Lesson 4: Intermolecular Forces Flashcards
Intermolecular Forces
Types
- Hydrogen Bonds.
- Permanent Dipole-Dipole Forces.
- Induced Dipole-Dipole Forces/London Forces.
This list is in order of strongest to weakest.
Intermolecular Forces
Intra- vs Intermolecular Forces
- An intramolecular force is a strong bond between two atoms in a compound (eg. covalent bond, HCl).
- An intermolecular force is a weak bond between two molecules (eg. HCl-HCl)
Intermolecular Forces
Hydrogen Bond
- Strongest type of IM force but 10 times weaker than a covalent bond.
- Between: very electronegative atom (F, O, N) that has a lone pair of electrons and a hydrogen atom covalent bonded.
- Eg. H₂O-H₂O
Hydrogen Bond
Drawing the Bond
- Draw the molecules.
- Show any lone pairs and dipoles.
- Draw a straight dotted line between the lone pair on one atom and the hydrogen atom.
Intermolecular Forces
Permanent Dipole-Dipole Forces
Weak forces of attraction between permanently polar molecules (bonds between elements of differing electronegativities).
Intermolecular Forces
Induced Dipole-Dipole Forces
- Also called Temporary Dipole-Dipole Forces or London Forces.
- Created when an external negative field distorts the electron cloud of a neutral molecule.
- This gives the molecule a positive and negative end.
- Negative atoms are attracted to the positive end of the molecule.
Intermolecular Forces
Chloropropane
- Molecules can have both London Forces and Permanent Dipoles.
- The C-Cl bond forms Permanent Dipoles; the rest of the molecule (C-C and C-H) forms London Forces.
London Forces
Boiling Point
Electron Distortion
- Forces get stronger as the number of electrons increases down the group.
- Temporary dipoles are caused by electron distortion.
- The more electrons there, the greater the distortion.
- Therefore, the forces are stronger.
Intermolecular Forces
Boiling Points
Down the Group
The boiling points of noble gases & Group 4 hydrides increase down the group as e- shells are added and formula mass increases.
Unusual Properties of Water
Low Density as Ice
- Solid water floats on liquid.
- Solid particles are usually more dense than liquid, so they normally sink.
- Water forms 4 hydrogen bonds per molecule, which holds the molecules apart in an open lattice.
Unusual Properties of Water
High Boiling Point
- Water is formed out of Hydrogen Bonds.
- This means the intermolecular forces are strong and require more energy to break.
Unusual Properties of Water
High Surface Tension & Low Viscosity
- Water molecules are attracted to each other by hydrogen bond.
- The bonds are strong and create a high surface tension.
- Despite this, molecules can flow over each other in the liquid, allowing it to flow smoothly (low viscosity).
