Corrosion
Intermediate 2
Unit 3(c)
WHAT IS CORROSION?
Potassium
• 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
R
E
A
T
I
V
I
T
Y
S
E
R
I
E
S
CHEMICALS FOR RUSTING
Rusting
requires
OXYGEN
and
WATER
Rusting is
speeded
up by
SALT
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?
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
Fe + O2
Iron(III) oxide
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
CHEMISTRY OF RUSTING 2
•
Corrosion occurs when metals lose electrons –
OXIDATION
•
When iron rusts, there are two steps.
The IRON is
OXIDISED
Iron atoms into iron(II) ions
Iron(II) ions into iron(III) ions
Write ion-electron equations for both steps
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
4OHGAIN of electrons = REDUCTION
•Dissolved chemicals in the water called ELECTROLYTES
help the electrons to be transferred
•Give 2 examples of these dissolved chemicals
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
TUTORIAL QUESTIONS
True or false:
1.
2.
3.
4.
5.
6.
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.
(a) Write an ion-electron equation for the rusting of iron
(b)Why does rusting occur faster in salt solution than pure water?
(c) Suggest why rusting also occurs faster in acid solution than in pure
water
Practical
Set up the experiment shown bellow, using two metals of your choice.
Use a U-tube instead of a beaker.
A
TUTORIAL QUESTION
Relative
rate of
rusting of
iron
Concentration of dissolved electrolytes
1. Suggest units for the horizontal axis
2. What is the relationship between the rate of rusting of iron and the
concentration of dissolved electrolytes.
3. Suggest why the graph does not start from a rate of zero.
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
MORE REDOX
A
Iron
Tin
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:
a) The colour of the solution around the iron
b) The mass of the iron in the cell
c) Does the iron rust?
MORE REDOX 2
A
Iron
When a cell is made from two metals,
ELECTRONS FLOW FROM THE METAL
HIGHEST IN THE ELECTROCHEMICAL
Magnesium
SERIES TO THE METAL LOWER IN THE
SERIES.
Which metal in this cell loses electrons?
Which metal is oxidised?
Describe what happens to:
a) The colour of the solution around the
iron
b) The mass of the iron in the cell
c) Does the iron rust?
MORE REDOX 3
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:
Electrons move from the carbon rod to
the iron
Electrons flow through the solution
A
Iron
Carbon
A blue colour appears at the iron
electrode
The mass of the iron electrode
decreases
The iron is oxidised
• SG textbook, p118-119
Set up an experiment to
answer the above questions.
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)
ELECTROPLATING
DC
-ve
+ve
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
Copper
Gold
So the copper is coated in gold atoms
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.
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)
CHEMICAL PROTECTION
• Sacrificial protection
A metal high in the electrochemical series
is connected to the metal to be protected
and is sacrificed
A
Iron
Magnesium
- This is used in underground pipes
Mg
In terms of electrons, explain how sacrificial
protection works.
• Galvenising
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 12.1 + 12.2
PRACTICE EXAM
QUESTIONS
SG textbook, p120-121
Questions for SG textbook, p43-44