Science - Combined (0653) | Metals

Chemistry Topic C9: Metals

Hello future chemists! This chapter is all about Metals – the backbone of modern civilisation. From the structure of your phone to the strength of skyscrapers, metals play a huge role. We will explore what makes metals special, how they react, and why we often mix them to create stronger materials called alloys. Don't worry if some concepts seem tricky; we'll break them down with simple explanations and real-world examples!

C9.1 General Physical and Chemical Properties of Metals

Metals are a large group of elements, and they generally share a distinct set of characteristics, setting them apart from non-metals.

Physical Properties: Metals vs. Non-metals (Core)

Metals usually have a few standout physical features:

• High Melting and Boiling Points: Most metals require a lot of energy to melt or boil (except for mercury, which is liquid at room temperature!). Non-metals generally have lower melting points.
• High Density: Metals are usually heavy for their size. Non-metals generally have low density.
• Good Thermal Conductivity: They transfer heat quickly. This is why metal pans heat up efficiently when cooking.
• Good Electrical Conductivity: They conduct electricity well. This is essential for wiring. Non-metals are usually electrical insulators (except carbon/graphite).
• Malleability: Metals can be hammered or pressed into shape without breaking. Think about shaping aluminium foil or sheet steel.
• Ductility: Metals can be stretched into thin wires. Think about drawing copper into electrical cables. Non-metals are usually brittle (they shatter).

Quick Review: The M&D Trick
Remember the special properties of metals: they are Malleable and Ductile.

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C9.4 The Reactivity Series and Reactions

The Reactivity Series is a list of metals ordered by how easily they react (their chemical reactivity). A more reactive metal displaces a less reactive metal from its compounds.

Order of Reactivity (Core)

It is vital to know this order. The most reactive metal is at the top, and the least reactive is at the bottom.

K (Potassium) > Na (Sodium) > Ca (Calcium) > Mg (Magnesium) > Al (Aluminium) > C (Carbon) > Zn (Zinc) > Fe (Iron) > H (Hydrogen) > Cu (Copper) > Ag (Silver) > Au (Gold)

Memory Aid (Mnemonic):

Please Stop Calling Me A Cute Zebra Instead How Can She Get.

(P=Potassium, S=Sodium, C=Calcium, M=Magnesium, A=Aluminium, C=Carbon, Z=Zinc, I=Iron, H=Hydrogen, C=Copper, S=Silver, G=Gold)

Chemical Properties: Reactions with Water, Steam, and Acid (Core)

The position of a metal in the series determines its general chemical behaviour:

1. Metals with Cold Water (K, Na, Ca):
   

   These are the most reactive metals. They react quickly with cold water to form a metal hydroxide and hydrogen gas.

   Example: Sodium reacts violently with cold water.
   \(\text{Sodium} + \text{Water} \rightarrow \text{Sodium Hydroxide} + \text{Hydrogen}\)

2. Metals with Steam (Mg, Al, Zn, Fe):
   

   These metals are less reactive than K, Na, and Ca. They react only when heated strongly with steam (not cold water) to form a metal oxide and hydrogen gas.

   Example: Magnesium glows brightly when heated in steam.
   \(\text{Magnesium} + \text{Steam} \rightarrow \text{Magnesium Oxide} + \text{Hydrogen}\)

3. Metals with Dilute Acids (Metals above Hydrogen):
   

   Any metal that is positioned above Hydrogen (H) in the series (K down to Fe) will react with dilute acids (like dilute hydrochloric acid) to produce a salt and hydrogen gas.

   Example: Zinc reacts with dilute acid.
   \(\text{Zinc} + \text{Dilute Hydrochloric Acid} \rightarrow \text{Zinc Chloride} + \text{Hydrogen}\)

   Metals below Hydrogen (Cu, Ag, Au) will not react with dilute acids.

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C9.6 Extraction of Metals

Metals exist in the Earth’s crust often combined with other elements in rocks called ores. We need to use specific methods to extract the pure metal, and this choice depends entirely on the metal's position in the reactivity series (C9.6 Core).

1. High Reactivity Metals (K, Na, Ca, Mg, Al)

These metals form very stable compounds. Their chemical bonds are very strong, meaning it is extremely difficult to separate them using cheaper methods like heating with carbon.

Extraction Method: Electrolysis (passing an electric current through the molten ore).

Example: Aluminium is extracted from its ore, bauxite, using electrolysis (C9.6 Core).

2. Medium Reactivity Metals (Zn, Fe)

These metals are below Carbon in the reactivity series (or just above it, like zinc). This means they can be extracted by heating their oxides with carbon, as carbon is reactive enough to displace them.

Extraction Method: Reduction by Carbon or Carbon Monoxide.

Example: Iron is extracted from its ore, hematite (which contains iron(III) oxide, Fe₂O₃), in the blast furnace (C9.6 Core).

