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
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