electrolysis module c2 splitting up ionic compounds (f) molten compounds ionic solutions &...
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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
+
-
+-+
- +
-+-
+-
+ +
-
Cl- ION
Na+ ION
SPLITTING UP IONIC COMPOUNDS 1
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
+ --
++ --+
- +
DISSOLVE
H2O800°C 20°C
SEPARATING THE IONS 1
+ _
Battery pulls electrons off one
electrode and pushes them onto the other
Metal ELECTRODEELECTRON
This IS SHORT OF electrons so becomes
POSITIVELY CHARGED “ANODE”
This HAS EXTRA electrons so becomes
NEGATIVELY CHARGED “CATHODE”
+
+ -+
- +
-
-
+
--
+ +
-
+
MOLTEN IONIC COMPOUND
+ ANODE - CATHODE
When the battery is switched on,
the + IONS move to the – CATHODE
the – IONS move to the + ANODE
SEPARATING THE IONS 2
This gives a way to SPLIT UP IONIC COMPOUNDS: “ELECTROLYSIS”
Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)
+
Cl-
Cl-
Cl-
Cl-
ClCl
Cl Cl
- = chloride ION, extra 1 electronCl- Cl
chlorine ATOM, NEUTRAL
Cl Cl Cl2
molecule
- chloride IONS lose their extra electrons and turn into neutral chlorine ATOMS
At ANODE: Cl- e- + Cl
then: Cl + Cl Cl2 (gas)
Both together:
2Cl- → 2e- + Cl2
Na+
Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)
+
At CATHODE: Na+ + e- Na
Na+
Na+ Na+
Na
NaNa
Na
= sodium ION, missing1 electron
sodium ATOM, NEUTRALNa++ Na
+ sodium IONS gain an extra electron and turn into neutral sodium ATOMS
molten sodium metal sinks to
bottom
Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)
MOLTEN SODIUM
CHLORIDE
+ ANODE - CATHODE
CHLORINE gas Cl2
SODIUM metal Na
At ANODE: Cl- e- + Cl
At CATHODE: Na+ + e- Na
Cl + Cl Cl2 (gas)
ELECTRONS
Cl- Na+
Example 2: Splitting up MOLTEN LEAD BROMIDE PbBr2
MOLTEN LEAD
BROMIDE
+ ANODE - CATHODE
BROMINE gas Br2
LEAD Metal Pb
At ANODE: Br- e- + Br
At CATHODE: Pb2+ + 2e- Pb
Br + Br Br2 (gas)
ELECTRONS
Br- Pb2+
Both together:
2Br- → 2e- + Br2
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)
Cl-
Na+
OH-H+Na+
Cl-OH-H+
H+
OH-Cl-
Na+
IONIC SOLUTION
Cl-
Na+
OH- H+Na+
Cl-
OH-
H+
H+
OH-Cl- Na+
Which ions gain or lose electrons (“get discharged”) and which stay in solution?
IONIC SOLUTIONS: At the CATHODE
+
At CATHODE: 2H+ + 2e- H2
sodium ION, missing 1 electron
Hydrogen ATOM, NEUTRALNa+ H
Na+
Na+
H+
H+
H
H
H+
hydrogen ION, missing 1 electron
As HYDROGEN is LESS REACTIVE than SODIUM, it is discharged. The sodium ions stay in solution.
which ions?
+
Cl-
Cl-
Cl-
Cl-
ClCl
Cl Cl
chloride ION, extra 1 electronCl- Cl
chlorine ATOM, NEUTRAL
At ANODE: 2Cl- 2e- + Cl2
IONIC SOLUTIONS: At the CATHODE – halogen compounds
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
OH
O HOH
O H
O Hhydroxide ION, from water extra electron
which ions?
+
nitrate ION, extra 1 electronNO3
-Oxygen atom
At CATHODE: 4OH- 2H2O + O2 + 4e-
OH
H
IONIC SOLUTIONS: CATHODE – non halogen compounds
NO3-
NO3-
O Hhydroxide ION, OH- from water, extra electron
O
NO3-
NO3-
OH
O H
O
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?
+ ANODE
Attracts – ions (‘Anions’)
If – ions are HALOGENS ie
chloride Cl-
bromide Br-
iodide I-
the HALOGEN is produced.If – ions are NOT HALOGENS
Eg sulphate SO42-,
nitrate NO3-
carbonate CO32-
OXYGEN 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
RULES FOR IONIC SOLUTIONS
potassium chloride
Compound State Anode (+)Cathode (-)Ions
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 solution Zn+ SO42- H+ OH- hydrogen oxygen
(REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ )
(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
To DOUBLE the MASS of substance discharged at electrodes:
2 x CURRENT (2x batt. voltage)
2 x TIME current flows for
(Q = I t)
-
+
+-
Cl-
Cl-
Cl-
Cl- Na+ Na+
Na+ Na+
OILRIG
- 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’
Oxidation is loss, reduction is gain
‘OILRIG’
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 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
INDUSTRIAL ELECTROLYSIS OF BRINE
ANODEOH- and Cl-
2Cl- 2e- + Cl2
OH- left in solution so concentration grows
CATHODEH+ and Na+
2H+ + 2e- H2
Na+ left in solution so concentration grows
Chlorine gas Hydrogen gas
Sodium chloride solution (neutral) slowly changed to
sodium hydroxide solution (alkaline)
BRINE (NaCl solution)
IMPURE COPPER ANODE
PURE COPPER CATHODECopper sulphate
CuSO4 solution
PURIFYING COPPER
Cu2+
Cu
CuCu2+
Cu2+Copper atoms from impure copper are OXIDISED to copper ions
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.