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Electrolysis

Electrolysis. To revise electrolysis of molten compounds: aluminium oxide To revise electrolysis of aqueous solutions: brine and copper sulfate. Electrolysis. The decomposition of ionic compounds using electricity

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Electrolysis

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  1. Electrolysis • To revise electrolysis of molten compounds: aluminium oxide • To revise electrolysis of aqueous solutions: brine and copper sulfate

  2. Electrolysis • The decomposition of ionic compounds using electricity • Only works with molten or dissolved ionic compounds because the ions are free to move

  3. - + Electrolysis • Negative ions are attracted to the positive electrode (anode) – here they lose electrons = OIL • Positive ions are attracted to the negative electrode (cathode) – here they gain electrons = RIG PANIC!!!

  4. 6.1 Electrolysis Electrolysis: splitting up using electricity Ionic substance - molten (l) - dissolved (aq) Non-metal ion Metal ion

  5. Electrolysis With molten compounds only the elements that are present will be produced at the electrodes, e.g. Zinc chloride zinc + chlorine at the cathode Zn2+ + 2e- Zn at the anode 2Cl- - 2e- Cl2

  6. Electrolysis • With aqueous solutions, the water solvent must be taken into account • It is also possible to have hydrogen or oxygen gas produced • If a halide is present then the halogen gas will be produced

  7. 6.2 Changes at the electrodes Solutions Water contains the ions: H+ and O2- The less reactive element will be given off at electrode

  8. Electrolysis of sodium chloride solution: important industrial process

  9. Sodium Chloride splits to give you: Na+ions and Cl- ions Some water molecules also split up into: H+ ions and OH- ions When Salt Dissolves

  10. Where will the ions go? D.C. current + - Positive Anode Negative Cathode Cl- H+ Na+ OH- Na+ H+ OH- Cl-

  11. Cl2 At the anode D.C. current + - Positive Anode Negative Cathode Cl- H+ 2e- Na+ OH- Na+ H+ OH- Cl-

  12. H2 At the cathode D.C. current 2e- + - Positive Anode Negative Cathode Cl- H+ Na+ OH- Na+ H+ OH- Cl-

  13. H2 Cl2 Where will the ions go? D.C. current + - Positive Anode Negative Cathode Cl- H+ 2e- Na+ OH- H+ OH- Na+ Cl-

  14. H2 Cl2 Products • The three main products are: • Hydrogen gas • Chlorine gas • Sodium hydroxide solution Na+ OH-

  15. Electrolysis of sodium chloride solution • At the negative electrode: hydrogen gas is produced (this comes from the water solvent, hydrogen is less reactive than sodium) 2H+ + 2e- H2 • At the positive electrode: chlorine gas is produced 2Cl- - 2e- Cl2

  16. 6.3 Electrolysing brine

  17. Electrolysis of copper sulfate solution using carbon electrodes • The products are different because water solvent is present: • At the negative electrode: copper metal is produced (it is less reactive than hydrogen) Cu2+ + 2e- Cu • At the positive electrode: oxygen gas is produced (from the water solvent) 4OH- O2 + 2H2O + 4e-

  18. Electrolysis • This technique can be used to electroplate cheaper objects with a layer of a more expensive metal, e.g. Gold-plated jewellery

  19. Purifying copper metal • Electrolysis of copper sulfate solution using copper electrodes • This shows how very pure copper is produced from crude copper metal. • It needs to be very pure for things like ELECTRICAL WIRES so that they conduct very well.

  20. 6.4 Purifying copper

  21. Electrolysing aluminium

  22. Electrolysis of Aluminium Aluminium oxide has a very high melting point. It melts at 2050oC. So, chemists mix another ionic compound called Cryolite with the aluminium oxide. The molten mixture can now be electrolysed at 850-950oC.

