C3.4 – Electrolysis and cells

1. 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

2. Name all the properties of metals that you can

3. catalysts density ductile heat high properties transition 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.

4. 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

5. H2O MELT DISSOLVE - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + SPLITTING UP IONIC COMPOUNDS To make ions move: 800°C 20°C

6. 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”

7. Define the terms: • Electrolyte • Electrode • Anode • Cathode • Inert

8. Electrolysis Practical • Select one of the test substances and connect it into the circuit. • Switch on the power supply. Does the bulb light? • 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.

9. 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”

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

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

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

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

14. What happens when the ionic compounds are dissolved in water? Na+ Na+ Na+ OH- OH- OH- H+ H+ H+ Cl- Cl- Cl- 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)

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

16. H Na+ Na+ H H Na+ H+ H+ H+ At CATHODE: 2H+ + 2e- H2 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.

17. IONIC SOLUTIONS: At the CATHODE – halogen compounds Cl- H H H H H Cl Cl Cl Cl Cl At ANODE: 2Cl- 2e- + Cl2 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?

18. IONIC SOLUTIONS: CATHODE – non halogen compounds NO3- NO3- NO3- NO3- O O O O H H H H H At CATHODE: 4OH- 2H2O + O2 + 4e- 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?

19. RULES FOR IONIC SOLUTIONS + ANODE Attracts – ions (‘Anions’) - CATHODE Attracts + ions (‘Cations’) If + ions (metals) are MORE REACTIVE than hydrogen K, Na, Ca, Mg, Zn, Fe Then HYDROGEN is produced 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. If + ions (metals) are LESS REACTIVE than hydrogen Cu, Ag, Au Then the METAL is produced

20. Zinc atoms turn into ions, and electrons travel around the circuit. • When they reach the metals cap they travel down the positive pole. • The manganese dioxide absorbs the electrons. • There is an electrolyte (ammonium chloride in this case) which is kept damp and which the ions can travel through.

21. Recipe Time Write a recipe of the lesson (or of your learning)