Chemistry Key Stage 4  Tutorial 4 - PowerPoint Presentation

1. ChemistryKey Stage 4 

2. Tutorial 4.3Electrolysis in industryIn this tutorial we will look at:Describing the extraction of aluminium by electrolysis Explaining the formation of the products in the electrolysis of copper sulfate solution, using copper electrodes, and how this electrolysis can be used to purify copper. Page 100 in your handbook

3. Electrolysis in industry– Starter activityWhat is a metal ore?You do not need to remember any names of ores and minerals, but do you know of any examples? STARTER QUESTIONSPage 100 in your handbook

4. Electrolysis in industryFor a metal below carbon in the reactivity series (see lesson 1), the cheapest method extracted the ore by reduction is often to heat it with carbon. Metals more reactive than carbon (above zinc in the reactivity series) are extracted by electrolysis. Aluminium is extracted by electrolysis of a molten mixture of aluminium oxide and cryolite. The cryolite is used to make the process more economical. Pure aluminium oxide melts at 2000 oC but if it is dissolved in molten cryolite the whole process can be carried out at around 950 oC. Cryolite is also a better conductor of electricity. This saves a lot of energy and therefore also saves a lot of money.Page 100 in your handbook

5. Electrolysis in industryAt the cathode the reaction is: Al3+ + 3e- à Al (reduction)At the anode the reaction is: 2O2- à O2 + 4e- (oxidation)The oxygen gas reacts with the hot carbon anodes. It makes carbon dioxide gas. This burns away the anodes, which must be replaced quite often.Page 100 in your handbook

6. Electrolysis in industryEXAMPLE 1a) Write a full balanced chemical equation for the electrolysis of aluminium oxide.b) Explain why aluminium oxide is dissolved in molten cryolite and then electrolysed.c) List some of the main expenses in the electrolysis of aluminium oxide.Answer:a) 2Al2O3  4Al + 3O2b) Dissolving in molten cryolite brings down the melting point of aluminium oxide. Cryolite is also a better conductor of electricity. This saves a lot of energy and therefore also saves a lot of money.c) Heat required to melt aluminium oxide – energy costThe anodes need to replaced often.Electricity for the electrolysis process.Page 101 in your handbook

7. ElectroplatingGraphite electrodes are often used to investigate the electrolysis of molten salts, and of aqueous solutions of ionic compounds. Graphite electrodes are inert electrodes because they do not take part in the electrolysis reactions. Instead, they provide a surface on which these reactions can happen. Electroplating is the process of coating a thin layer of a metal onto another metal object using electrolysis. The electrodes used in electroplating are non-inert - they take part in the electrolysis reactions.Page 101 in your handbook

8. Improving propertiesOne use of electroplating is to improve their appearance and may be used to produce gold-plated jewellery. Or to improve the resistance of metal objects to corrosion. Car rims are electroplated to make them shiny and protection from rusting. Chromium, nickel, tin, cadmium and zinc are often used to plate copper, iron and steel objects. To carry out electroplating:the negatively charged cathode is the object to be platedthe positively charged anode is the plating metalthe electrolyte contains ions of the plating metalFor example, steel cutlery can be electroplated with silver using a silver anode and silver nitrate solution.

9. Activity:Label the diagramAnother use of electroplating is to purify copper on an industrial scale for use in electrical wiring. The copper extracted from its ores is not pure enough: The cathode is pure copperthe anode is impure copper, produced from copper orethe electrolyte is copper(II) sulfate solutionPage 101 in your handbookACTIVITY

10. Activity:Label the diagramWhen the dc power supply is turned on:at the cathode, copper ions from the electrolyte gain electrons and become copper atoms and plate the cathode: Cu2+(aq) + 2e- → Cu(s)at the anode, copper atoms lose electrons and become copper ions in the electrolyte: Cu(s) → Cu2+(aq) + 2e-Over time, the pure copper cathode increases in mass (gets bigger) and the impure copper anode decreases in mass (gets smaller and smaller). Impurities from the anode drop to the bottom.Page 101 in your handbookACTIVITY