  23. Positive Electrode (Anode) O O Al3+ Al3+ O2- Al3+ O2- Al Al Negative Electrode (Cathode) Al

  24. Acids, bases & salts • To revise the difference between acids, bases and alkalis. • To revise the different methods used to make salts.

  25. Acids and bases Metal oxides and metal hydroxides are bases e.g. sodium hydroxide, copper oxide. If the hydroxides are soluble they are called alkalis Sodium + Chloride Water Hydrochloric + Sodium acid hydroxide The name of the salt produced when an acid and alkali react depends on the acid used and the metal in the base

  26. Acids and bases Ammonia (NH3) dissolves in water to form an alkali solution, NH3 (aq). It is used to produce ammonium salts These salts are used as fertilisers

  27. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 More acid More H+ ions More alkali More OH- ions Acids and bases The presence of H+ ions makes solutions acidic. The presence of OH- ions make solutions alkali.

  28. Acids and bases • In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water. Salt + Water Acid + Alkali  potassium + nitrate Water nitric + potassium acid hydroxide H+(aq) + OH-(aq)  H2O (l)

  29. 7.1 Acids and alkalis Acids = H+ ions Alkalis = OH- ions Alkalis = soluble bases

  30. Making salts H+(aq) + OH-(aq)  H2O (l) State symbols are shown after substances in reactions (s)= solid (l)= liquid (aq) = aqueous (dissolved in water) (g) = gas

  31. Making salts Soluble salts can be made by reacting acids with: 1. metals – not all metals are suitable; some are too reactive and others are not reactive enough 2. insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off (copper oxide to make copper sulphate) 3. alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. (neutralisation)

  32. Making salts Metal + Acid  Salt + Hydrogen Soluble salts can be made by reacting acids with: 1. metals – not all metals are suitable; some are too reactive and others are not reactive enough (MASH)

  33. Making salts • Hydrochloric acid produces metal chlorides e.g. Sodium chloride • Sulphuric acid produces metal sulphates e.g. Sodium Sulphate • Nitric acid produces metal nitrates e.g. sodium nitrate

  34. 7.2 + 7.3 Salts

  35. Making salts Soluble salts can be made by reacting acids with: 2. insoluble bases – the base (copper oxide) is added to the acid until no more will react and the excess solid is filtered off (copper sulphate)

  36. Making salts Acid + Alkali  Salt + Water Soluble salts can be made by reacting acids with: 3. alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. (neutralisation) Sodium + Chloride Water Hydrochloric + Sodium acid hydroxide H+(aq) + OH-(aq)  H2O (l)

  37. Making salts Salts solutions (aq) can be crystallised to produce solid salts (s)

  38. 7.2 + 7.3 Salts – metals, bases and alkalis Metals: Metal(s) + acid(aq) salt(aq) + hydrogen(g) Bases:Acid(aq) + base(aq) salt(aq) + water(l) Alkalis:Acid(aq) + alkali(aq) salt(aq) + water(l) Ionic equation (neutralisation): H++ OH- H2O

  39. Making salts Soluble except when with silver, mercury or lead. Insoluble salts can be made by reacting two soluble salts together. This is called a precipitation reaction. All soluble Soluble except when with silver, lead, mercury, barium, strontium and calcium.

  40. Making salts All insoluble except for NH4+, barium and those of group 1 elements All insoluble except for calcium, barium and group 1 Insoluble salts can be made by reacting two soluble salts together. This is called a precipitation reaction. All insoluble except when with NH4+ and group 1 elements

  41. Making salts Silver + Sodium nitrate chloride Sodium + nitrate Silver chloride AgNO3(aq) + NaCl(aq) → NaNO3(aq) +AgCl(s) An example precipitation reaction

  42. Making salts Precipitation reactions are used to remove unwanted ions from solutions, for example to treat drinking water or treating effluent

  43. 7.3 Salts – solutions Solutions: solution(aq) + solution(aq)  precipitate(s) + solution(aq) Solid precipitate is filtered off and dried

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