2 WHAT IS CORROSION?PotassiumREATIVITYSERIESWhen the surface of a metal changes from being an element into a compoundThe surface goes from being shiny to dullNearly all metals corrodeThey don’t all corrode at the same rateSuggest a metal which does NOT corrodeIronGold
3 CHEMICALS FOR RUSTING Rusting requires OXYGEN and WATER Rusting is speeded up by SALT
4 RUSTING The corrosion of iron is called rusting Iron is the only metal which rustsView Rusting practical demoWhy did the water rise up the tube?How would the rate of the water rising compare if the wool had been soaked in water instead of acid?
5 CHEMISTRY OF RUSTINGIron metal reacts with oxygen to produce iron(III) oxide (rust)Write a word equation and a chemical equation for this processIron + oxygen Iron(III) oxideFe + O Fe2O3This is an example of a REDOX reactionWriting ion-electron equation practice:Sodium atoms losing electrons to form sodium ionsSulphur atoms gaining electrons to become sulphide ionsCopper(II) ions being reducedBromine atoms being reduced
6 CHEMISTRY OF RUSTING 2Corrosion occurs when metals lose electrons – OXIDATIONWhen iron rusts, there are two steps.Iron atoms into iron(II) ionsIron(II) ions into iron(III) ionsWrite ion-electron equations for both stepsThe IRON is OXIDISED
7 REDOX REACTIONSWhenever there is oxidation there must also be reductionWhat two other chemicals are involved in the rusting of iron?Oxygen and waterThe reduction step involves these chemicals:2H2O + O2 + 4e OH-GAIN of electrons = REDUCTIONDissolved chemicals in the water called ELECTROLYTES help the electrons to be transferredGive 2 examples of these dissolved chemicals
8 DETECTING RUSTING Ferroxyl indicator can be used to detect rusting The chemical changes colour in the presence of Fe2+ ionsIt changes from yellow/green to blue
9 TUTORIAL QUESTIONS True or false: Corrosion is the rusting of iron pH indicator should be used to test for rustingRusting takes place in the presence of oxygen onlyRusting will not take place if water is not presentSalt is required for rusting to take placeRusting will occur faster in sea water than tap waterProblem:Rusting occurs faster in salt solution than in pure water.Write an ion-electron equation for the rusting of ironWhy does rusting occur faster in salt solution than pure water?Suggest why rusting also occurs faster in acid solution than in pure water
10 PracticalSet up the experiment shown bellow, using two metals of your choice.Use a U-tube instead of a beaker.A
11 TUTORIAL QUESTION Relative rate of rusting of iron Concentration of dissolved electrolytesSuggest units for the horizontal axisWhat is the relationship between the rate of rusting of iron and the concentration of dissolved electrolytes.Suggest why the graph does not start from a rate of zero.
12 FLOWING ELECTRONSA chemical cell can be used to show electrons moving away from iron when it rustsAFe Fe2+ + 2eOxidationReductionElectrons leave the iron atoms when it rusts
13 MORE REDOXWhen a cell is made from two metals, ELECTRONS FLOW FROM THE METAL HIGHEST IN THE ELECTROCHEMICAL SERIES TO THE METAL LOWER IN THE SERIES.Which metal in this cell loses electrons?Which metal is oxidised?Describe what happens to:The colour of the solution around the ironThe mass of the iron in the cellDoes the iron rust?AIronTin
14 MORE REDOX 2When a cell is made from two metals, ELECTRONS FLOW FROM THE METAL HIGHEST IN THE ELECTROCHEMICAL SERIES TO THE METAL LOWER IN THE SERIES.Which metal in this cell loses electrons?Which metal is oxidised?Describe what happens to:The colour of the solution around the ironThe mass of the iron in the cellDoes the iron rust?AIronMagnesium
16 Set up an experiment to answer the above questions. TUTORIAL QUESTIONSWrite an ion-electron equation to show the rusting of iron.A cell is made using an iron nail and a carbon rod, in a solution of ferroxyl indicator. Decide which statements are true:SG textbook, pElectrons move from the carbon rod to the ironElectrons flow through the solutionA blue colour appears at the iron electrodeThe mass of the iron electrode decreasesThe iron is oxidisedAIronCarbonSet up an experiment to answer the above questions.
17 Carry out electroplating practical (7.2) PREVENTING CORROSIONThere are two types of way to help prevent corrosion:Physical protectionChemical protectionPHYSICAL PROTECTIONStops oxygen and water coming into contact with the metal:PaintOil or greasePlastic coatingTin plating – covering in molten tinGalvanising – covering in molten zinc (more expensive)Cathodic protectionElectroplatingCarry out electroplating practical (7.2)
18 ELECTROPLATING The metal to be coated is at the NEGATIVE terminal POSITIVE ions of the OTHER METAL are in the solutionThese ions are attracted to the negative terminal, and when they meet it turn into atomsSo the copper is coated in gold atomsDC-ve+veCopperGoldGold ions, Au+Write an ion-electron equation for the reaction occurring at the negative electrodeIs this oxidation or reduction?Check-test 12.2 ext.
19 Carry out Nails in Contact practical (7.3 core) Cathodic ProtectionWhen a metal corrodes it ………………. electronsIf electrons were forced back to the metal, it would not corrodeHow could these be supplied?Connecting to NEGATIVE terminal of any electrical supply- Used in carsCarry out Nails in Contact practical (7.3 core)
20 CHEMICAL PROTECTION Sacrificial protection A metal high in the electrochemical series is connected to the metal to be protected and is sacrificed- This is used in underground pipesAIronMagnesiumMgIn terms of electrons, explain how sacrificial protection works.
22 TUTORIAL QUESTIONS1. a) Why does coating steel with plastic prevent corrosion?b) Chromium-plated steel corrodes quickly if scratched. What does this tell you about the reactivity of chromium?2. Explain how zinc gives sacrificial protection to steel.3. a) Why should a copper roof not be held in position with iron nails?b) Suggest a metal which the nails should be made of. Why?4. Explain what is meant by “electroplating”5. What name is given to the process where steel is protected by coating it with zinc?6. Assessment test
23 PRACTICE EXAM QUESTIONS SG textbook, pQuestions for SG textbook, p43-44