Polymer molecules (AQA GCSE Chemistry Combined Science): Revision Notes
Polymer molecules
What are polymer molecules?
Polymer molecules are extremely large molecules that form when atoms connect together through covalent bonds. These huge molecules are built from identical units that repeat over and over again. The word "polymer" comes from "poly" (meaning many) and "mer" (meaning parts).
Think of polymers like a very long chain made up of identical links. Each link is called a repeating unit, and when you join hundreds or thousands of these units together, you get a polymer.
Structure of polymers
Long-chain molecules
Polymers have a long-chain structure that gives them their unique characteristics. The structure works as follows:
- Each repeating unit connects to the next one through strong covalent bonds
- The chain can contain hundreds or thousands of repeating units
- This creates a very long molecule that extends in one direction
Repeating units
The repeating unit is the smallest part of the polymer that shows the pattern. Understanding this concept is essential for working with polymer chemistry.
Polymer Chain Structure Example:
A simple polymer might have a repeating unit of
This unit repeats many times:
The repeating unit is shown in brackets with a subscript 'n' to show it repeats many times:
Properties of polymers
Size and intermolecular forces
The large size of polymer molecules has a direct impact on their properties. Because polymer molecules are so large:
- There are relatively strong intermolecular forces between the polymer chains
- These forces are stronger than those between smaller covalent molecules
- The large size means more surface contact between molecules
The connection between molecular size and intermolecular forces is crucial for understanding why polymers behave differently from smaller molecules.
Physical properties
This relationship between size and intermolecular forces leads to specific properties:
- High melting points - you need lots of energy to break the strong intermolecular forces
- Solid at room temperature - the strong forces keep the molecules together
- Strong materials - the long chains and strong forces make them tough
Understanding formulas
When working with polymers, it's important to distinguish between different types of formulas:
- Molecular formula: Shows the actual repeating unit, like
- Empirical formula: Shows the simplest ratio of atoms, like
Formula Comparison Example:
If a polymer has the repeating unit , the empirical formula would be (the simplest ratio of carbon to hydrogen).
Molecular formula: Empirical formula:
Examples
Understanding polymers becomes clearer when we look at real-world examples. A common example is polyethene:
- Repeating unit:
- Empirical formula:
- Common uses: Used to make plastic bags and bottles
Polyethene is one of the most widely produced polymers in the world and demonstrates all the key principles we've discussed.
Key Points to Remember:
- Polymers are very large molecules made from repeating units joined by covalent bonds
- Long-chain structure gives polymers their unique properties
- Strong intermolecular forces between polymer chains lead to high melting points
- Solid at room temperature due to these strong forces
- Empirical formula shows the simplest ratio of atoms in the repeating unit