2. What you need to be able to
do and understand:
• State that electrolysis is the chemical • Relate the products of electrolysis to
effect of electricity on ionic the electrolyte and electrodes
compounds, causing them to break up used, exemplified by the specific
into simpler substances, usually examples in the Core together with
elements. aqueous copper(II) sulfate using
• Use the terms carbon electrodes and using copper
electrode, electrolyte, anode and electrodes (as used in the refining of
cathode. copper).
• Describe electrolysis in terms of the • Describe the electroplating of
ions present and the reactions at the metals, using laboratory apparatus.
electrodes. • Predict the products of the electrolysis
• Describe the electrode of a specified binary compound in the
products, using inert electrodes, in the molten state, or in aqueous solution.
electrolysis of: • Describe, in outline, the chemistry of
• molten lead(II) bromide the manufacture of:
• aqueous copper chloride, • aluminium from pure aluminium oxide in
• dilute sulfuric acid. molten cryolite,
• State and use the general principle • chlorine, hydrogen and sodium
that metals or hydrogen are formed at hydroxide from concentrated aqueous
the negative electrode (cathode), and sodium chloride.
that nonmetals (other than hydrogen)
are formed at the positive electrode
(anode).
3. Before we begin with this unit we need a quick
review of electricity, conductors and insulators.
Electricity is a stream of electrons in a circuit.
Insulators are substances that resist a flow of electrons
Conductors are substances that allow the flow of electrons.
The only solids that conduct are metals and graphite.
To review all
these we are
going to
practice with
this animation
– the circuit
construction kit
4. So then… how do batteries work and
What is electrolysis?
Ionic substances do not conduct when solid (solid
lead). But they do conduct when melted or
dissolved in water – and they decompose at the
same time. This is the basis of how batteries can
power things (make electrons move).
Ionic substances have IONS (something that has
a charge) and electricity is the movement of
charged particles- specifically electrons.
Decomposition brought about by electricity is called electrolysis. In
other words, electrolysis is the chemical effect of electricity on ionic
compounds, causing them to break up into simpler substances, usually
elements.
5. Electrolysis: breaking down To start here are a few animations –
by electricity some fun and some serious
A simple animation to show A bit on the fun side
the electrolysis of water
Any liquid that contains ions will
conduct electricity. Now think about
this… what conducts electricity
better – fresh or salt water?
How your car batteries work
6. Anatomy of a battery
Flow of electrons
Electrons
flow from
the
positive
terminal The negative
The positive end
to the end is the
is the anode - it
negative cathode - it
connects to the
terminal connects to the
positive end of
negative end of
the battery
the battery
Flow of electrons
The solution inside a battery
is called the electrolyte
Remember PA These rods are called electrodes
Positive Anode
7. The electrolysis of molten lead bromide
The rods are graphite
Flow of electrons (carbon). They are inert
(remain unchanged)
At the anode (+) the Br-
ions give up electrons.
You can see red-brown At the cathode (-) the Pb2+
bromine gas bubbles. ions accept electrons. The
Bromine is a non-metal cathode looks like it is growing
as lead collects on it. Lead is a
metal
Lead(II)bromide is the electrolyte (the solution). It has Pb2+ and Br-
ions. Here the ions move to the electrode of opposite charge
8. What is the result?
Lead(II)bromide has decomposed. The chemical reaction is:
lead(II)bromide lead bromide
PbBr2(aq) Pb(l) Br2(g)
Things to remember:
• Electrons carry current through the wires and electrodes, but the
ions carry it through the electrolyte (liquid)
This pattern is the SAME for all other molten ionic compounds of two
elements.
Electrolysis decomposes ionic compounds into its elements, giving
the metal at the cathode, and the non-metal at the anode.
