Using materials (chemistry only)

Welcome to "Using Materials"!

In this chapter, we are going to explore why we choose certain materials for specific jobs. Whether it's the gold in a ring, the glass in a laboratory beaker, or the fertilisers used to grow our food, chemistry is the secret ingredient behind them all. This section is exclusive to Chemistry students, so it’s a great chance to dive deeper into how we use the Earth's resources to build our modern world.

Don’t worry if some of these terms seem new—we’ll break them down step-by-step with simple examples you see every day!

1. Corrosion and its Prevention

Corrosion is the destruction of materials by chemical reactions with substances in the environment. The most famous example is rusting.

What is Rusting?

Rusting only happens to iron (and alloys of iron, like steel). For iron to rust, two things must be present: air (oxygen) and water. If you take away one of these, the iron won't rust!

Equation: \( \text{iron} + \text{oxygen} + \text{water} \rightarrow \text{hydrated iron(III) oxide} \)

How can we stop it?

We can prevent corrosion by using a "barrier" to keep the air and water away. Think of it like wearing a raincoat! Common methods include:
• Greasing or oiling (used for moving parts like bike chains).
• Painting (used for large structures like bridges).
• Electroplating (coating the metal with a thin layer of a different metal).
• Aluminium’s secret weapon: Unlike iron, aluminium doesn’t flake away when it corrodes. It forms a very thin layer of aluminium oxide that sticks tightly to the surface, acting as a natural shield against further corrosion.

Sacrificial Protection and Galvanising

Sometimes we use a more reactive metal to protect a less reactive one. This is called sacrificial protection. For example, zinc is more reactive than iron. If we coat iron in zinc, the zinc reacts with the oxygen and water instead of the iron. This specific process of coating iron in zinc is called galvanising.

Quick Review: Iron needs BOTH air and water to rust. Aluminium protects itself with an oxide layer. Zinc "sacrifices" itself to save iron.

Key Takeaway: Preventing corrosion is all about keeping oxygen and water away from the metal surface using barriers or more reactive metals.

2. Alloys as Useful Materials

Pure metals are often too soft for everyday use because their atoms are arranged in neat layers that slide over each other easily. Alloys are mixtures of a metal with other elements. These different-sized atoms distort the neat layers, making it harder for them to slide. This makes alloys harder than pure metals.

Common Alloys You Need to Know:

• Bronze: A mixture of copper and tin. Used for medals and statues.
• Brass: A mixture of copper and zinc. Used for door fittings and musical instruments.
• Gold Jewellery: Pure gold (24 carat) is too soft. It is usually alloyed with silver, copper, and zinc to make it tougher.
Did you know? 24 carat is 100% gold, while 18 carat is 75% gold (\( \frac{18}{24} \times 100 \)).
• Steels: Alloys of iron containing specific amounts of carbon and other metals.
     - High carbon steel: Strong but brittle (breaks easily).
     - Low carbon steel: Softer and easier to shape.
     - Stainless steel: Contains chromium and nickel. It is hard and resistant to corrosion (it doesn't rust!).
• Aluminium alloys: These are low density, which makes them perfect for building aeroplanes.

Key Takeaway: Alloys are harder than pure metals because the different-sized atoms stop the layers from sliding.

3. Ceramics, Polymers, and Composites

Ceramics

• Glass: Most glass we use is soda-lime glass. It’s made by heating a mixture of sand, sodium carbonate, and limestone. Borosilicate glass (often called Pyrex) is made from sand and boron trioxide. It melts at higher temperatures than soda-lime glass, which is why we use it for ovenware and lab flasks.
• Clay Ceramics: Things like pottery and bricks are made by shaping wet clay and then heating it in a furnace. This hardens the material.

Polymers

The properties of a polymer depend on the monomers used to make it and the conditions (like temperature and pressure) used. For example, ethene can produce:
• Low density (LD) poly(ethene): Flexible, used for carrier bags.
• High density (HD) poly(ethene): Stronger and more rigid, used for water pipes.