11. Comparing inert and non-inert electrodesPage 102 in your handbook Carbon electrodes (inert)Copper electrodes (non-inert)At the cathode (-)(the reactions are the same)Cu2+(aq) + 2e- → Cu(s)Cu2+(aq) + 2e- → Cu(s)At the anode (+)4OH- → 2H2O + O2 +4e-Oxygen gas is given offCu(s) → Cu2+(aq) + 2e-no oxygen is seen

12. Electrolysis in industrySTRETCH QUESTIONAn electrolysis reaction is set up with silver nitrate solution as the electrolyte, silver anode and an aluminium glass frame as the cathode that needs to be coated in silver.a) Silver atoms form silver ions at the anode. Write an ionic half equation for the reaction taking place at the anode.b) Write the ionic half equation for the reaction where the glass frame becomes coated in silver at the cathode.c) Explain whether it is oxidation or reduction taking place at the cathode.d) Plating the glass frame in silver improves its appearance. State another reason for plating a glass frame in silver. Answer:a) Ag à Ag+ + e-b) Ag+ + e- à Agc) As the silver ions gain electrons to form silver atoms, reduction is taking place at the cathode.d) Increases corrosion resistanceYou can get the same look as real silver, however, for less money.

13. Electrolysis in industryEXAMPLE 2In the purification of copper, impure copper is used as the anode and pure copper is collected at the cathode. The electrolyte used is copper sulfate solution. a) Copper ions form copper atoms at the cathode. Write the ionic half equation for this reaction.b) Explain the type of reaction taking place at the cathode.c) Explain why the electrolyte has to be a copper salt solution  Answer:a) Cu2+(aq) + 2e- → Cu(s)b) Reduction, as the copper ions gain electronsc) If the electrolyte contained a different cation example Ag+ , this ion will be attracted to the cathode and Ag atoms would form at the cathode which would contaminate the copper.Page 102 in your handbook

14. Summary Questions1. Give the definition for an ore. [1 mark] 2. Explain the two ways in which metals can be extracted from their ores. [2 marks] 3. Aluminium is extracted from its ore aluminium oxide through electrolysis. Write the ionic half equations for the reactions taking place at the anode and cathode during the electrolysis of molten aluminium oxide. [2 marks]Pages 102-103 in your handbookPRACTICE

15. Summary Questions4. Electrolysis is used in the purification of copper where the anode is impure copper, cathode is pure copper, and the electrolyte is an aqueous solution of copper sulfate. Sam weighed the anode (impure copper) and cathode (pure copper) before setting up the electrolysis experiment for the purification of copper and weighed them again after electrolysing for about an hour. a) The mass of the cathode went up during the electrolysis reaction. Explain this observation. [2 marks] b) The mass of the anode went down during the reaction. Explain this observation. [2 marks] c) The blue colour of the solution stayed the same during the reaction. Explain this observation. [2 marks] d) A solid collected under the anode. Explain what the solid could be. [1 mark] e) Write the ionic half equations for the reactions taking place at each electrode in this reaction. [2 marks]Pages 102-103 in your handbookPRACTICE

16. Summary Questions1. An ore is a rock that contains enough metal to make extraction worthwhile. [1 mark]2. Reduction using carbon/carbon monoxide – this method is used to extract metals below carbon in the reactivity series as here carbon can displace the metal from its compound. [1 mark]Electrolysis – this method is used for metals above carbon as carbon can’t displace them. [1 mark]3. Anode: 2O2- → O2 + 4e- [1 mark] Cathode: Al3+ + 3e- → Al [1 mark]Pages 102-103 in your handbookANSWERS

17. Summary Questions4. a) Copper ions attracted to the cathode gained electrons at the cathode to form copper atoms. Copper solid was deposited on the electrode causing the mass to increase. [1 mark] Cu2+ + 2e- → Cu [1 mark] b) copper atoms at the anode lost electrons to form copper ions. The copper ions entered the solution. As copper atoms were being lost, the mass of the electrode decreased. [1 mark] Cu → Cu2+ + 2e- [1 mark] Pages 102-103 in your handbookANSWERS

18. Summary Questions4. c) The amount of copper ions entering the solution at the anode is the same amount of copper ions removed from the solution at the cathode. [1 mark] No extra copper ions are removed from the copper sulfate solution. The concentration of copper ions in solution stays constant so colour does not change. [1 mark] d) The solid is impurities that were part of the anode (impure copper) [1 mark] e) Cathode: Cu2+ + 2e- → Cu [1 mark] Anode: Cu → Cu2+ + 2e- [1 mark] TOTAL = 14 marks Pages 102-103 in your handbookANSWERS