9. Let’s work together to fill in the blanks for the electrolysis
of aqueous copper(II)chloride
The rods are graphite
Flow of electrons (carbon). They are inert
(remain unchanged)
At the ________ (+)
the ______ ions
_________ electrons.
At the _________ (-) the _____
_____ is a
ions ________ electrons. ____
___________
is a ________
________________ is the electrolyte (the solution). It has ______ and
______ ions. Here the ions move to the electrode of __________charge
10. Let’s work together to fill in the blanks for the electrolysis
of aqueous copper(II)chloride
The rods are graphite
Flow of electrons (carbon). They are inert
(remain unchanged)
At the anode (+) the Cl-
ions give up electrons.
You can see chlorine gas At the cathode (-) the Cu2+ ions
bubbles. Chlorine is a accept electrons. The cathode
non-metal looks like it is growing as copper
collects on it. Copper is a metal
Copper(II)chloride is the electrolyte (the solution). It has Cu2+ and Cl- ions.
Here the ions move to the electrode of opposite charge
11. What is the result?
Copper(II)chloride has decomposed. The chemical reaction is:
copper(II)chloride copper chloride
CuCl2(aq) Cu(s) Cl2(g)
Now on your own draw a diagram and write the equation
for the electrolysis of dilute sulfuric acid (H2SO4).
This will make you think…
12. Electrolysis of dilute dilute sulfuric acid (H2SO4).
At the anode the concentration of
hydroxyl ions is too low to maintain
a reaction and the sulphate ions are
not oxidized but remain on in
solution at the end. Water molecules
must be the species reacting at the
anode.
At the anode (+) the OH- ions give At the cathode (-) the H+ ions
up electrons. You can see oxygen accept electrons. Hydrogen
gas bubbles. Oxygen is a non-metal gas is given off. Hydrogen acts
as a metal
Sulfuric acid is the electrolyte (the solution). It has H+ and SO42- ions. BUT…
water is present as well. It has H+ and OH- ions. Here the ions move to the
electrode of opposite charge
13. What is the result?
The chemical reactions are:
Sulfuric acid is a strong electrolyte is fully dissociated in aqueous solution.
H2SO4(aq) H+ SO42-
Water is a weak electrolyte and is only slightly dissociated.
H2O (aq) H+ OH-
During electrolysis, the hydrogen ions migrates towards the cathode, and are
discharged there (i.e. they gain an electron and are converted to hydrogen gas).
4H+ 4e- 2H2(g)
At the anode the concentration of hydroxyl ions is too low to maintain a reaction
and the sulphate ions are not oxidized but remain on in solution at the end.
Water molecules must be the species reacting at the anode.
2H2O (aq) O2(g) 4H+ 4e-
14. What are the rules for the electrolysis of a solution?
At the cathode(-), either a metal or hydrogen forms.
• The more reactive an element, the more it likes to ‘exist’ as ions. So
if a metal is more reactive than hydrogen its ions STAY in solution
(it does not form a product) and instead hydrogen gas is produced.
• But if the metal is less reactive than hydrogen, the metal forms.
At the anode(+), a non-metal other than hydrogen forms.
• If it is a concentrated solution of a halide (something that has Cl-, Br-
, I-… ions), then the respective halogen forms.
• But if the halide solution is dilute, or there is no halogen (i.e. the
sulfuric acid example), oxygen forms.
A non-metal A metal or hydrogen
forms here forms here
15. Now knowing the general rules determine what
forms at the anode and cathode when a solution
of copper(II)sulfate is the electrolyte and
graphite are the electrodes.
At the anode (+) the OH- ions give
up electrons. You can see oxygen
gas bubbles. Oxygen is a non-metal.
At the cathode (-) the Cu2+
Copper(II)sulfate is the electrolyte. It has ions accept electrons.
Cu2+, SO42-, H+, and OH- ions. The blue colour Copper coats the
of the solution fades as copper ions are electrode. Copper is a
discharged. metal.
You may be asked to write half-equations for the reactions at the electrodes.
