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

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

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

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

9. Br- e- + Br Pb2+ + 2e- Pb At ANODE: At CATHODE: Br + Br Br2 (gas)
Example 2: Splitting up MOLTEN LEAD BROMIDE PbBr2
- CATHODE
+ ANODE
ELECTRONS
LEAD Metal Pb
BROMINE gas Br2
MOLTEN LEAD BROMIDE
Pb2+
Br-
At ANODE:
Br e- + Br
At CATHODE:
Pb2+ + 2e Pb
Br + Br Br2 (gas)
Both together:
2Br- → 2e- + Br2

10. SODIUM CHLORIDE SOLUTION NaCl (aq)
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) H+
OH-
Cl-
Na+ H+
Na+
Na+ H+
OH-
Cl-
Cl-
OH-

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

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

13. IONIC SOLUTIONS: At the CATHODE – halogen compounds
hydroxide ION, from water extra electron
chloride ION, extra 1 electron
Cl-
chlorine ATOM,
NEUTRAL O H Cl +
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 H O O H Cl Cl
Cl-
Cl- H O O H Cl Cl
Cl-
Cl-
which ions?
At ANODE: Cl e- + Cl2

14. + At CATHODE: 4OH- 2H2O + O2 + 4e-
IONIC SOLUTIONS: CATHODE – non halogen compounds
hydroxide ION, OH- from water, extra electron
Oxygen atom
nitrate ION, extra 1 electron O
NO3- O H +
NO3-
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 O H
NO3-
NO3- O H O H
which ions?
At CATHODE: 4OH H2O + O2 + 4e-

15. 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

16. (REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ )
Compound
State
Ions
Cathode (-)
Anode (+)
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+ )

17. - ELECTROLYSIS makes a CIRCUIT Complete electric 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. Oxidation is loss, reduction is gain
OILRIG
Oxidation is loss, reduction is gain
‘OILRIG’
Na+
Na+
Cl-
Cl-
Na+
Na+
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’

19. 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

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

21. Industrial chlorine production from electrolysis of brine

22. Copper sulphate CuSO4 solution
PURIFYING COPPER
IMPURE COPPER ANODE
PURE COPPER CATHODE
Copper sulphate CuSO4 solution
Cu2+
Cu2+
Copper atoms from impure copper are OXIDISED to copper ions
Copper ions from impure copper are REDUCED to copper atoms Cu
Cu2+ Cu
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.

23. Why will the concentration of the solution stay the same?
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?