3. Low Reactivity Metals (Cu, Ag, Au)

These metals are so unreactive that they are often found uncombined in the ground, making extraction very simple.

Extraction Method: Simply collecting the element (sometimes needing just heat or simple physical separation).

🔥 Step-by-Step: Iron Extraction in the Blast Furnace (C9.6 Supplement)

Iron extraction is a crucial industrial process. The raw materials used are hematite (iron ore), coke (carbon), and limestone (calcium carbonate).

Step 1: Providing Heat and Making Carbon Dioxide
Coke (carbon) burns fiercely in the hot air (oxygen) blown into the furnace, generating intense heat and producing carbon dioxide:

\(\text{C} + \text{O}_2 \rightarrow \text{CO}_2\)

Step 2: Generating the Reducing Agent
The carbon dioxide then reacts with unburnt coke higher up in the furnace to produce Carbon Monoxide (\(\text{CO}\)). Carbon monoxide is the main reducing agent.

\(\text{C} + \text{CO}_2 \rightarrow 2\text{CO}\)

Step 3: Reduction of Iron(III) Oxide
The carbon monoxide reduces the iron(III) oxide (the chemical term for rust/ore) to molten iron metal.

\(\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2\)

Did you know? The limestone is added to remove acidic impurities (like silicon dioxide) in the ore. It forms molten slag, which floats on the molten iron.

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C9.5 Corrosion of Metals (Rusting)

Corrosion is the destructive process that occurs when metals react with substances in the environment, like oxygen and water. The most common example is the rusting of iron.

Conditions Required for Rusting (Core)

Rusting is the specific term for the corrosion of iron. Two conditions must be present simultaneously:

1. Oxygen (\(\text{O}_2\))
2. Water (\(\text{H}_2\text{O}\))

If you remove either the oxygen or the water, rusting cannot happen.

Barrier Methods to Prevent Rusting (Core)

Since we know rusting requires both air and water, barrier methods work by simply excluding these substances from the iron surface (C9.5 Core).

• Painting: Used for cars, bridges, and gates. The paint layer forms a physical barrier.
• Greasing/Oiling: Used for moving parts of machines (like bicycle chains) or tools. The grease forms a protective, water-repelling layer.
• Coating with Plastic: Used for garden furniture or electrical appliance casings. Provides a tough, durable barrier.

Don't worry about sacrificial protection or galvanising for this syllabus—just focus on the simple barrier methods!

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C9.3 Alloys and Their Properties

A pure metal is often too soft or too reactive for certain uses. To improve their properties, metals are often mixed with other elements (which can be metals or non-metals) to form alloys (C9.3 Core).

What is an Alloy?

An alloy is defined as a mixture of a metal with one or more other elements, where the resulting material has metallic properties.

Key Examples of Alloys (Core)

• Brass: An alloy of Copper (Cu) and Zinc (Zn).
• Stainless Steel: An alloy of Iron (Fe) with other elements like Chromium (Cr), Nickel (Ni), and Carbon (C).

Why are Alloys Useful?

Alloys are typically harder and stronger than the pure metals from which they are made (C9.3 Core). This makes them much more useful for applications where strength and durability are key.

• Example: Stainless steel is used for cutlery because of its hardness and excellent resistance to rusting.

⚙️ Why are Alloys Stronger? (C9.3 Supplement)

To understand why alloys are stronger, we must look at the structure of pure metals:

1. Pure Metal Structure: Pure metals consist of atoms of the same size arranged neatly in layers. These layers can easily slide over each other when a force is applied. This easy sliding is what makes pure metals malleable and soft.
2. Alloy Structure: When you mix in atoms of a different element to make an alloy, these atoms are usually a different size (either bigger or smaller).
3. Result: The different-sized atoms disrupt the neat arrangement of the layers. This disruption means the layers can no longer slide over each other easily when a force is applied. This makes the alloy much harder and stronger.

Analogy: Imagine trying to slide a neat stack of plates (pure metal). Easy! Now imagine the stack has small cups and large bowls mixed in (alloy). Trying to slide the stack now causes everything to jam up. That jamming is the strength of the alloy!

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C9.2 Uses of Metals Related to Their Properties

Metals are chosen for specific jobs based on their unique physical properties (C9.2 Core).

Aluminium Uses

Aluminium has several fantastic properties, making it very versatile:

• Aircraft Manufacture: Used because of its low density (lightweight) combined with good strength.
• Overhead Electrical Cables: Used because of its low density and good electrical conductivity. Being light means less strain on the supporting pylons.
• Food Containers/Foil: Used because of its excellent resistance to corrosion. Aluminium forms a thin, protective layer of aluminium oxide on its surface that prevents further reaction.

Copper Uses

Copper is known for its electrical excellence:

• Electrical Wiring: Used primarily because of its good electrical conductivity. Copper is slightly better at conducting electricity than aluminium (and much cheaper than silver or gold!).