This document provides information about electrolysis. It discusses: 1. How electrolysis can be used to split up ionic compounds by melting them or dissolving them in water. Positive ions move to the negative cathode and negative ions move to the positive anode. 2. The rules for what products are formed at the electrodes during electrolysis of molten ionic compounds and ionic solutions. Hydrogen, metals, halogens, oxygen may be produced depending on the ions present. 3. Industrial applications of electrolysis including producing chlorine and sodium hydroxide from salt water, and purifying copper.

1. ELECTROLYSIS
+
- + - +
- + - + - + - +
+
Module C2
Splitting up ionic compounds (F)
Molten compounds
Ionic solutions & discharge rules
Q = It and OILRIG
Brine and purifying copper

2. SPLITTING UP IONIC COMPOUNDS 1
Cl- ION
Na+ ION
Ionic compounds (eg sodium chloride) are made from:
POSTIVE IONS (atoms which LOST negative electrons)
NEGATIVE IONS (atoms which GAINED negative electrons)
As these ions have OPPOSITE CHARGES they attract each
other strongly to form IONIC BONDS

3. SPLITTING UP IONIC COMPOUNDS 2
2 ways to split up the ions:
MELT
+
- + - +
- + - + - + - +
+
800°C
- +
+ - +
+ -
-
+
-
DISSOLVE
+
-
+ - +
+
+ -
-
-
+
20°C
H2O

4. SEPARATING THE IONS 1
+
_
Metal
ELECTRODE
ELECTRON
Battery pulls
electrons off one
electrode and
pushes them
onto the other
This IS SHORT OF
electrons so becomes
POSITIVELY CHARGED
“ANODE”
This HAS EXTRA
electrons so becomes
NEGATIVELY CHARGED
“CATHODE”

5. SEPARATING THE IONS 2
+
MOLTEN IONIC
COMPOUND
-
+ -
+ ANODE
+
+
-
+
-
+
+
+
- CATHODE
When the battery is switched on,
the + IONS move to the – CATHODE
the – IONS move to the + ANODE
This gives a way to SPLIT UP IONIC COMPOUNDS: “ELECTROLYSIS”

6. Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)
- =
Cl-
chloride ION,
extra 1 electron
Cl
chlorine ATOM, Cl Cl
NEUTRAL
Cl2
molecule
+
Cl
Cl-
ClCl
ClCl
- chloride IONS
lose their extra
electrons and
turn into neutral
chlorine ATOMS
ClCl
At ANODE:
Clthen:
Cl + Cl
e- + Cl
Cl2 (gas)
Both together:
2Cl- → 2e- + Cl2

7. Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)
+ = Na+
sodium ION,
missing1 electron
Na
sodium ATOM,
NEUTRAL
+
+ sodium IONS
gain an extra
electron and turn
into neutral
sodium ATOMS
At CATHODE: Na+ + e-
Na
Na+
Na
Na+
Na
Na+
Na
Na+
Na
molten sodium
metal sinks to
bottom

8. Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)
- CATHODE
+ ANODE
ELECTRONS
SODIUM
metal Na
CHLORINE
gas Cl2
Cl-
MOLTEN
SODIUM
CHLORIDE
At ANODE:
Cle- + Cl
Cl + Cl
Cl2 (gas)
Na+
At CATHODE:
Na+ + eNa

9. Example 2: Splitting up MOLTEN LEAD BROMIDE PbBr 2
- CATHODE
+ ANODE
ELECTRONS
LEAD
Metal Pb
BROMINE
gas Br2
Br-
MOLTEN
LEAD
BROMIDE
At ANODE:
Bre- + Br
Br + Br
Br2 (gas)
Pb2+
At CATHODE:
Pb2+ + 2eBoth together:
2Br- → 2e- + Br2
Pb

10. What happens when the ionic compounds are dissolved
in water?
Here, water molecules break up into HYDROGEN IONS, H+
and HYDROXIDE IONS OH-
H2O  H+ + OHSo, in an ionic solution (eg sodium chloride solution), there will be
FOUR types of ion present:
TWO from the ionic compound and TWO from the water ( H+
SODIUM CHLORIDE
SOLUTION NaCl (aq)
H+
OH-
ClNa+
OHH+
H+
Na+
Cl
-
-
OH
Na+
Cl-
+ OH-)

11. IONIC SOLUTION
H+
OH
OH-
Cl-
+
Na
H+
+
Na
Cl-
H+
Na+
OHCl-
Which ions gain or lose electrons (“get discharged”)
and which stay in solution?