1. Name the ions present, and then the products
2. Write each half-equation correctly while adding symbols for the states
Do this for the above
reactions
At the anode: 4OH-(aq) → 2H2O(l) + O2(g) + 4e-
At the cathode: 2Cu2+(aq) + 4e- → 2Cu(s)
16. Now apply the same rules but instead of an
.
inert graphite electrode we can use copper
electrodes and copper(II)sulfate.
At the anode (+), the anode
dissolves, giving copper ions in the
solution. These move to the cathode, to
form copper. So copper moves from the
anode to the cathode. The colour of the At the cathode (-) the Cu2+ ions
solution does not fade. accept electrons. Copper still
Write the half-reactions
coats the electrode.
At the anode: Cu(s) → Cu2+(aq) + 2e-
At the cathode: Cu2+(aq) + 2e- → Cu(s)
This method is the basis for two very
important uses of electrolysis: refining
copper and electroplating.
When you refine copper, the anode is impure
copper, while the cathode is pure. The copper
in the anode dissolves and then builds up on
the cathode while the impurities drop to the
floor of the container.
17. This animation is about a
Galvanic cell. You don’t need to
know about Galvanic cells but it’s
a good animation on what is
going on at the molecular level at
the anode and cathode when you
have an active metal as the
electrode.
http://www.mhhe.com/physsci/ch
emistry/essentialchemistry/flash/g
alvan5.swf
A battery is an example of a
Galvanic cell. It is an
electrochemical cell in which
a spontaneous reaction
generates a flow of current.
18. Electroplating means using electricity to coat one metal with another.
There are many uses of this including making cheap jewellery look more
expensive by coating it with a thin layer of sliver or gold.
To electroplate:
At the cathode: the object to be electroplated
At the anode: metal X (the metal you want to
coat the object with)
Electrolyte: an aqueous solution of compound X
This animation is excellent for
Anode – the metal Cathode – viewing electroplating
we want to coat the object
the object with
This is a good
video on gold
plating
19. Chemistry in action: How do you extract aluminum?
Steps in aluminum extraction:
1. Mine bauxite
2. Purify bauxite to get aluminum oxide
(alumina).
3. Alumina is mixed with cryolite (sodium
fluoride and aluminum fluoride). This is
This video talks about done because you need a lower
the whole process. temperature to melt the aluminum this way.
(908°C vs 2040°C)
4. This mixture is then electrolysed to form
aluminum metal (at the cathode) and
oxygen gas is formed at the anode, where it
reacts with the carbon in the anode to give
carbon dioxide gas.
This one is more specific on
Another good animation the electrolysis.
on the electrolysis
20. Cryolite (added to Bauxite (mainly Oxygen and
reduce melting point) in aluminum oxide) in carbon dioxide out
Al3+ move
towards the O2- move
cathode towards the
anode
Graphite anodes
Graphite accept
cathode gives electrons, reacts
electrons, to with O2 to form
Al3+ to form Al(l) CO2
Write the
half-reactions
At the anode (2 reactions): Write the overall reaction:
6O2-(l) → 3O2(g) + 12e- & C(s) + O2(g) → CO2(g)
At the cathode: 4Al3+(l) +12e- → 4Al(l) 2Al2O3(l) → 4Al(s) + 3O2(g)
21. Chemistry in action #2:
Electrolysis of concentrated sodium
chloride (brine, NaCl), to make
H2(g), Cl2(g) and NaOH(l) A short
video
At the anode (+) the Cl- ions give up
electrons. You can see chlorine gas
bubbles.
Sodium chloride is the electrolyte. It has
Na+, Cl-, H+ and OH- ions. Here the ions
At the cathode (-) the H+
move to the electrode of opposite charge.
ions accept electrons.
When the hydrogen and chlorine bubble
Hydrogen gas is given off.
off, Na+ and OH- ions are left behind, so a
solution of sodium hydroxide (NaOH) is
formed.
What are the half-reactions?
At the anode: 2Cl-(aq) → Cl2(g) + 2e-
At the cathode: 2H+(aq) +2e- → H2(g)
