Intermolecular Forces (Leaving Cert Chemistry): Revision Notes
Intermolecular Forces
What are intermolecular forces?
Understanding the difference between intramolecular and intermolecular forces is crucial for grasping this topic. Think of the Latin roots to help you remember:
Etymology Helper:
- Intra = inside or within
- Inter = between
This simple memory trick will help you distinguish between forces within molecules versus forces between molecules.
Intramolecular bonding refers to the bonding that takes place within a molecule. These are the bonds that hold atoms together, such as covalent bonds and polar covalent bonds. Think of it as the "glue" that keeps a single molecule intact.
Intermolecular forces are completely different - these are the forces of attraction that exist between separate molecules. These forces determine how molecules interact with each other.
Understanding intermolecular vs intramolecular bonding
Helpful Analogy: To visualise this difference, imagine an intravenous injection. The needle goes inside the vein (intramolecular = inside), whilst the attraction between the syringe and the patient's arm would be like intermolecular forces (between separate objects).
Intramolecular bonding is the bonding that occurs inside a molecule - it holds the molecule together. Intermolecular forces are attractions between different molecules.
Van der Waals forces
The most important type of intermolecular forces are called van der Waals forces, named after Dutch scientist Johannes van der Waals who discovered them.
Historical Context: In 1910, van der Waals received the Nobel Prize in Physics for his groundbreaking work on these intermolecular forces, highlighting their fundamental importance in understanding molecular behaviour.
Key characteristics of van der Waals forces
Van der Waals forces have several important features that you need to understand:
- They are much weaker than covalent or ionic bonds
- They occur between all molecules, regardless of whether the molecules are polar or non-polar
- The strength decreases greatly as the distance between molecules increases
- Although they're weak individually, they're present in large numbers
Why van der Waals forces matter
Despite being weak compared to covalent or ionic bonds, van der Waals forces play a crucial role in determining the physical properties of substances. These properties include:
- Boiling points
- Melting points
- Solubility
This is because van der Waals forces are present in large numbers throughout substances, so their combined effect becomes significant.
Three main types of van der Waals forces
There are three main categories of van der Waals forces that you need to know:
1. London dispersion forces
These are the weakest type of intermolecular force but are present between all molecules. They arise from temporary fluctuations in electron distribution around atoms and molecules.
2. Permanent dipole-dipole forces
These occur between molecules that have permanent dipoles (polar molecules). A special and particularly important type of dipole-dipole force is hydrogen bonding, which occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine.
3. Ion-dipole forces
These forces occur between ions and polar molecules. They're particularly important when ionic compounds dissolve in polar solvents like water.
Common Mistakes to Avoid:
- Don't confuse intermolecular forces with the bonds that hold atoms together in molecules
- Remember that van der Waals forces are always weaker than ionic or covalent bonds
- Don't forget that these forces become extremely important when explaining physical properties of substances
Key Points to Remember:
- Intermolecular forces exist between separate molecules, whilst intramolecular bonds hold atoms together within molecules
- Van der Waals forces are weak intermolecular attractions that exist between all molecules
- There are three main types: London dispersion forces, permanent dipole-dipole forces (including hydrogen bonding), and ion-dipole forces
- Although individually weak, these forces are crucial for determining physical properties like boiling points, melting points, and solubility
- Van der Waals forces get weaker as distance increases between molecules