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Electrolysis uses an electric current to break down compounds which containions, producing elements.
The basic idea is a simple one:
when atoms lose electrons they become positive ions (this happens to metals,
and to hydrogen). During electrolysis the positive ions may be given electrons,
turning them back into atoms.
when atoms gain electrons they become negative ions (this happens to non-
metals). During electrolysis, the negative ions may give up their electrons,
turning them back into atoms also.
Remember: this only works for ionicsubstances not giant covalent or
simple molecular.
In addition, the ions must be free to
moveso it doesnt work with solidionic substances because the ionsare fixed in the giant ionic lattice.The substance must be molten or
dissolved in solution.
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The ionic substance we want to break down is called the electrolyte.
We dip two electrodes into the molten or dissolved electrolyte, and these areconnected to a source of electric current. It is these electrodes which will supply or
remove electrons from the ions in the electrolyte.
Like the ions, the electrodes also have a charge. The electrode connected to thepositive side of the power supply is positively charged, and called the ANODE.
The electrode connected to the negative supply is called the CATHODE and isnegatively charged.
Remember that like charges repel, and opposite charges attract ?
The positive ions in the electrolyte are attracted to the negative
electrode (CATHODE) where they can gain electrons.
The negative ions in the electrolyte are attracted to the positiveelectrode (ANODE) where they can give up electrons.
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Electrolysis experiments:
+ -+ -anode
cathode
molten electrolyteelectrolyte solution
source of electric current
Electrolysis in industry used to obtain
aluminium from aluminium ore (bauxite)
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We will need to be able to write equations which show what is going on at the
electrodes. To do this, we are going to need to be able to show not only what the
different chemical substances are (using their formulae) but also what state they are
in.
We use state symbols, written in brackets, subscript, at the end of a formula.
We use (s) for solid,
(l) for a pure liquid (i.e. not a solution)
(aq) for a substance dissolved in water
(g) for gas
e.g. PbBr2(s) solid lead bromidea giant ionic lattice
PbBr2(l) liquid lead bromide after heating and melting (molten)
PbBr2(aq) lead bromide dissolved in water, making a solution
Br2(g) bromine gas
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Electrolysis of ionic compounds after melting
The simplest situation to consider is what happens when an ionic compound is
melted and electrolysis is carried out. The only ions present are those in the ionic
compound. Well consider copper chloride as our example.At the negative electrode (cathode):The positively charged copper ions (Cu2+) are attracted.
Here they can receive the electrons they need to turn them back into atoms in this
case two electrons. We show this with a half-equation:
Cu2+ + 2e- Cu What we would SEE is a pinkish layer ofcopper metal being deposited on thecathode.
At the positive electrode (anode):Negatively charged chloride ions (Cl-)are attracted
At the electrode they each give up one electron to form chlorine atoms
Chlorine atoms on their own have unfilled outer shells, so pairs of chlorine atoms
form a covalent bond and making Cl2 molecules. The half-equation is:
2Cl-
Cl2 + 2e- What we would SEE is a stream of bubbles of pale
green chlorine gas being produced at the anode.
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When electrons are gained, we call this REDUCTION. Since ions receive electronsat the cathode, this is where they are being reduced to elements
e.g. Pb2+(l) + 2e- Pb(l) lead ions gain two electrons and are reduced to lead
When electrons are lost, we call this OXIDATION. Since ions give up electrons atthe anode, this is where they are being oxidised to form elements
e.g. 2Br-(l) Br2(g) + 2e- bromide ions lose electrons and are oxidised to bromine
gas. Its a gas because the electrolyte is hot !
Remember:
Oxidation
Is
Loss (of electrons)
Reduction
Is
Gain (of electrons)reduction of lead
ions to lead metal
at the cathode
oxidation of bromide
ions to bromine gas
at the cathodeHEAT
molten lead bromide
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More about half-equations:A half equation is used to show the reduction (gain of electrons) which is takingplace at the cathode.
A second half-equation is used to show the oxidation (loss of electrons) which istaking place at the anode.
They always balance:- there are the same number and type of atoms/ions on each side
- the charges always add to the same on each side
For metals they show the number of electrons gained to turn the ion into an atom:
Na+(l) + e- Na(l) Cu
2+(aq) + 2e
- Cu(s) Al3+
(l) + 3e- Al(l)
For non-metals they show the number of electrons needed to turn the ions into
atoms. They also show the non-metals bonding to form molecules:
2Cl-(l) Cl2(g) + 2e- 2H+(aq) + 2e- H2(g) 2O-(l) O2(g) + 2e-
Hydroxide ions give up electrons forming oxygen and water:
4OH-(aq) 4e- + O2(g) + 2H2O(l)
The data sheet you get in your exam shows you what charge different ions have.
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Electrolysis of ionic compounds dissolved in water:
When an ionic substance is dissolved in water the ions are free to move around, rather
than being locked up in the giant ionic lattice, so they can move to the electrodes.
In a solution, however, the ions from the ionic compound are NOT the only ions
present. There are also hydrogen ions, H+(aq) and hydroxide ions, OH-(aq), from the
water.
There is therefore a competition at the negative cathode between the positively
charged metal ions from the ionic compound and the hydrogen ions.
