ELECTROLYSIS
+
- + - +
- + - + - + - +
+
Splitting up ionic compounds (F)
Molten compounds
Ionic solutions & discharge rules
Q = It and OILRIG
Brine and purifying copper
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
SPLITTING UP IONIC COMPOUNDS 2
2 ways to split up the ions:
MELT
+
+
+
+
- + - + - + - +
800℃
- +
+ - +
+ -
+
-
DISSOLVE
+
-
+ - +
+
+ -
-
-
+
20℃
H2O
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”
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”
Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)
-
=
Cl-
chloride ION,
extra 1 electron
Cl
chlorine ATOM, Cl Cl
NEUTRAL
Cl2
molecule
+
Cl
Cl-
Cl-
-
Cl-
Cl
Cl
At ANODE:
Clthen:
Cl + Cl
- chloride IONS
lose their extra
electrons and
turn into neutral
chlorine ATOMS
e- + Cl
Cl2 (gas)
Both together:
2Cl- → 2e- + Cl
2
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
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+ + e-
Na
Example 2: Splitting up MOLTEN LEAD BROMIDE PbBr2
- CATHODE
+ ANODE
ELECTRONS
LEAD
Metal Pb
BROMINE
gas Br2
Br-
MOLTEN
LEAD
BROMIDE
At ANODE:
Bre- + Br
Br + Br
Pb2+
At CATHODE:
Pb2+ + 2e-
Br2 (gas)
Both together:
2Br- → 2e- + Br
2
Pb
What happens when the ionic compounds are dissolved
in water?
Here, water molecules partly break up into HYDROGEN
IONS, H+ and HYDROXIDE IONS OH-
H+ + OH-
H2O
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+
SODIUM CHLORIDE
SOLUTION NaCl (aq)
Cl-
H+
OH-
Na+
OHH+
Na+
Cl-
H+
OH-
Na+
Cl-
+ OH-)
IONIC SOLUTION
H+
OH-
ClNa+
OHH+
Na+
Cl-
H+
Na+
OHCl-
Which ions gain or lose electrons (“get discharged”)
and which stay in solution?
IONIC SOLUTIONS: At the CATHODE
Na+
sodium ION,
missing 1
electron
H+
hydrogen ION,
missing 1
electron
H
Hydrogen ATOM,
NEUTRAL
+
As HYDROGEN is
LESS REACTIVE
than SODIUM, it is
discharged.
The
sodium ions stay in
solution.
At CATHODE: 2H+ + 2e-
which
ions?
Na+
H+
H+
H
Na+
H2
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
Cl
Cl-
H
O H
Cl-
O
O H
-
Cl
Cl
which
ions?
Cl-
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
IONIC SOLUTIONS: CATHODE – non halogen compounds
nitrate ION, extra
NO3
1 electron
hydroxide ION, OHO H from water, extra
electron
O
Oxygen
atom
+
NO3H
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 ANODE: 4OH-
2H2O + O2 + 4e-
RULES FOR IONIC SOLUTIONS
+ ANODE
Attracts – ions (‘Anions’)
If – ions are HALOGENS ie
chloride Clbromide Briodide Ithe HALOGEN is produced.
If – ions are NOT HALOGENS
Eg sulphate SO42-,
nitrate NO3carbonate CO32OXYGEN is produced.
