The study of alloys and polymers offers a unique window into the dynamic world of materials science. In this section, we will delve into their definitions, properties, and the critical aspects that set them apart in the realm of chemistry.
Alloys
Definition
Alloys are mixtures comprised of a metal and one or more additional elements, which can be either metals or non-metals. This combination often results in a material with superior properties compared to the pure metal, thanks to the non-directional bonding nature of metals.
Common Examples of Alloys and Their Properties
- Bronze: A combination of copper and tin. It is harder than either of its components and was historically significant in the creation of weapons, coins, and tools.
Coins made up of bronze.
Image courtesy of Amisos / BNF
- Brass: Made up of copper and zinc. This alloy is highly malleable, resists corrosion, and is often used in musical instruments and decorative items.
Picture of tuba, a brass instrument.
Image courtesy of Kreuzschnabel
- Stainless Steel: An alloy of iron, chromium, and often nickel and carbon. It is renowned for its resistance to rust and corrosion, making it essential in kitchenware, medical instruments, and construction.
A stainless steel kitchen container.
Image courtesy of Yapparina
Polymers
Introduction to Polymers
Polymers are large molecules, often referred to as macromolecules, which are formed by the repetition of smaller units called monomers.
Types of Polymers
- Natural Polymers: These occur naturally and can be extracted from plants and animals. Examples include DNA, proteins, cellulose, and natural rubber.
- Synthetic Polymers: These are man-made polymers, often derived from petroleum-based chemicals. Common examples include nylon, polyethylene, and polystyrene.
Image courtesy of Christinelmiller
Formation of Addition Polymers
Addition polymers form when monomers, possessing double bonds, react to form long chains. In this process:
- The double bond between carbon atoms in the monomer breaks.
- The monomers join end-to-end, with each monomer contributing to the growing chain without the elimination of any other atom or molecule.
Representing Repeating Units
For addition polymers, the repeating unit is a segment of the polymer chain that reflects the structure of the original monomer, minus the double bond. For instance, ethene (C2H4) polymerises to form polyethene, with a repeating unit derived from ethene but in a saturated form.
Ethene (C2H4) polymerising to form polyethene.
Image courtesy of V8rik
Functional Groups in Monomers
Certain functional groups in molecules can act as reactive sites, enabling these molecules to function as monomers in addition reactions. Common functional groups include:
- Double Bonds: Found in alkenes like ethene and propene.
Image courtesy of OpenStax
- Nitrile Groups: Present in molecules like acrylonitrile.
A structural formula of acrylonitrile.
Image courtesy of Chem Sim 2001
Atom Economy in Addition Polymerisation
In addition polymerisation, every atom in the monomer becomes part of the polymer. This means there are no wasted atoms or by-products formed. Therefore, the atom economy for an addition polymerisation reaction is 100%. This is significant as a high atom economy is desirable in sustainable manufacturing, reducing waste and environmental impact.
FAQ
Polymers typically exhibit flexibility due to the nature of their structure and bonding. While the covalent bonds within individual polymer chains are strong, the forces between different chains (intermolecular forces) are generally weaker van der Waals forces. This means that under stress, the chains can slide past each other, providing flexibility. However, it's worth noting that cross-linking, where covalent bonds form between different chains, can reduce this flexibility and make the polymer more rigid.
Certain polymers, known as thermoplastics, can be repeatedly melted and remoulded without undergoing significant chemical change. When heated, the weak intermolecular forces between the polymer chains are overcome, allowing the chains to slide past one another, making the polymer malleable. Upon cooling, these forces re-establish, causing the material to solidify. This property contrasts with thermosetting polymers, which become permanently hard when set and cannot be reshaped by heating.
Most commercial addition polymers, like polyethylene and polypropylene, are non-biodegradable. This means they do not break down easily in the environment. The primary reason is their carbon-carbon backbone, which is resistant to the microbial enzymes that break down natural organic substances. Additionally, these polymers lack functional groups that make them susceptible to microbial attack. However, with advances in polymer science, some newer addition polymers are designed to be biodegradable or more environmentally friendly.
The non-directional nature of metallic bonding means that the bonding electrons are free to move and are not tied to any particular atom. This type of bonding results in a sea of delocalised electrons surrounding a lattice of positively charged metal ions. In the context of alloys, the non-directional characteristic is crucial because when another element is introduced, it can easily fit into the lattice without disrupting the bonding. This adaptability is what allows for the creation of alloys with varied and enhanced properties compared to the pure metals.
The properties of alloys are influenced by the size of the atoms in the alloy, the proportion of the component metals, and the type of atomic bonding that occurs between them. For instance, when atoms of one metal are much larger than those of the other, it can lead to distortion in the metallic lattice, preventing layers of atoms from sliding over each other, thereby increasing the alloy's hardness. Additionally, the proportion in which metals are mixed can also determine properties like malleability, electrical conductivity, and melting point.
Practice Questions
Alloys are mixtures composed of a metal combined with other elements, which could be metals or non-metals. Their formation often enhances the material's properties compared to the pure metal because of the non-directional nature of metallic bonding. One prominent example of an alloy is stainless steel, composed of iron, chromium, and often some amounts of nickel and carbon. Stainless steel is particularly valued for its impressive resistance to rust and corrosion, properties not found in pure iron. Its durability and resistance make it indispensable in applications such as kitchenware, medical instruments, and construction.
Natural polymers are macromolecules that occur naturally and can be derived from plants or animals. An example is cellulose, a polymer found in plant cell walls, formed by the repetition of glucose monomers through glycosidic linkages. On the other hand, synthetic polymers are man-made macromolecules, usually derived from petroleum-based precursors. A classic example is polyethylene, which forms when ethene monomers undergo addition polymerisation. In this process, the double bond in the ethene monomers breaks, and the molecules link end-to-end without the elimination of any other atom or molecule, resulting in a long-chain polymer.
