Applied Chemistry: Corrosion, Rusting, Alloys, Polymers, Detergents, Soaps, Cleaning Agents

Fundamentals of Applied Chemistry

This unit connects chemistry to everyday life. You will learn how metals corrode, how alloys improve properties, how plastics are made, and how soaps and detergents clean. These topics are important for exams and real-world applications.

1. Corrosion and Its Prevention

Corrosion is the destruction of metals by chemical reactions with their environment. The most common example is rusting of iron.

How Does Corrosion Happen?

• Metals like iron react with oxygen and moisture in the air.
• The reaction forms iron oxide (rust).
• Rust is weak and flaky, so it does not protect the metal underneath.

Chemical Equation for Rusting:

Factors Affecting Corrosion

• Presence of moisture (water)
• Presence of oxygen
• Acidic environment (e.g., rainwater with CO₂ forms carbonic acid)
• Impurities in metals (e.g., carbon in steel speeds up rusting)

Methods to Prevent Corrosion

Example:

• Galvanized iron is iron coated with zinc. Zinc corrodes first because it is more reactive than iron, protecting the iron underneath.

2. Alloys: Stronger and Useful Metals

An alloy is a mixture of two or more metals (or a metal and a non-metal) to improve properties like strength, hardness, and resistance to corrosion.

Common Alloys and Their Uses

classDiagram
    class Alloy {
        <<abstract>>
        +Name
        +Composition
        +Uses
    }
    class Steel {
        +Fe + C (0.1%–2%)
        +Strong, hard, used in buildings, tools
    }
    class StainlessSteel {
        +Fe + Cr (10–20%) + Ni
        +Resistant to rust, used in cutlery, surgical tools
    }
    class Brass {
        +Cu + Zn (10–40%)
        +Yellow, used in musical instruments, pipes
    }
    class Bronze {
        +Cu + Sn (5–12%)
        +Hard, used in statues, bearings
    }
    class Duralumin {
        +Al + Cu + Mn + Mg
        +Light, strong, used in aircraft
    }
    Alloy <|-- Steel
    Alloy <|-- StainlessSteel
    Alloy <|-- Brass
    Alloy <|-- Bronze
    Alloy <|-- Duralumin

Why Are Alloys Better Than Pure Metals?

• Stronger (e.g., steel is stronger than pure iron).
• Harder (e.g., brass is harder than copper).
• More resistant to corrosion (e.g., stainless steel does not rust easily).
• Better electrical conductivity (e.g., copper alloys in wires).

Example:

• Stainless steel contains chromium (Cr), which forms a thin oxide layer that prevents further corrosion.

3. Polymers: Plastics and Their Types

Polymers are large molecules made of repeating units (monomers). They can be natural (e.g., rubber, cellulose) or synthetic (e.g., plastics).

Types of Polymers

How Are Polymers Made?

• Addition Polymerization: Many small molecules (monomers) join together in a chain. Example: Ethene (C₂H₄) → Polyethylene (C₂H₄)ₙ

• Condensation Polymerization: Monomers join, releasing a small molecule (like water). Example: Nylon-6,6 is made from hexamethylenediamine and adipic acid, releasing water.

Example of Polymerization:

Properties and Uses of Polymers

Advantages of Polymers: ✅ Lightweight ✅ Cheap to produce ✅ Resistant to corrosion ✅ Can be molded into different shapes

Disadvantages of Polymers: ❌ Non-biodegradable (cause pollution) ❌ Some release toxic gases when burned ❌ Harmful to marine life

4. Soaps and Detergents: How They Clean

Soaps and detergents help remove dirt and grease by emulsification (breaking grease into small droplets that mix with water).

Structure of a Soap Molecule

A soap molecule has:

• A hydrophilic (water-loving) head (polar, attracts water).
• A hydrophobic (water-hating) tail (non-polar, attracts grease).

How Soap Works

1. The hydrophobic tail attaches to grease/oil (non-polar).
2. The hydrophilic head attaches to water (polar).
3. This forms micelles, which suspend grease in water so it can be washed away.

Soap vs. Detergent

Why Do Soaps Not Work Well in Hard Water?

• Hard water contains Ca²⁺ and Mg²⁺ ions.
• These ions react with soap to form insoluble scum (calcium/magnesium stearate), which is useless for cleaning.

Example Reaction:

Detergents vs. Soaps in Hard Water:

• Detergents have sulfate groups (SO₄²⁻) instead of carboxyl groups (COO⁻), so they do not form scum.

5. Cleaning Agents: Bleaching and Disinfectants

Bleaching Agents

Bleaching removes color by oxidizing colored compounds into colorless ones.

Common Bleaching Agents:

Example Reaction (Bleaching Powder):

Disinfectants

Disinfectants kill or prevent growth of microorganisms (bacteria, viruses).

Common Disinfectants:

Exam Tip: How to Score Full Marks

1. Understand the Basics:

   • Know how rusting happens and how to prevent it.
   • Differentiate between soaps and detergents (especially in hard water).
   • Remember types of polymers and their uses.

2. Diagrams and Equations:

   • Draw micelle formation for soaps.
   • Write balanced equations for rusting and bleaching.
   • Show alloy compositions in tables.

3. Application-Based Questions:

   • Explain why stainless steel does not rust.
   • Describe how detergents work better than soaps in hard water.
   • Give real-life examples of polymers (e.g., PVC in pipes, nylon in clothes).

4. Common Mistakes to Avoid:

   • ❌ Confusing addition and condensation polymers.
   • ❌ Forgetting that soaps form scum in hard water.
   • ❌ Not mentioning both hydrophilic and hydrophobic parts in soap molecules.

5. Practical Questions:

   • If asked about preventing rust, mention painting, galvanization, and alloying.
   • For polymers, relate them to everyday objects (e.g., plastic bottles, nylon ropes).

NEB Board-Style Questions (Practice)

Short Answer Questions

1. What is corrosion? Give two methods to prevent it.
2. Why does stainless steel not rust easily?
3. What is the difference between a soap and a detergent?
4. How does a soap molecule help in cleaning?
5. Name two synthetic polymers and their uses.

Long Answer Questions

6. Explain the process of rusting of iron with a balanced chemical equation. How can rusting be prevented?
7. Describe the structure of a soap molecule. Why do soaps not work well in hard water? How are detergents better?
8. What are alloys? Give examples of three alloys and their uses.
9. What are the advantages and disadvantages of synthetic polymers? Give two examples of each.
10. How do bleaching agents work? Give two examples of bleaching agents and their uses.

Summary Table for Quick Revision

Final Advice:

• Relate chemistry to daily life (e.g., why pipes are made of PVC, why cars rust).
• Practice drawing diagrams (rusting, micelle, polymer chains).
• Memorize key reactions (rusting, soap scum, bleaching).

Good luck with your exams! 🚀