- CATHODE
Attracts + ions (‘Cations’)
If + ions (metals) are MORE
REACTIVE than hydrogen
K, Na, Ca, Mg, Zn, Fe
Then HYDROGEN is
produced
If + ions (metals) are LESS
REACTIVE than hydrogen
Cu, Ag, Au
Then the METAL is produced
(REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ )
Ions
Cathode (-) Anode (+)
potassium chloride molten
K+ Cl-
potassium
chlorine
aluminium oxide
Al3+ O2-
aluminium
oxygen
Compound
State
molten
copper chloride
2+ Cl- H+ OHCu
solution
copper
chlorine
sodium bromide
solution Na+ Br- H+ OH-
hydrogen
bromine
silver nitrate
solution Ag+ NO3- H+ OH-
silver
oxygen
hydrogen
chlorine
hydrogen
oxygen
potassium chloride solution
zinc sulphate
K+ Cl- H+ OH-
solution Zn2+ SO42- H+ OH-
(REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ )
ELECTROLYSIS makes a CIRCUIT
Complete electric circuit:
Current carried by:
ELECTRONS in electrodes/wires
IONS in the electrolyte
+
+
-
OILRIG
Cl-
Cl-
Cl-
Cl-
Oxidation is loss,
reduction is gain
‘OILRIG’
‘OXIDATION’ occurs
at the anode
Na+
Na+
Na+
Na+
‘REDUCTION’
INDUSTRIAL USES OF ELECTROLYSIS
1. To extract reactive metals such as Al, Na, Mg from their
compounds. This is EXPENSIVE due to the large amounts
of electrical energy needed.
2. Electrolysis of BRINE
3. Electrorefining of copper
4. Electroplating
see below
see below
C
C
C
C
C
ELECTROLYSIS OF BRINE
--- CHLOR-ALKALI INDUSTRY(P126, P267)
Chlorine gas
Hydrogen gas
BRINE
(NaCl solution)
CATHODE
H+ and Na+
ANODE
OH- and Cl2Cl-
2e-
2H+ + 2e-
+ Cl2
Sodium chloride
solution (neutral)
slowly changed to
sodium hydroxide
solution (alkaline)
H2
ELECTROLYSIS OF BRINE
--- CHLOR-ALKALI INDUSTRY
 at the anode: 2Cl-(aq)
Cl2(g) + 2e-
 at the cathode: 2H+(aq) + 2e2H2O(l) + 2e-
H2(g)
H2(g) + 2OH-(aq)
 The solution becomes sodium hydroxide, NaOH.
 Overall: 2Cl-(aq) + 2H2O(l)
Cl2(g) + H2(g) + 2OH-(aq)
Uses of the products (P268)
chlorine
hydrogen
Sodium hydroxide
to make HCl
in the Haber process
to make soaps
to treat drinking
water
to make margarine
in extraction of
aluminum
a bleach
a fuel in fuel cells
to make detergents
to make
plastics, PVC
to make nylon
to make paper and
textiles
Industrial chlorine production from electrolysis of brine
ELECTROREFINING OF 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 are
REDUCED to
copper atoms
ELECTROREFINING OF COPPER
• the anode: impure copper
Cu(s)
Cu2+(aq) + 2e-
• the cathode: pure copper
Cu2+(aq) + 2e-
Cu(s)
• the electrolyte: aqueous copper(II) sulfate (CuSO4solution)
 Over time, the anode dissolves away and the impurities
sink to the bottom;
 The cathode increases in size as more pure copper is
deposited on it;
 No change in concentration of aqueous copper(II) sulfate.
Copper is transferred from the anode to the cathode.
ELECTROREFINING OF COPPER
IMPURE
COPPER
ANODE
PURE
COPPER
CATHODE
IMPURE
COPPER
ANODE
PURE
COPPER
CATHODE
(anode ‘mud’ / ‘sludge’ / ‘slime’
containing Ag, Au & Pt).
ELECTROPLATING(P126)
--- to improve the appearance (highly polished shiny finish);
--- to protect from corrosion and abrasion (e.g. chromium plating).
Cathode(-) metal object to be plated, e.g. spoon
Anode (+) the metal
M being used for plating,
e.g. silver
Electrolyte containing the
metal ions Mn+ being used for
plating, e.g. silver nitrate
Sample Questions
1. Describe how you would chromium plate a steel tap by
completing the following.
The anode is…………
The cathode is …………
The electrolyte is …………
The half equation at the anode……………………………..
The half equation at the cathode……………………………
[5 marks]