Thermosoftening vs. Thermosetting Polymers

• Thermosoftening polymers: These melt when heated and can be reshaped. They are like chocolate. Their polymer chains are not linked together.
• Thermosetting polymers: These do not melt when heated. They are like a cooked egg—once they are "set," they stay that way. This is because they have cross-links (strong covalent bonds) between the polymer chains.

Composites

A composite is made of two materials: a matrix (the binder) surrounding and binding together fibres or fragments of another material called the reinforcement. Examples include fiberglass, carbon fiber, and concrete.

Key Takeaway: Borosilicate glass handles heat better than soda-lime glass. Thermosetting polymers have cross-links that prevent them from melting.

4. The Haber Process (Chemistry Only)

The Haber Process is a massive industrial reaction used to manufacture ammonia (\( NH_3 \)). This ammonia is then used to make nitrogen-based fertilisers to help grow enough food for the world.

The Raw Materials

1. Nitrogen: Easily obtained from the air.
2. Hydrogen: Usually obtained from natural gas (methane).

The Process

The purified gases are passed over an iron catalyst at a high temperature (about 450°C) and a high pressure (about 200 atmospheres). The reaction is reversible:

\( \text{nitrogen} + \text{hydrogen} \rightleftharpoons \text{ammonia} \)

Because the reaction is reversible, some of the ammonia breaks back down into nitrogen and hydrogen. To make it efficient:
• The mixture is cooled so the ammonia liquefies and can be removed.
• The remaining nitrogen and hydrogen are recycled back into the reactor so nothing is wasted!

(Higher Tier Only) The Trade-off

The conditions used (450°C and 200 atm) are a compromise.
• Temperature: The forward reaction is exothermic. A lower temperature would give a higher yield (more ammonia), but the reaction would be too slow. 450°C is used to get a reasonable amount of ammonia quickly.
• Pressure: Higher pressure increases the yield and the rate, but it is very expensive and dangerous to build high-pressure pipes. 200 atm is a safe, economic middle ground.

Key Takeaway: The Haber process uses nitrogen from air and hydrogen from natural gas to make ammonia using an iron catalyst.

5. NPK Fertilisers (Chemistry Only)

Plants need three main elements to grow well: Nitrogen (N), Phosphorus (P), and Potassium (K). NPK fertilisers are formulations containing salts of these three elements.

Where do the ingredients come from?

• Potassium: Potassium chloride and potassium sulfate are obtained by mining. They can be used directly as fertilisers.
• Nitrogen: Ammonia is used to produce ammonium salts (like ammonium nitrate) and nitric acid.
• Phosphorus: Phosphate rock is obtained by mining, but it cannot be used directly because it is insoluble. It must be treated with acids to produce soluble salts:

1. Treated with nitric acid \( \rightarrow \) produces phosphoric acid and calcium nitrate.
2. Treated with sulfuric acid \( \rightarrow \) produces single superphosphate (a mixture of calcium phosphate and calcium sulfate).
3. Treated with phosphoric acid \( \rightarrow \) produces triple superphosphate (calcium phosphate).

Industrial vs. Lab Production

In the lab, we make fertilisers using titration and crystallisation on a small scale. It's slow and produces small amounts. In industry, the process is continuous, uses giant vats, and the heat released from the reaction is used to evaporate the water, making it much more efficient.

Key Takeaway: NPK fertilisers provide essential nutrients. Phosphate rock must be treated with acid to make it soluble so plants can actually absorb it.

Final Quick Check!

• Can you list the two things needed for iron to rust? (Air and Water)
• Why is an alloy harder than a pure metal? (Distorted layers cannot slide)
• What catalyst is used in the Haber process? (Iron)
• What do the letters N, P, and K stand for? (Nitrogen, Phosphorus, Potassium)

Great job! You've just covered the essentials of how we use chemistry to create and improve the materials around us.