CorrosionStandard Grade Chemistry

Topic 12

WHAT IS CORROSION?

• 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

Potassium

Iron

Gold

REATIVITY

SERIES

Concept Cartoons: Rusting

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 Iron(III) oxide Fe + O2 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 sulphur 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.

Iron atoms into iron(II) ions

Iron(II) ions into iron(III) ions

Write ion-electron equations for both steps

The IRON is OXIDISED

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

2H2O + O2 + 4e 4OH-

GAIN of electrons = REDUCTION

The reduction step involves these chemicals:

•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 QUESTIONSTrue or false:

1. Corrosion is the rusting of iron2. pH indicator should be used to test for rusting3. Rusting takes place in the presence of oxygen only4. Rusting will not take place if water is not present5. Salt is required for rusting to take place6. 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

A

Set up the experiment shown bellow, using two metals of your choice.

Use a U-tube instead of a beaker.

TUTORIAL QUESTION

Concentration of dissolved electrolytes

Relative rate of rusting of iron

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 Magnesium

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 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:

• SG textbook, p118-119

A

Iron Carbon

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

Set up an experiment to answer the above

questions.

PREVENTING CORROSIONThere are two types of way to help prevent corrosion:• Physical protection• Chemical protection

PHYSICAL PROTECTIONStops 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

Copper Gold

Gold ions, Au+

DC-ve +v

e

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

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 ………………. electronsIf 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

Iron Magnesium

A metal high in the electrochemical series is connected to the metal to be protected and is sacrificed

- 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