19. Exam-Style Questions1. The diagram shows how aluminium is extracted in industry.a) i) Name the process used to extract aluminium. [1 mark]ii) Identify the element used to make the electrodes labelled G and explain why it is chosen. [2 marks]iii) State whether electrode H is positive or negative. [1 mark]Page 103 in your handbookEXAM PRACTICE

20. Exam-Style Questionsiv) Liquid L contains aluminium oxide and one other substance. [2 marks]Name this other substance and give one reason for its use in the extraction of aluminium.v) State the product collected at electrode H and explain why it is collected in liquid state. [2 marks]b) The product formed at electrode G reacts with the electrode to form carbon monoxide and carbon dioxide.i) Identify this product. [1 mark]ii) State why carbon monoxide is poisonous. [1 mark]Page 104 in your handbookEXAM PRACTICE

21. Exam-Style Questionsc) The extraction of aluminium can be represented by the chemical equation:2Al2O3 → 4Al + 3O2Write the two ionic half-equations that can also be used to represent this extraction.[2 marks]d) State the major reason for the high cost of extracting aluminium [1 mark]Page 104 in your handbookEXAM PRACTICE

22. Exam-Style Questions1. a) i) Electrolysis [1 mark] ii) Carbon/graphite [1 mark] Graphite conducts electricity [1 mark] iii) Negative [1 mark] iv) Cryolite [1 mark] Solvent (for alumina) OR To lower operating temperature/to lower melting point of mixture/electrolyte [1 mark] v) Aluminium is collected at electrode H [1 mark] The melting point of aluminium is below the temperature of the set up so aluminium melts. [1 mark]Pages 103-104 in your handbookANSWERS

23. Exam-Style Questions1. b) i) Oxygen/O2 [1 mark] ii) Decreases capacity of blood to carry oxygen [1 mark] c) Cathode: Al3+ + 3e- → Al [1 mark] Anode: 2O2- → O2 + 4e- [1 mark] d) Cost of electricity [1 mark] TOTAL = 13 marksPage 104 in your handbookANSWERS

24. Title/subheadingOver to you!<<Insert learning activities here>>

25. Module Learning ObjectivesIn this module we looked at:Understanding how metals can be arrange into a reactivity series based on their reactivity and displacement traits. ​Understanding and using the terms reduction, oxidation, redox, oxidising agent and reducing agent. ​Outlining the experimental processes of electrolysis including half equations at each electrode. ​Describing electrolysis of aqueous solutions and gaseous tests of products. ​Using purification of copper and aluminium extraction to describe electrolysis. ​

26. Knowledge CheckTo complete the Knowledge Check go to this link:[TUTOR TO ADD LINK]Page 105 in your handbook

27. Knowledge Check1. What are the products of electrolysing potassium iodide solution at cathode and anode?Hydrogen- IodineHydrogen- OxygenPotassium- IodinePotassium- OxygenPage 105 in your handbook

28. Knowledge Check2. Amy carried out an experiment between magnesium and hydrochloric acid. She did two experiments. In the first one she used small pieces of magnesium ribbon and in the second one she used magnesium powder. She kept all other variables the same. She noticed that the second reaction was faster. Choose the best explanation for the observation.Magnesium pieces have greater surface area than magnesium powder Magnesium powder has more surface area than magnesium pieces. Magnesium powder has more energy than magnesium pieces. The hydrochloric acid has more energy in the second one than the firstPage 105 in your handbook

29. Knowledge Check3. Magnesium can be obtained by the electrolysis of magnesium chloride. Solid magnesium chloride is obtained from seawater. The magnesium chloride is melted and then electrolysed. The positive electrode is made of graphite and the negative electrode is made of steel. Magnesium forms at the negative electrode. Chlorine forms at the positive electrode. Why is steel is not used for the positive electrode? A. The chlorine produced at the positive electrode will react with the iron in steel B. The chlorine produced at the positive electrode will not react with the iron in steel Page 105 in your handbook

30. ReflectionPage 106 in your handbook

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