Electrolysis L.O. I know and can use the terms electrolyte, electrode, anode and cathode.

1. Electrolysis • L.O. • I know and can use the terms electrolyte, electrode, anode and cathode. • I can demonstrate the electrolysis of sodium chloride and test the gases given off • I can explain the electrolysis of dilute sulphuric acid. 21/09/2014

2. What is electrolysis? An ionic compound conducts electricity when it is molten or in solution. The current causes the ionic compound to split up and form new substances. This process is called electrolysis, a word which comes from Greek and means “splitting by electricity”. Electrolysis has many uses, including: • purifying copper • plating metals with silver and gold • extracting reactive metals, such as aluminium • making chlorine, hydrogen and sodium hydroxide.

3. heat What happens during electrolysis? In electrolysis, the substance that the current passes through and splits up is called the electrolyte. The electrolyte contains positive and negative ions. What happens to these ions during electrolysis? Negative ions move to the positive electrode and lose electrons. This is oxidation. Positive ions move to the negative electrode and gain electrons. This isreduction.

4. Electrolysis of molten lead bromide

5. leadbromidelead + bromine PbBr2(l)Pb(l) + Br2(g) Electrolysis of molten PbBr2 – redox equations What redox processes occur at the electrodes during the electrolysis of molten lead bromide (PbBr2)? At the negative electrode: Pb2+ + 2e- Pb (reduction) At the positive electrode: 2Br-  Br2 + 2e- (oxidation) What is the overall equation for the electrolysis of molten lead bromide?

6. Electrolysis of molten PbBr2 – summary

7. What is Electrolysis? When an electric current is passed through an ionic compound dissolved in water, the compound is split up. This is the process of electrolysis. The solution that conducts the electricity is called an electrolyte. To pass the electricity through, a positive electrode (anode) and a negative electrode (cathode) are placed in the electrolyte and connected to a supply of electricity. 21/09/2014

8. SO2- 4 What is Electrolysis? (cont.) Negative ions are attracted towards the _______ and positive ions are attracted towards the __________. Because of the electrode the ions are attracted to, negative ions are called _______ and positive ions are called ________. Which of these ions are anions and which are cations? Na+ Cl- Cu2+ Al3+ O2- SO2- Ag+ Anions:- Cations:- anode cathode anions cations 4 O2- Cl- Na+ Cu2+ Al3+ Ag+

9. Watch the demo then complete the worksheet. Half equations are:- At the cathode H+ + e- H2 At the anode OH- - e- H2O + O2 Electrolysis of Dilute Sulphuric Acid 2 2 4 4 2

10. Three quarters of the salt we use comes from rock salt. As the earth’s surface changed stretches of sea-water became landlocked. Evaporation of this water along with further geological changes led to deposits of rock salt in many countries including the UK. Rock salt

11. Negative ions Salt consists of sodium ions (Na+) and chloride ions (Cl-). Chloride ions go to the anode where they lose an electron The neutral chlorine atoms produced join up into pairs Electrolysis of salt 1 Chlorine gas is formed 2Cl- - + 2e- Cl2

12. Positive ions Na+ are not the only + ions present. There are also H+ ions because some water molecules split up into H+ and OH- ions. H+ accepts electrons more easily than Na+ does. This has important consequences at the cathode. Na+ Cl- H H H O O- Electrolysis of salt 2 H+

13. Electrolysis of salt 3 • Na+ ions move to the cathode but do not accept electrons. • It is the hydrogen ions that gain electrons • As a result hydrogen gas is formed at the cathode. 2H+ + 2e-  H2

14. No change “Spectator ion” Na+ Changed into chlorine gas at anode H Changed into hydrogen gas at cathode O- No change “Spectator ion” Electrolysis of salt 4 • What happens to the various ions? Cl- H+

15. Na+ Cl Cl Na+ Cl- H H H H O- O- H+ Solution Electrodes Electrolysis of salt 5 • Sodium hydroxide is what is left in the solution at the end of electrolysis

16. Electrolysis of salt

17. Electrolysis of salt - summary

18. Questions about salt Some salt is dug out of underground deposits of rock salt but most salt is obtained by pumping river water into the salt deposits which may be about 2000m below ground. The salt dissolves to form a solution called brine which travels up a second pipe to the surface. It is then stored in a brine reservoir prior to being used for various purposes. • Name one important winter use of solid rock-salt. • Draw a diagram illustrating the production of brine and its subsequent electrolysis. • Draw a diagram showing how you could remove the gritty impurities in rock-salt in the laboratory.

19. Brine 2000m - + Dissolved salt. up to surface Rock Salt dissolves Hydrogengas Chlorine gas Sodium Hydroxide Electrolysis Cell Answers about salt 1 • An important winter use of solid rock-salt is to treat icy roads. • Diagram of brine production to electrolysis. pump River

20. Residue: Gritty impurities Filter paper Filter funnel Filtrate: brine Conical flask Answers about salt 2 • Removal of gritty impurities from Rock Salt

21. Sterilisation of water e.g. swimming pools and drinking water Bleaching agent, e.g. paper industry Uses of chlorine 1 electrolysis Sodium chloride ‘rock salt’ solution Chlorine gas

22. Manufacture of HCl continued Uses of chlorine 2 Sterilisation of water e.g. swimming pools and drinking water electrolysis Sodium chloride ‘rock salt’ solution Bleaching agent, e.g. paper industry Chlorine gas Manufacture of Cl-containing organic chemicals

23. Uses of chlorine 3 Manufacture of PVC and other plastics Manufacture of HCl

24. Uses of chlorine 4 Manufacture of PVC and other plastics Manufacture of HCl Pesticides Manufacture of Cl-containing organic chemicals Solvents, e.g. solvent for tippex, & ‘dry cleaning’ dyes

25. Uses of sodium hydroxide 1 Extraction of aluminium Manufacture of soap Sodium chloride ‘rock salt’ solution electrolysis Sodium hydroxide Manufacture of Paper

26. Uses of sodium hydroxide 2 Extraction of aluminium Manufacture of soap Sodium chloride ‘rock salt’ solution Sodium hydroxide solution electrolysis Manufacture of Paper Textiles (wool, cotton)

27. Uses of sodium hydroxide 3 Extraction of aluminium Manufacture of soap Sodium chloride ‘rock salt’ solution Sodium hydroxide solution electrolysis Manufacture of Paper Textiles (wool, cotton) Neutralisation of acid effluents

28. Uses of Cl and NaOH

29. bleach Sodium hydroxide soap Aluminium extraction Acid neutralisation salt chlorine Hydrochloric acid hydrogen Organic chlorides pvc dyes Uses of products from salt Join the appropriate substances with arrows.

30. What is formed at the cathode in the electrolysis of aqueous rocksalt? Sodium Chlorine Hydrogen Sodium chloride

31. What is formed at the anode in the electrolysis of aqueous rocksalt. Sodium Chlorine Hydrogen Sodium hydroxide

32. What is left in the solution as a result of the electrolysis of aqueous rocksalt? Sodium Chlorine Hydrogen Sodium hydroxide

33. What is chlorine gas NOT used in the manufacture of? Alkali Hydrochloric acid P.V.C. Bleach

34. What is sodium hydroxide NOT used in the manufacture of: Textiles Aluminium Soap Cable insulation