMINIUM

Cryolite is added to lower melting point of aluminium oxide (2405 900oC)

Cathode : Al3+ (l) + 3e- Al (l)Anode : 2O2- (l) O2 (g) + 4e-

Overall : 2 Al2O3 (l) 4Al (l) + 3O2 (g)

Also, liberated oxygen gas react with carbon electrode (anode) to form carbondioxide

C (s) + O2 (g) CO2 (g)

Molten aluminium oxidewith cryolite, Na3AlF6

Molten aluminium


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EXTRACTIONOF SODIUM

cathode: steel

Na+

(l) +e-

Na (l)anode: graphite

2Cl- (l) Cl2 (g) + 2e-

Overall: 2NaCl (l) 2Na (l)+ Cl2 (g)

CaCl2 is added to lower meltingpoint of NaCl (900600oC)

+ CaCl2


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PURIFICATIONOFMETALS

To purify the extracted metals from minor impurities

Impure metal: anode

Pure metal : cathode

Electrolyte : aqueous salt solution of metal

Anode : Cu (s) Cu2+ (aq) + 2e-

Cathode : Cu2+ (aq) + 2e- Cu (s)

Observations:

impure copper becomes thinner

pure copper becomes thicker

impurities fall to the bottom of beaker


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ELECTROPLATINGOFMETALS

A process of coating a layer of metal on another substanceusing electrolysis

Objects are electroplated for protection from corrosion

In electrolysis: Cathode : object that is being coated

Anode : plating material

Electrolyte : ions of plating material

Other considering factors for good quality plating:

Object to be plated must be clean & free of grease

Concentration of electrolyte must be low

Small electric current

Object must be turned steadily


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EXAMPLEOFELETROPLATING

PROCESS

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

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

Observations:

Silver electrode becomes thinner

the spoon is coated with a silver layer

anodecathode


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PROS & CONSOF ELECTROLYSISIN INDUSTRIES

Advantages Disadvantages

Pure metals can be

obtained from ores

May cause pollution

Purification of metals canbe done

Expensive process requires energy input

Coating of object is easilydone

Chemistry Lesson23 ( Electrochemistry3)

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