C6 - Electrolysis

6.1 intro to electrolysis

breaking down of ionic compounds using electric current

the compound broken down is the ELECTORATE

setting up

2 ELECTRODES that dip into electorate, with gap between them

ELECTRODES are conducting rods

ANODE - connects to POSITIVE terminal of power supply

CATHODE - connects to NEGATIVE terminal of power supply

made of unreactive/INERT substances (GRAPHITE / PLATINUM METAL)

so that it doesn't react with electorate of products made

during electrolysis

negative charged ions move to anode

positively charged ions move to cathode

when the ions reach the electrodes they loose their charge and become elements

the electrodes may give of gas or deposit metals

depends on compound used and whether it is molten or dissolved in water

MICHAEL FARADAY was 1st person to explain, 200 years ago

EXAMPLE

Capture

zinc chloride --> zinc + chlorine ZnCl2(l) --> Zn(s/l) + Cl (g)

zinc chloride is ionic compound

ionic compounds dont conduct electricity when solid (ions in FIXED position in giant lattice)

once melted, ions are free to move around within the hot liquid and carry their charge towards electrodes

the positive zinc ions (Zn2+) move towards cathode

negative chlorine ions (Cl1-) move towards anode

state symbols

zinc chlorine is molten, liquid at the temperature in evaporating dish

solid zinc coats tip of 1 electrode (melts if temp reaches 420c)

chorine gas is given off at other electrode

electrolysis of solutions

many ionic substances have very high melting points

takes lots of energy to melt them and free ions so they can move to electrodes

some dissolve in water, allowing ions to move around

when electrolysing ionic compounds that have been dissolved, it is more difficult to predict what will be formed

due to water also creating ions

only metals of very low reactivity (below hydrogen in reactivity series) are deposited form their aqueous solutions

copper bromide --> copper + bromide

copper ions move to cathode

bromide ions move to anode

copper bromide is split into its elements at the elctrodes

6.3 the extraction of aluminum

uses of aluminum

pans

overhead power cables

planes

foil

cans

window and door frames

bikes and car bodies

aluminum is very reactive

less than magnesium

more than zinc and iron

to extract aluminum - carbon is not reactive enough so we use electrolysis

compound electrolyzed is ALUMINUM OXIDE Al2O3

found in ORE - BAUXITE

mined by open cast mining and digging directly from surface

Contains mainly Al2O3

needs to be separated from ore

sometimes contains other rock impurities

contain lot of iron oxide

makes waste solution rusty brown

has to be stored in large lagoons

electrolysis of aluminum oxide

requires lots of energy and electrcity

once purified, Al2O3 must be melted

allows ions to move to electrodes

aluminum has high melting point 2050c

looses energy

chemist found way to save energy

mix Al2O3 with molten cryolite

ionic compound

can be electrolyzed at 850c

kept molten by the electrical energy in electrolysis cells

overall reaction :

Aluminum Oxide --> aluminum + oxygen

2Al O --> 4Al + 3O

2

3

2

half equations

cathode -

Al ions gain 3 electrons

turn into Al atoms

ions are reduced (gain electrons)

Al + 3e --> Al

3+

_

Al metal formed is molten at temp of cell and collects at bottom

Siphoned or tapped off

anode +

each O ion looses 2 electrons

turn into O atoms

O ions are oxidized (lose electrons)

2O --> O +4e

2-

2

_

oxygen reacts with carbon anode

produces carbon dioxide

carbon anodes burn away, have to be replaced

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