Group 13 Aluminium

8/6/2019 Group 13 Aluminium

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AluminiumSymbol : Al

Proton number : 13

Electronic configuration : 2.8.3

Melting point : 660 oC

Boiling point : 2450 oC

Density : 2.70 g cm-3

Standard electrode potential : -1.66 VElectronegativity : 1.5

Lattice structure : Hexagonal close-pack


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Most abundant metal on earth

3rd most abundant element on earth

Makes up about 8% of the Earths crust Very reactive element, hence does not occur naturally in the

uncombined state

Ores containing aluminium:

Bauxite Al2O3.2H2OGibbsite Al(OH)3

Diaspore AlO(OH)

Corundum Al2O3

Cryolite Na3AlF6

Kaolinite Al2Si2O7.2H2O

Feldspar KAlSi3O8


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Extraction of

A l u m I n I u m


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Purification of Bauxite


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Electrolysis of Bauxite


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Use this

a molten mixture of

cryolite (sodium

hexafluoroaluminate)with calcium and

aluminium fluoride

Melting point of aluminium oxide: 2050 oC

Melting point of aluminium oxide + molten cryolite: 900 oC - 950 oC

LOWER THE MELTING POINT OF ALUMINIUM OXIDE!


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Hall-Heroult Process


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At the anode (graphite):

Oxide ions are discharged

~ forms oxygen gas~ oxygen liberated reacts with carbon electrode

~ produces carbon dioxide and carbon monoxide

~ anode is corroded slowly and must be replaced from time to time

At the cathode (graphite):

Al3+ is reduced to liquid aluminium

~ melting point = 660 oC

~ denser than the electrolyte and sinks to the bottom of the cell~ drains off into moulds to cool

The overall chemical reaction is:


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Note:Very high current (about 40,000 A) is used to maintain the temperature of the

electrolyte at 900 oC to prevent it from freezing

Low voltage (about 4.0 V) is used to prevent the fluoride ions from being

discharged. Fluorine and fluorine containing compounds are poisonous


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Aluminium chloride: White solid at room conditions

Sublimes at 180 C

Soluble in water and organic solvents

Fumes in moist air

Covalent compound


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Exist as a dimer at room conditions.

When heated. It will sublime as the dimer.

Dimer starts to disso

ciate at >400

Cto

AlC

l3 (g)

Complete dissociation at 800 C. *No MORE DIMER!!!!*


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Undergoes hydrolysis in water.

Produces acidic solution

Al2Cl6 + 6H20 2Al(OH)3 + 6HCl

Anhydrous aluminium chloride is a catalyst.

WHEN??????

Benzene ring electrophilic substitution.


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PREPARATIONOF ALUMINIUM CHLORIDE

Dry chlorine gas/ HCl gas pass over heated

aluminium powder.

2Al(s) +3Cl2(g) Al2Cl6(s) (white solid when cool)


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Al ion has HIGHCHARGE DENSITYand empty orbitalsof valence shells.

Thus it can form complexes by receiving lone pair

from ligands.


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FACTS on ALUMINIUM!!!

Most abundant metal on the earths surface

Light

Resistant to corrosion

100% recyclable and recycling process uses very little energy

One of the most difficult metal to extract, despite its

abundance


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Used in:

High tension cables high electrical conductivity. Light.

Firefighter suits good thermal insulator. (Reflect heat)

DID you know???

Doctors wrap premature babies in foil to keep baby warm and

prevent heat loss from body.

Used in extractionofCHROMIUM and MANGANESE

(Thermite process) aluminium has high affinity for oxygen.

Alum K2SO4.Al2(SO4)3.24H2O. Cleans water.

Precipitates suspended particles.


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Make CERAMICS: hard and stable even at high temperature

Cans// ThinSheets// Wires: very malleable and ductile


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ANODISATION

Aluminium is a REACTIVEMETAL (based on E.C.S)

- It will react actively with O2 and dilute acid

- This will then form an OXIDE layer on metal surface

ANODE: 4OH- 2H20 +O2+ 4e

Oxygen liberated oxidises aluminium to aluminium oxides.


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Function of anodisation:

- Used in industries. To FURTHERTHICKENthe aluminium

- Protects aluminium from further attack

- Also trap dyes: colourful!!! due to oxide layer. Waterproof.


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Chemical

Properties of

Aluminum


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Aluminum is a strong reducing agent and a

reactive metal

proof .^

~ large ve value in the standard electrodepotential (-1.66V)

The electron configuration

aluminum is 1s22s22p63s23p.

The outer three electrons occupy three s2p hybrid orbital that point in orthogonal

directions.

These electrons easily form covalent bonds, as in anhydrous AlCl3.


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Reaction with non-metallic elements

Aluminum reacts directly when heated!

e.g. 4Al(s)+ 3O2(g)>>>>>2Al2O3(s)

2Al(s)+3S(s)>>>>>>>Al2S3(s)

2Al(s)+N2(g)>>>>>>>2AlN(s)

4Al(s)+3C(s)>>>>>>>Al4C3(s)

2Al(s)+3Cl2(s)>>>>>>Al2Cl6(s)


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Reaction with sulphuric acid

Aluminium reacts with hot, dilute/concentrated sulphuric (VI) acid, liberating sulphur

dioxide gas

Reaction with dilute sulphuric acid is slow.

