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Topic 12 -

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Topic 12 -

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    1. Topic 12 - Corrosion

    2. 1. The Corrosion of Metals Most metals react with their surroundings to form oxides and hydroxides. E.g. copper forms copper hydroxide. iron rusts to form iron oxide.

    3. There are only a few metals that resist corrosion. These include:- Gold stainless steel (iron + carbon + chromium) platinum

    4. Corrosion - is the changing of the surface of a metal element into a compound. Silver + oxygen silver oxide Rusting - is the special name given to the corrosion of iron. Iron + oxygen iron (III) oxide

    5. Corrosion and the Reactivity series Potassium Sodium Lithium Calcium Magnesium Aluminium Zinc Iron Tin Lead Hydrogen Copper Mercury Silver

    6. Would you want to be wearing a ring made of rubidium when you go for a swim?

    7. Experiment: Conditions For Rusting

    8. Experimental Conclusions

    9. 2. What Happens When Metals Corrode? Corrosion is a chemical reaction. It involves the metal atoms losing electrons. Ag(s) Ag+ + e- Metals corroding are examples of . reactions.

    10. 2. What Happens When Iron Rusts? Rusting involves iron atoms (metal) losing electrons to form ions. It occurs over two steps Fe(s) Fe2+(aq) + 2e- () Fe2+(aq) Fe3+(aq) + e- (....) The electrons lost from the iron are accepted by the water and oxygen. 2H2O(l) + O2(g) + 4e- 4OH-(aq) ()

    11. Experiment 2: Testing For Rusting

    12. Protection Against Corrosion

    13. Methods of Protection There are two main methods of protecting metals from corrosion:- Physical protection - placing a barrier to water and oxygen on the surface of the metal. Chemical Protection - providing the metal with a source of electrons to prevent oxidation.

    14. Questions Name the two types of corrosion prevention. What two chemicals are required for rusting to occur? What else is required for rusting to occur?

    15. Physical Protection

    16. Questions Give one advantage and one disadvantage of each type of physical protection:- Plastic Oil and grease. Paint Tin-plating

    17. Physical Protection cont- Electro-plating - coating the iron with a less reactive metal.

    18. Questions What metals are used to plate steel and iron? Where do most of these metals sit in the electrochemical series? What terminal on the power pack is the metal to be coated connected? Write the ion-electron equation for the reduction of copper(II) ions.

    19. Problems with Electroplating

    20. Questions When does electro-plating prevent rusting? When does electro-plating cause rusting to occur faster? When rusting occurs, what metal is losing electrons? What metal is being protected from corrosion?

    21. Physical Protection cont- Galvanising - coating the iron with zinc.

    22. Questions What metal is used in galvanising? What type of protection does this offer the iron/steel? Give 3 examples of galvanising being used to prevent rusting. Which metal is higher in the electrochemical series - iron or zinc?

    23. When a metal corrodes it loses electrons. Mg Mg2+ + 2e- Chemical protection supplies the metal with a flow of electrons. This prevents corrosion. This could be from a direct current power supply or a metal higher in the E.C. series. Chemical Protection

    24. Questions What happens to a metal when it corrodes? How can we prevent this loss of electrons from a corroding metal?

    25. Direct Electrical Protection This involves connecting the iron to the negative terminal of a battery or power supply.

    26. Questions What do you use to provide direct-electrical protection to a metal surface? Are there any disadvantages to this method? Give two examples of where this is used.

    27. Sacrificial Protection What happens when you connect a more reactive metal to a less reactive metal in a simple cell?

    28. Questions Where must the metal used for sacrificial protection be on the E.C. series? What is the next most suitable metal for sacrificial protection after zinc? Which metal would provide the best protection out of these two? What problems may there be from using these metals?

    29. Sacrificial Protection cont.

    32. Past Paper Questions The following past paper questions are from Topic 12.

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