Separate chemistry 1 Revision Notes

Atomic Structure of Pure Metals

Pure metals consist of atoms of roughly the same size, arranged in a highly regular, ordered lattice structure. This uniform arrangement allows layers of atoms to slide over each other with relative ease when a force is applied. This characteristic contributes to the ductility and malleability of pure metals.

How Alloying Strengthens Metals

Alloys are formed by mixing a primary metal with one or more other elements, often other metals, which typically have different atomic radii. These 'foreign' atoms disrupt the regular crystal lattice of the pure metal. The size difference creates points of irregularity. When a stress is applied, these irregularities act as barriers, preventing layers of atoms from sliding past each other as easily as they would in a pure metal. This increased resistance to atomic layer movement results in a stronger, harder material.

Impact on Properties

The increased strength and hardness of alloys come at the cost of some ductility. While pure metals are generally soft and easily deformed, alloys are designed to be much more rigid and resistant to deformation, making them suitable for structural applications where strength is crucial.

Limitations of Pure Iron

Pure iron is a relatively soft and malleable metal. Its atomic layers can slide over each other quite easily, which means it isn't very strong and can bend or deform under stress. It also reacts readily with oxygen and water in the presence of air to form rust (hydrated iron(III) oxide), which degrades its structural integrity and appearance.

The Purpose of Alloying Iron

Iron is alloyed with other elements, primarily carbon, to produce steels. Alloying involves mixing metals with other elements to change their properties. The main reasons for alloying iron are to increase its strength and hardness, improve its resistance to corrosion, and generally enhance its durability and suitability for various applications. Different alloying elements provide different benefits.

How Alloying Changes Properties

When other atoms (like carbon, chromium, nickel) are introduced into the iron lattice, they disrupt the regular arrangement of iron atoms. These foreign atoms, often of different sizes, prevent the layers of iron atoms from sliding over each other smoothly. This 'pinning' effect significantly increases the material's strength and hardness. For example, carbon atoms, being smaller, fit into the gaps between iron atoms, making the structure more rigid. Larger atoms replace iron atoms or distort the lattice, achieving similar effects.

Examples of Alloying Elements and Their Effects

Common alloying elements include carbon, which primarily increases strength and hardness. Chromium significantly improves corrosion resistance, forming stainless steels. Nickel enhances toughness and ductility. Manganese increases strength and impact resistance. Molybdenum and vanadium can improve hardness and heat resistance. The specific combination and proportions of these elements determine the final properties of the alloy steel, allowing for a wide range of applications from construction to surgical instruments.

Aluminium and Magnalium

Aluminium is valued for its low density and corrosion resistance. Its surface forms a protective oxide layer that prevents further corrosion. These properties make it suitable for aircraft and drink cans. Magnalium, an alloy of aluminium and magnesium, is lighter and stronger than pure aluminium, used in lightweight components.

Copper and Brass

Copper is an excellent conductor of electricity and heat, and it is ductile. These properties lead to its use in electrical wiring and water pipes. Brass is an alloy of copper and zinc. It is harder and more resistant to corrosion than pure copper, making it suitable for musical instruments, taps, and decorative objects.

Gold

Gold is a highly unreactive metal, meaning it doesn't easily corrode or tarnish. It is also very malleable and does not react with oxygen. These properties make it ideal for jewellery and high-quality electrical connectors where reliability and corrosion resistance are essential.

General Principles

The properties of metals, such as density, strength, conductivity, and reactivity, dictate their practical applications. Alloying can modify these properties, often enhancing strength or corrosion resistance, to suit specific uses. Understanding the relationship between properties and uses is key to material selection.

Theoretical vs. Actual Yield

Theoretical yield is the maximum possible product from a reaction, calculated stoichiometrically. Actual yield is the amount of product obtained experimentally. Actual yield is almost always less than theoretical yield due to various factors.

Reasons for Lower Actual Yield

Actual yield is lower than theoretical due to: 1. Incomplete reactions (not all reactants convert to products). 2. Practical losses (product left in apparatus, lost during transfer or purification). 3. Competing side reactions (reactants form undesired products, reducing the desired yield).

Impact of Incomplete Reactions

Sometimes a chemical reaction does not proceed to completion, meaning that some of the reactants remain unreacted. This can be due to equilibrium effects where the forward and reverse reactions are constantly occurring, preventing 100% conversion to products.