12. IONIC SOLUTIONS: At the CATHODE
Na
+
sodium ION,
missing 1
electron
H+
+
hydrogen ION,
missing 1
electron
As HYDROGEN is
LESS REACTIVE
than SODIUM, it is
discharged.
The
sodium ions stay in
solution.
At CATHODE: 2H+ + 2e-
H
Hydrogen ATOM,
NEUTRAL
which
ions?
Na+
H+
H
H+
Na+
H2

13. IONIC SOLUTIONS: At the CATHODE – halogen compounds
Cl-
chloride ION,
extra 1 electron
hydroxide ION,
O H from water
extra electron
chlorine ATOM,
NEUTRAL
Cl
+
H
O
O H
Cl
Cl-
H
ClCl
O
O H
ClCl
which
ions?
ClCl
At ANODE:
If the – ion is a
HALOGEN (Cl, Br, I)
it is discharged and
chlorine (or Br or I)
is given off and the
OH - ions stay in
solution
2Cl-
2e- + Cl2

14. IONIC SOLUTIONS: CATHODE – non halogen compounds
nitrate ION, extra
NO3 1 electron
hydroxide ION, OHO H from water, extra
electron
O
Oxygen
atom
+
NO3
H
-
O
NO3H
O
which
ions?
NO3-
O H
NO3-
If the – ion is NOT a halogen
(eg nitrate, sulphate etc) then
the HYDROXIDE ions from the
water are discharged to make
WATER and OXYGEN gas.
The other ions stay in solution.
O H
At CATHODE: 4OH-
2H2O + O2 + 4e-

15. RULES FOR IONIC SOLUTIONS
+ ANODE
Attracts – ions (‘Anions’)
- CATHODE
Attracts + ions (‘Cations’)
If – ions are HALOGENS ie
If + ions (metals) are MORE
REACTIVE than hydrogen
chloride Clbromide Br
K, Na, Ca, Mg, Zn, Fe
-
iodide Ithe HALOGEN is produced.
If – ions are NOT HALOGENS
Eg sulphate SO4 ,
2-
Then HYDROGEN is
produced
If + ions (metals) are LESS
REACTIVE than hydrogen
nitrate NO3-
Cu, Ag, Au
carbonate CO32-
Then the METAL is produced
OXYGEN is produced.

16. (REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ )
Ions
Cathode (-) Anode (+)
potassium chloride molten
K+ Cl-
potassium
chlorine
aluminium oxide
molten
Al3+ O2-
aluminium
oxygen
copper chloride
solution
Cu2+ Cl- H+ OH-
copper
chlorine
sodium bromide
solution
Na+ Br- H+ OH-
hydrogen
bromine
silver nitrate
solution
Ag+ NO3- H+ OH-
silver
oxygen
potassium chloride solution
K+ Cl- H+ OH-
hydrogen
chlorine
zinc sulphate
Zn+ SO42- H+ OH-
hydrogen
oxygen
Compound
State
solution
(REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ )

17. ELECTROLYSIS makes a CIRCUIT
Complete electric circuit:
Current carried by:
ELECTRONS in electrodes/wires
+
+
-
IONS in the electrolyte
-
To DOUBLE the MASS of substance
discharged at electrodes:
2 x CURRENT (2x batt. voltage)
2 x TIME current flows for
(Q = I t)

18. OILRIG
Cl-
Cl-
Cl-
Cl-
Oxidation is loss,
reduction is gain
‘OILRIG’
Na+
Na+
Na+
Na+
- ions LOSING electrons
+ ions GAINING electrons
to become atoms is called to become atoms is called
‘OXIDATION’
‘REDUCTION’
(even though oxygen may
not be involved)

19. INDUSTRIAL USES OF ELECTROLYSIS
1. To extract reactive metals such as ALUMINIUM, sodium,
magnesium etc from their compounds. This is EXPENSIVE
due to the large amounts of electrical energy needed.
Aluminium is extracted from bauxite (Al2O3).
2. Electrolysis of BRINE (salt solution) to produce
see below
CHLORINE (for disinfectants and plastics)
HYDROGEN (for ammonia fertilisers, margarine)
SODIUM HYDROXIDE (for soap and cleaning agents)
3. Purifying copper. The copper for wiring etc needs to be
more pure than that produced in a blast furnace. see below
Electrolysis is used to convert impure copper to pure
copper

20. INDUSTRIAL ELECTROLYSIS OF BRINE
Chlorine gas
Hydrogen gas
BRINE
(NaCl solution)
CATHODE
H+ and Na+
ANODE
OH- and Cl2Cl
-
2H+ + 2e-
2e + Cl2
-
OH- left in solution
so concentration
grows
Sodium chloride
solution (neutral)
slowly changed to
sodium hydroxide
solution (alkaline)
H2
Na+ left in solution
so concentration
grows

21. Industrial chlorine production from electrolysis of
brine

22. PURIFYING COPPER
IMPURE COPPER
ANODE
Copper atoms
from impure
copper are
OXIDISED to
copper ions
PURE COPPER
CATHODE
Copper sulphate
CuSO4 solution
Cu2+
Cu
Cu2+
Cu2+
Cu
Copper ions transported
from anode to cathode
Copper ions
from impure
copper are
REDUCED to
copper atoms
As the atoms of the impurities are not
transported, the copper that builds up on the
anode is extremely pure.

23. IMPURE
COPPER
ANODE
PURE
COPPER
CATHODE
IMPURE
COPPER
ANODE
PURE
COPPER
CATHODE
Over time, the impure anode dissolves away and the
impurities sink to the bottom. The pure cathode grows as
more pure copper is deposited on it.
Why will the concentration of the solution stay the same?