C3.4 – Electrolysis and cells
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Transcript of C3.4 – Electrolysis and cells
C3.4 – Electrolysis and cells
Learning Objectives:
• To describe the properties of metals
• To explain the processes involved in electrolysis
• To describe the principle behind a dry cell
Name all the properties of metals that you can
Iron and copper are both __________ metals. They have __________ melting points, they are good conductors of __________ and electricity and they have a high __________. Transition metals have uses linked to their __________. For example, copper is __________ and so is used to make wires. Some transition metals are used as __________ , speeding up chemical reactions.
catalysts density ductileheat
high properties transition
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
To make ions move:
-+ +- + -
-+ +
- + -
+
-
+-+
- +
-+-
+-
+ +
-
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”
Define the terms:
•Electrolyte•Electrode•Anode •Cathode•Inert
Electrolysis Practical1. Select one of the test substances
and connect it into the circuit.2. Switch on the power supply. Does
the bulb light?3. Look carefully for any signs of
reaction. These might include:
• gases being given off• changes taking place on the
surface of the electrodes • changes taking place in the liquid.
+
+ -+
- +
-
-
+
--
+ +
-
+
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
1. Zinc atoms turn into ions, and electrons travel around the circuit.
2. When they reach the metals cap they travel down the positive pole.
3. The manganese dioxide absorbs the electrons.
4. There is an electrolyte (ammonium chloride in this case) which is kept damp and which the ions can travel through.
Recipe Time
Write a recipe
of the lesson (or of your learning)