Chapter 3: Oxidation and Reduction Form 5 Chemistry

Title: Rusting As A Redox Reaction

Section A: General understanding (Fill in the blanks)

1. __________________ is the oxidation of a metal while ________________ is the oxidation of iron when the metal or iron interact with the environment by losing electron.

2. The presence of _______________ and ________________ will cause iron to rust, the rusting can be accelerated by adding _________________ or _____________.

3. Copper forms a green coating as a result of corrosion, it contains _______________ ions.

4. Metals like ______________ and ____________________ forms a very tough oxide which adhere tightly to the surface of the metal preventing further oxidation, thus protecting the metals from further corrosion.

5. To prevent corrosion, metal A can be coated with a layer of metal B which is more _________________________ than A. Metal B will corrode first, thus preventing metal A from corrosion. In this situation, metal B is also called the ____________________ metal.

6. If iron is in contact with another ______________ which is less electropositive than iron, the rate of rusting for iron will be _________________.

7. Zinc is used to protect iron or steel by coating a thin layer of zinc onto it, this process is called ___________________________. In industry, zinc is chosen to serve the purpose rather than other metals because zinc is ___________________ in cost.

8. Some household and bathroom equipments are coated with a layer of shiny finishes, the metals usually used for these coatings are ____________________ and __________________ .

Section B: Learn to write the equations (Guided steps)

The process of rusting occurs in three stages.

1. The formation of iron(II) ions from the metal:

When iron contacts with water, the iron surface oxidizes to form iron(II) ions.

Equation: ______________________________________________________

2. The formation of hydroxide ions:

Electrons travel to the edges of the water droplets, where there is high concentration of dissolved oxygen. Water and oxygen molecules receive the electrons, they are reduced to form hydroxide ions.

Equation: ______________________________________________________

Fe2+readily combines with OH- to form Fe(OH)2

Equation: ______________________________________________________

3. With excess oxygen, the rust is formed:

The Fe2+ ions are further oxidized to form Fe3+ ions, which reacts with OH- ions to form the hydrated iron oxide, Fe2O3. xH2O, known as ‘Rust’.

Equation: ______________________________________________________

Answers Title: Rusting As a Redox Reaction

Section A

1. Corrosion, rusting

2. water, oxygen, salt, acid

3. copper(II)

4. zinc, aluminium

5. electropositive, sacrificial

6. metal, faster

7. galvanization, cheaper

8. chromium, nickel

Section B

1. Fe ---- Fe2+ + 2 e-

2. O2 + 2 H2O + 4 e- --- 4 OH –

* Fe 2+ + 2 OH - ---- Fe(OH)2

3. 2 Fe(OH)2 + O2 ---- Fe2O3. x H2O + H2O

( value of x varies depending on the condition)