Chapter 1 : Oxidation & Reduction

Question 1

Definition of Rust

Answer 1

is a redox reaction between iron, oxygen and water to form a brown substance.

Question 2

What is Rust? Detailed H2O?

Answer 2

• HYDRATED IRON(III) OXIDE, Fe203•xH20

- Composition of H2O in Rust is not constant

Question 3

Condition for Rusting

Answer 3

Water and Air

Question 4

Explain how rusting occurs
In terms of,
- terminal

Answer 4

TERMINAL

• In the centre of the water droplet, the concentration of oxygen is low. Therefore, it acts as the negative terminal (anode）
• At the edge of the water droplet, the concentration of oxygen is high. Therefore, it acts as the positive terminal (cathode)

Question 5

Explain how rusting occurs
In terms of,
- redox reaction
- equation

Answer 5

1) At the negative terminal,
> Iron rust via the oxidation process to form iron(II) oxide.
>The electrons flows to the edge of the water droplet through the iron surface.

Fe > Fe2+ + 2e-

2) At the positive terminal,
>oxygen accepts electrons from the oxidation of iron and undergoes reduction to form hydroxide ions.

O2 + 2H2O + 4e- > 4OH-

3) The Fe2+ and OH-ions in the water droplet combine to form iron(II) hydroxide.

Fe2+ + 2OH- > Fe(OH)2

4) The iron(II) hydroxide produced is oxidised by oxygen to form iron (iii) hydroxide, which then decompose to hydrated iron(iii) oxide.

4Fe(OH) 2 + 2H2O + O2 > 4Fe(OH)3 (oxidation)

Fe ( OH) 3 > Fe2O3 • xH2O ( decompose)

Question 6

Equation of Rust

Answer 6

4Fe + 3O2 + 2xH20 > 3Fe2O3•xH20

Question 7

Method to Prevent Rust

1) Using a Protective Layer
- how
- why

Answer 7

Apply a layer of paint, oil or grease
Wrap with a plastic coating (fences)

To prevent iron from being in contact with water and air.

Question 8

Method to Prevent Rusting

2) Galvanization
- how
- why
- 4 sentence

Answer 8

Iron/steel is coated with zinc to prevent rusting.

The zinc coat provides a protective oxide layer that is hard, strong, non porous.

When galvanized iron is scratched, the iron beneath does not rust.

It is because zinc which is more electropositive will corrode instead of iron.

Question 9

What is iron coated with zinc called?

Answer 9

Galvanised iron

Question 10

How to prevent rusting in ships?

Answer 10

Fixing bars of zinc metal to the part of the ship that submerged in water.

Zinc is more electropositive than iron, thus it will be oxidised and corrode first.

Question 11

How to prevent rusting in underground pipes?

Answer 11

Attach blocks of magnesium to the iron pipes

Magnesium is more electropositive, thus it will be oxidised and corrodes first in reference to iron.

Question 12

Methods to prevent rusting

3) Using of Alloys

Answer 12

In stainless steel, iron is allowed with carbon, nickel and chromium

Chromium and nickel creates a shiny, protective and corrosion proof layer.

The oxide layer formed protects the iron from being exposed to water and air to prevent rusting.

Question 13

Uses of alloy

Answer 13

Surgical instrument and kitchen utensils

Question 14

Method to prevent rusting

4) Sacrificial Protection
- how
- why
- become what?

Answer 14

Iron is attached to a more electropositive metal, such as Mg and Zn

When iron is attached to a more electropositive metal, the more electropositive metal corrodes first and rusting can be prevented.

The more electropositive metal becomes a sacrificial metal.

Question 15

How to increase rusting?

Answer 15

1) Use of strong electrolyte

2) In contact with less electropositive metal

Question 16

How strong electrolyte can increase rusting?

• near beach
• industrial area

Answer 16

Near beach
- sea contains sodium chloride and magnesium chloride

Industrial area

• air is polluted by acidic gases
• such as sodium dioxide and nitrogen dioxide

Question 17

Example of strong electrolyte that can increase rusting? Why?

Answer 17

Acid and Salt

- increase the electrical conductivity of water