1. Corrosion
Intermediate 2
Unit 3(c)

2. WHAT IS CORROSION?
Potassium
R E A T I V I T Y
S E R I E S
When the surface of a metal changes from being an element into a compound
The surface goes from being shiny to dull
Nearly all metals corrode
They don’t all corrode at the same rate
Suggest a metal which does NOT corrode
Iron
Gold

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 rusts
View Rusting practical demo
Why 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 RUSTING
Iron metal reacts with oxygen to produce iron(III) oxide (rust)
Write a word equation and a chemical equation for this process
Iron + oxygen Iron(III) oxide
Fe + O Fe2O3
This is an example of a REDOX reaction
Writing ion-electron equation practice:
Sodium atoms losing electrons to form sodium ions
Sulphur atoms gaining electrons to become sulphide ions
Copper(II) ions being reduced
Bromine atoms being reduced

6. CHEMISTRY OF RUSTING 2
Corrosion occurs when metals lose electrons – OXIDATION
When iron rusts, there are two steps.
Iron atoms into iron(II) ions
Iron(II) ions into iron(III) ions
Write ion-electron equations for both steps
The IRON is OXIDISED

7. REDOX REACTIONS
Whenever there is oxidation there must also be reduction
What two other chemicals are involved in the rusting of iron?
Oxygen and water
The reduction step involves these chemicals:
2H2O + O2 + 4e OH-
GAIN of electrons = REDUCTION
Dissolved chemicals in the water called ELECTROLYTES help the electrons to be transferred
Give 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+ ions
It 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 rusting
Rusting takes place in the presence of oxygen only
Rusting will not take place if water is not present
Salt is required for rusting to take place
Rusting will occur faster in sea water than tap water
Problem:
Rusting occurs faster in salt solution than in pure water.
Write an ion-electron equation for the rusting of iron
Why does rusting occur faster in salt solution than pure water?
Suggest why rusting also occurs faster in acid solution than in pure water

10. Practical
Set 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 electrolytes
Suggest units for the horizontal axis
What 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 ELECTRONS
A chemical cell can be used to show electrons moving away from iron when it rusts A
Fe Fe2+ + 2e
Oxidation
Reduction
Electrons leave the iron atoms when it rusts

13. MORE REDOX
When 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 iron
The mass of the iron in the cell
Does the iron rust? A
Iron
Tin

14. MORE REDOX 2
When 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 iron
The mass of the iron in the cell
Does the iron rust? A
Iron
Magnesium

15. MORE REDOX 3

16. Set up an experiment to answer the above questions.
TUTORIAL QUESTIONS
Write 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, p
Electrons move from the carbon rod to the iron
Electrons flow through the solution
A blue colour appears at the iron electrode
The mass of the iron electrode decreases
The iron is oxidised A
Iron
Carbon
Set up an experiment to answer the above questions.

17. Carry out electroplating practical (7.2)
PREVENTING CORROSION
There are two types of way to help prevent corrosion:
Physical protection
Chemical protection
PHYSICAL PROTECTION
Stops oxygen and water coming into contact with the metal:
Paint
Oil or grease
Plastic coating
Tin plating – covering in molten tin
Galvanising – covering in molten zinc (more expensive)
Cathodic protection
Electroplating
Carry 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 solution
These ions are attracted to the negative terminal, and when they meet it turn into atoms
So the copper is coated in gold atoms DC
-ve
+ve
Copper
Gold
Gold ions, Au+
Write an ion-electron equation for the reaction occurring at the negative electrode
Is this oxidation or reduction?
Check-test 12.2 ext.

19. Carry out Nails in Contact practical (7.3 core)
Cathodic Protection
When a metal corrodes it ………………. electrons
If electrons were forced back to the metal, it would not corrode
How could these be supplied?
Connecting to NEGATIVE terminal of any electrical supply
- Used in cars
Carry 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 pipes A
Iron
Magnesium Mg
In terms of electrons, explain how sacrificial protection works.

21. Galvenising

22. TUTORIAL QUESTIONS
1. 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, p
Questions for SG textbook, p43-44