ELECTROLYSIS

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ELECTROLYSIS Module C2
Splitting up ionic compounds (F)
Molten compounds
Ionic solutions discharge rules
Q It and OILRIG
Brine and purifying copper
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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
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SPLITTING UP IONIC COMPOUNDS 2
2 ways to split up the ions
800C
20C
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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
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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
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Example 1 Splitting up MOLTEN SODIUM CHLORIDE
(salt)

- chloride IONS lose their extra electrons and
turn into neutral chlorine ATOMS
Cl-
Cl-
Cl-
Cl-
Both together 2Cl- ? 2e- Cl2
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Example 1 Splitting up MOLTEN SODIUM CHLORIDE
(salt)

sodium IONS gain an extra electron and turn
into neutral sodium ATOMS
molten sodium metal sinks to bottom
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Example 1 Splitting up MOLTEN SODIUM CHLORIDE
(salt)
- CATHODE
ANODE
ELECTRONS
SODIUM metal Na
CHLORINE gas Cl2
MOLTEN SODIUM CHLORIDE
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Example 2 Splitting up MOLTEN LEAD BROMIDE PbBr2
- CATHODE
ANODE
ELECTRONS
LEAD Metal Pb
BROMINE gas Br2
MOLTEN LEAD BROMIDE
Both together 2Br- ? 2e- Br2
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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 OH-
So, 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 OH-)
SODIUM CHLORIDE SOLUTION NaCl (aq)
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IONIC SOLUTION
Which ions gain or lose electrons (get
discharged) and which stay in solution?
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IONIC SOLUTIONS At the CATHODE
sodium ION, missing 1 electron
hydrogen ION, missing 1 electron
Hydrogen ATOM, NEUTRAL
which ions?

As HYDROGEN is LESS REACTIVE than SODIUM, it is
discharged. The sodium ions stay in
solution.
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IONIC SOLUTIONS At the CATHODE halogen
compounds
hydroxide ION, from water extra electron
chloride ION, extra 1 electron
chlorine ATOM, NEUTRAL
O

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
O
O
Cl-
Cl-
O
O
Cl-
Cl-
which ions?
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IONIC SOLUTIONS CATHODE non halogen compounds
hydroxide ION, OH- from water, extra electron
Oxygen atom
nitrate ION, extra 1 electron
O
O
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.
which ions?
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RULES FOR IONIC SOLUTIONS
ANODE Attracts ions (Anions)
- CATHODE Attracts ions (Cations)
If ions are HALOGENS ie chloride Cl- bromide
Br- iodide I- the HALOGEN is produced.
If ions (metals) are MORE REACTIVE than
hydrogen K, Na, Ca, Mg, Zn, Fe Then HYDROGEN is
produced
If ions are NOT HALOGENS Eg sulphate SO42-,
nitrate NO3- carbonate CO32- OXYGEN is
produced.
If ions (metals) are LESS REACTIVE than
hydrogen Cu, Ag, Au Then the METAL is produced
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(REACTIVITY K Na Ca2 Mg2 Al3 Zn2
Fe3 H Cu2 Ag Au3 )
Compound
State
Anode ()
Cathode (-)
Ions
potassium chloride
molten
K Cl-
potassium
chlorine
aluminium oxide
molten
Al3 O2-
aluminium
oxygen
Cu2 Cl- H OH-
copper chloride
solution
copper
chlorine
sodium bromide
Na Br- H OH-
solution
hydrogen
bromine
silver nitrate
Ag NO3- H OH-
silver
oxygen
solution
potassium chloride
solution
K Cl- H OH-
hydrogen
chlorine
zinc sulphate
Zn SO42- H OH-
oxygen
solution
hydrogen
(REACTIVITY K Na Ca2 Mg2 Al3 Zn2
Fe3 H Cu2 Ag Au3 )
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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)
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OILRIG
Oxidation is loss, reduction is gain OILRIG
Cl-
Cl-
Cl-
Cl-

• ions LOSING electrons to become atoms is called
  OXIDATION
• (even though oxygen may not be involved)

ions GAINING electrons to become atoms is
called REDUCTION
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INDUSTRIAL USES OF ELECTROLYSIS

• 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 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. Electrolysis is used to convert
  impure copper to pure copper

see below
see below
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INDUSTRIAL ELECTROLYSIS OF BRINE
Chlorine gas
Hydrogen gas
BRINE (NaCl solution)
CATHODE H and Na
ANODE OH- and Cl-
2H 2e- H2
2Cl- 2e- Cl2
OH- left in solution so concentration grows
Na left in solution so concentration grows
Sodium chloride solution (neutral) slowly changed
to sodium hydroxide solution (alkaline)
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Industrial chlorine production from electrolysis
of brine
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PURIFYING COPPER
Copper atoms from impure copper are OXIDISED to
copper ions
Copper ions from impure copper are REDUCED to
copper atoms
Copper ions transported from anode to cathode
As the atoms of the impurities are not
transported, the copper that builds up on the
anode is extremely pure.
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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?