2Al(s)+6H2SO4(aq)>>>>>>Al2(SO4)3(aq)+ 3SO4(g)= 6H2O(l)


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Reaction of aluminium with air

Aluminium is a silvery white metal.

The surface of aluminium metal is covered with a thin layer of oxide thathelps protect the metal from attack by air.

If the oxide layer is damaged, the aluminium metal is exposed to attack.

Aluminium will burn in oxygen with a brilliant white flame to form thetrioxide alumnium(III) oxide, Al2O3.

4Al(s)+ 3O2(l) 2Al2O3(s)

Same goes to water.


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Reaction of aluminium with the halogens

Aluminium metal reacts vigorously with all the halogens to form aluminium halides.

It reacts with chlorine,Cl2 forming aluminium(III) chloride, AlCl3,

2Al(s)+ 3Cl2(l) 2AlCl3(s)

It reacts with Bromine,Br2 forming aluminium(III) bromide, AlBr3

2Al(s)+ 3Br2(l) 2AlBr3(s)

It reacts with iodine, I2, forming aluminium(III) iodide, AlI3.2Al(s)+ 3l2(l) 2All3(s)


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Reaction of aluminium with bases

Aluminium dissolves in sodium hydroxide with the evolution of hydrogen gas, H2, and

the formation of aluminates of the type [Al(OH)4]-.

2Al(s)+ 2NaOH(aq)+ 6H2O 2Na+(aq)+ 2[Al(OH)4]- + 3H2(g)


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Bonding inAluminum

compounds


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Generally, aluminum forms ionic compounds with

covalent character.

reason: Al3+ ions have a higher charge density which

can distort the electron cloud of the neighboring

anion.

degree of covalent character

Polarising effect Polarisibility of anion of cationelectron cloud

Cation highly anion is large

charged has stronger and highly chargedPolarizing effect

easily polarized


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For halide ions, polarisibility of an electron cloud

increases

Tendency to form covalent compound increases in

the following

F-


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Gen

erally, to determin

e the bon

d formed is ion

ic,covalent or polarlised, consider the elements

electronegativity

Difference of electronegativity, @E between the

bonded elements is

a)@E=0, the bond is covalent bond

b)0


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Aluminum oxide Al2O3

Aluminum oxide is a white solid with high

melting point of 2050C

INDICATION!

-Indicates bonding betweenAl3+ and O2- ions are

very strong

-lattice energy ofAl2O3

-is very high due to high charge and very small ionic

radius.

-RESULT!

-Al2O3 is insoluble in water

-Al2O3 has high melting point.

Corundum from crystalline Al2O3


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Covalent character in ionic bond causes aluminum

oxide to be amphoteric

Aluminium oxide forms salt with acid and alkalis

Al2O3(s)+ 6HCl (l) 2ALCl3(aq)+3H2O(l)

Al2O3(s)+2NaOH(aq)+3H2O 2NaAl(OH)4(aq)

Corundum from crystalline Al2O

3


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Uses ofAl2O3

a) Dehydration of alcohols to alkene

CH3CH2OH(l) C2H4(g)+H2O

Ethanol Ethene

b)Cracking of alkanes in petroleum industry

C6H14 C2H4(g)+C4H10(l)Hexane Ethene Butane

c)Refractory lining in the furnace

d)A

s an

adsorbin

g agen

t in

column

chromatography

Corundum from crystalline Al2O

3

350C


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Aluminum Hydroxide Al(OH)3

Aluminum hydroxide is a white solid

- Prepared by adding excess aqueous ammonia tosolution containing Al3+ ions

NH3+H20 NH4++OH-

Al3++3H2O Al(OH)3


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Aluminum oxide is amphoteric

- It dissolves in both acid and alkalis

Al(OH)3(s)+ 3H+(aq) NH4++OH-

Al3++ 3OH Al(OH)4-


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IfCO2 is passed through the aluminate solution, Al(OH)3

is reprecipitated

Prove: aluminum hydroxide is a weak acid

2Al(OH)4-(aq)+CO2(g) 2Al(OH)3(s)+CO3(aq)+H2O(l)


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application of aluminum hydroxide

Used as a morda

nt in

dyein

g in

dustry to help dye biteup the fabric

Making water-proof clothing


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Question time!

Remember!You get 1 mark for each

c

orrec

t answer!


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1. What is the function of anodisation?

- Forms oxide layer to protect the aluminium

from further attack

- Traps dyes (waterproof)

- Increase thickness of aluminium


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2. What form is aluminium chloride at room

temperature?

- Solid form. It is a white solid.


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Aluminium is resistant to corrosion because

A) The presence of impervious oxide layer on the surface of

aluminiumB) Aluminium oxide is porous

C) The ionisation energy of aluminium is very high

D) Aluminium is less reactive

Answer:

A)

Corrosion is the oxidation of a metal by moisture and air. The presenceof non-porous oxide layer on the surface of aluminium shields the

aluminium from attack by moisture and air.


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In the extraction of aluminium from bauxite, a current of 40000 A is used.

The function of the high current is to

A) Prevent the discharge of sodium ions from the dissociation ofcryolite

B) Increase the yield of aluminium

C) Keep the temperature of the electrolysis cell at 900 oC

D) Increase the rate of electrolysis

Answer:

C)

At temperature lower than 900o

C, the electrolyte will freeze. The highcurrent is necessary to generate heat to keep the temperature of the

electrolyte at 900 oC


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Thank you!

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Group 13 Aluminium