There is also a competition at the positive anode between the negatively charged non-
metal ions from the ionic compound and the hydroxide ions.
+ -
H+
Cu2+
OH-
Cl-
attraction of ions in a copper
chloride solution
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At the cathode:
The least reactive of the two positive ions is the one which is forced to leave thesolution, either as hydrogen gas, H2(g) , or by being deposited as the metal.
The reactivity series of the metals, including the position of hydrogen isgiven in your data sheet in the exam.
Calcium is more reactive than
hydrogen, so remains in the solution.
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At the anode:
A similar competition occurs. The basic rule is that ifchloride, bromide or iodideions are present they will be forced to leave the solution as the halogen(chlorine, bromide or iodine).
Other ions are able to stay in the solution, forcing the hydroxide ions to be oxidisedto oxygen gas and water.
4OH-(aq) 4e- + O2(g) + 2H2O(l)
e.g. 2Cl-(aq) 2e- + Cl2(g)
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Industrial uses of electrolysis 1: Electrolysis of brine
Brine is a concentrated solution of sodium chloride in water. There are therefore
four ions present in a brine solution. We can use the rules from the last section to
decide what will happen:
Positive ionsNa+(aq) more reactive - will remain in solution
H+(aq) less reactive - given off as H2 at the cathode 2H+
(aq) + 2e- H2(g)
Negative ionsCl-(aq) given off as the halogen Cl2 at the anode
OH-(aq) will remain in solution 2Cl-(aq) Cl2(g) + 2e
-
The ions remaining, Na+ and
OH- form a sodium hydroxide
solution.
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All three products of electrolysis are highly useful, so this is a
very large scale industrial process.
HYDROGEN is used:- in the Haber process for making ammonia- forhydrogenation of vegetable oils to make spreads etc.- as a fuel
CHLORINE is used:- for making bleach
- for sterilising (killing bacteria) in drinking water- for purifying swimming pool water
SODIUM HYDROXIDE is used:- in soap and paper manufacturing- to increase the pH of waste water (in order to remove
heavy metal ions by precipitating them as metal hydroxides)- for making bleach
Environmental:Lots of electrical energy is needed to do electrolysis. This has to be produced by
burning fossil fuels, so CO2 is released into the environment (global warming) and
finite reserves of crude oil are used up.
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Industrial uses of electrolysis 2: Electroplating
Electrolysis can be used to coat a conductive object in a layer
of another metal.
This might be done to provide a protective layer e.g. to
prevent corrosion, or may be to change the properties or
appearance of the metal.
The metal object to be coated is used as the negative
electrode, and the electrolyte is a solution containing ions ofthe metal which is to form the coating.
e.g.
chrome-plating of steel parts on automobiles. to
make them more corrosion-resistant
Plain steel cutlery is often electroplated with
silver
Inexpensive jewellery is often electroplated with
a layer of silver or gold
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Industrial uses of electrolysis 3: Extracting Aluminium from its ore
Aluminium ore (bauxite) is mainly aluminium oxide.
Because aluminium is more reactive than carbon it
cant be extracted by reduction in a blast furnace.
Aluminium ore has to be melted, then an electric
current is passed through it. Huge amounts of
electrical energy are needed to do this.
A mineral called cryolite (sodium aluminium fluoride)
is mixed with the aluminium ore. This makes the
aluminium ore melt at a lower temperature.
At the negative electrode:
Al3+
(l) + 3e-
Al(l)
At the positive electrode
2 O2-(l) 4e- O2(g)
The positive electrode is made of graphite (carbon).
This reacts with the oxygen produced, forming carbondioxide.
bauxite
cryolite
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Industrial uses of electrolysis 4: Purification of copper
This works in the same way as electroplating.
The basic idea is that we use electrolysis to remove
copper ions from a large impure copper anode, thendeposit them as pure copper on the cathode.
The impurities fall to the bottom of the electrolysis cell
as a sludge.
To make copper pure enough for electrical
wiring (99.99% pure) we have to remove the
impurities present in copper metal after
extracting it from its ore. Such copper is
typically 99% pure, containing valuable silver
and gold impurities, as well as toxic arsenic.
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At the anode:Copper atoms in the anode itself give up
electrons to form copper ions, and enter thecopper sulphate solution.
Cu(s) Cu2+
(aq) + 2e-
The impure copper anode therefore loses mass.
Underneath the anode, the impurities (silver,
gold, arsenic etc.) collect as sludge.
Electrolysis is carried out in a copper sulphate solution
At the cathode:Positively charged copper ions in the solution are attracted to the negative cathode.
Here they receive electrons, forming copper atoms and being deposited as pure
copper metal on the cathode.
Cu2+(aq) + 2e- Cu(s)
The cathode therefore gains mass as pure copper is deposited.
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Environmental:Electrolysis uses a lot of electrical energy, which is generated by burning fossil fuels
(CO2 given off, loss of finite resource)
The arsenic produced in the sludge is highly toxic and must not be released backinto rivers etc. but disposed of carefully.
In other electrolysis process where an ionic substance has to be melted (e.g.
extraction of aluminium from its ore) very high temperatures are needed to melt the
giant ionic structure of the electrolyte. This also means very high energy costs and
use of fossil fuels.
copper pure
enough to be
used in electrical
wiring
electrodes coated in
copper by the
purification process