Intermolecular Forces of Attraction (IMFs)

Definitions IMF: a force of attraction between molecules of the same type Bonds are INTRAmolecular forces because they chemically combine one atom to another atom •INTERmolecular forces do not chemically combine anything. But they do help one molecule “stick” to another

Types of IMFs London Dispersion Ion-dipole Dipole-Dipole Hydrogen Bonding • One strong force (not technically an IMF, but an actual bond!)  Ionic

London Dispersion Forces Weakest of the weak intermolecular forces Caused by random and temporary movement of electrons Short-lived, temporary force More electrons = greater attractions

Ion-dipole Forces Occur when an ion interacts with a polar substance.

Dipole-Dipole Still a very weak force, but stronger than London dispersion Molecules must be permanently polar with one positive and one negative side The positive side of one molecule attracts the negative of another molecule

Dipole-Dipole Continued One end of a molecule is permanently positive and one end is permanently negative Opposites attract

Hydrogen Bonding A weak force of attraction, but strongest compared to the previous two (London dispersion and dipole-dipole) Occurs when the most electronegative elements – N, O, and F – are directly bonded to H. (H-bonding is FON!!!) High electronegative elements attract electrons so strongly that the molecule becomes very polar

H-Bonding Continued The non-bonding pairs of electrons on the N, O, or F are strongly attracted to the highly positive H on another molecule. A “quarter bond” or “half bond” is formed between the H and the pair of electrons, resulting in a much stronger force than the previous forces. Requirements (both are necessary!) H bonded to N, O, F Non-bonded pair of electrons on the N, O, or F

Ionic This is an intra molecular force, NOT an intermolecular force of attraction! Actual bonds are formed between molecules forming a crystal lattice (or network) of atoms. Attractive forces between positive and negative ions are as strong between molecules as within molecules.

So Now What? – Let’s put it all together! How do you determine which intermolecular forces a compound has? First, determine whether the compound is made of ionic or covalent bonds. Ionic compounds will have only ionic attractions. Covalent compounds will have some sort of Intermolecular force.

All covalent molecules have London dispersion forces All covalent molecules have London dispersion forces. Every covalent molecule has electrons moving around. London dispersion forces are made when those electrons move. You need to draw an accurate LEWIS structure to verify if the molecule is POLAR or NON-POLAR

If the molecule is polar it has Dipole-Dipole forces, but if it also contains a bond between H–O, H–N, and/or H–F, then it also has Hydrogen Bonding. If the molecule is non-polar, it only has London Dispersion forces

Physical properties depend on these forces Physical properties depend on these forces. The stronger the forces between the particles, (a) the higher the melting point. (b) the higher the boiling point (c) the lower the vapor pressure (partial pressure of vapor in equilibrium with liquid or solid in a closed container at a fixed temperature). (d) the higher the viscosity (resistance to flow). (e) the greater the surface tension (resistance to an increase in surface area)

Effect of Intermolecular Forces on the Melting/Boiling Points of Covalent Compounds Melting/Boiling results from a weakening of the attractive forces between covalent molecules. The stronger the forces, the higher the melting/boiling points. If the molecule is non-polar, the more electrons it has the higher its melting/point due to only having London dispersion forces acting on it. If the molecule is polar – but does not contain H-bonding to F, O or N, it’s melting point will be higher than a non-polar substance, but lower that one that has H-bonding or is ionic.