Intermolecular Forces (IMF)

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Attractions Between Molecules Van der Waals Forces Intermolecular forces (also called van der Waals forces) describe the attraction between molecules. The strength of these attractions determine at what temperature a substance is a solid, liquid, or gas. These are the forces that must be overcome in order to melt a solid or vaporize a liquid.

Attractions Between Molecules Van der Waals Forces Although we sometimes use the word “bond” when describing these forces, these attractions between molecules are much weaker than the (intramolecular) covalent bonds joining atoms in a chemical compound. The breaking of intermolecular attractions does not produce a chemical change, but a physical one.

Attractions Between Molecules Van der Waals Forces 3 basic types from weakest to strongest (London) Dispersion forces Dipole interaction Hydrogen bonding

London Dispersion Forces Weakest of all molecular attractions Caused by the motion of electrons producing a temporary polarity. Electrons are constantly in motion. When all of the electrons are randomly on one side of a molecule, even nonpolar molecules will have a slightly negative and positive side.

London Dispersion Forces The slightly positive charge will attract its neighbor molecule’s electrons—causing them to gather on one side. So, for a split second, one molecule has a negative side; the neighbor molecule has a positive side, and an attraction occurs. This attraction is called a temporary dipole. A dipole is a separation of charge, and the dipole produced by dispersion forces is temporary because the electrons continue to move.

London Dispersion Forces Since all molecules have electrons, this force is experienced by all molecules—even the noble gases. The strength of dispersion forces increases as the number of electrons in the molecule increases.

London Dispersion Forces

Dipole Interaction Attraction that occurs between polar molecules Polar molecules have a permanent dipole. The slightly negative side of one molecule is attracted to the slightly positive side of its neighbor.

The image above shows that HCl only has one bond and that bond is polar, making HCl a polar molecule—a molecule having a permanent dipole. It is attracted to other HCl molecules by dipole interaction. Recall that HCl will also experience dispersion forces.

Hydrogen Bonds Hydrogen bonding is the strongest of the 3 intermolecular forces. Although it has the word “bond” in its name, it is still an attraction between molecules that has only about 5% of the strength of a covalent bond.

Hydrogen Bonds Hydrogen bonding occurs in very specific instances—when H is covalently bonded to O, N, or F. In other words, just because a molecule has hydrogen does not mean that hydrogen bonding is occurring.

Hydrogen Bonds Hydrogen is a very small molecule with only 1 electron. Oxygen, nitrogen, and fluorine are also relatively small and very electronegative. When hydrogen is bound to one of these three, the resulting covalent bond is very polar. Oxygen, nitrogen, and fluorine also have lone pairs that attract neighboring hydrogens.

Hydrogen Bonds + Very Important!! Water experiences hydrogen bonding. High surface tension High specific heat When frozen, more space between the molecules than liquid form Reason ice floats on water O H + 2 lone pairs

Hydrogen Bonds Responsible for the double helix of the DNA molecule

Hydrogen Bonds In essence, hydrogen bonding is a strong dipole interaction that will occur only in molecules in which H is bound to N, O, or F. For example, it will occur in ammonia (NH3) but not in PH3 or CH4.

Intermolecular Forces Compound Lewis Structure Intermolecular Forces Explanation NH3 Hydrogen Bonding Dispersion Since H is bound to an N, H-bonding occurs. All molecules experience dispersion. PH3 Dipole Interaction This is a polar molecule (lone pairs on central atom) so it experiences dipole interaction. H is not bound to an O, N, or F, so there is no H-bonding. CH4 This is a nonpolar molecule that can only experience dispersion. O2 There is obviously no H-bonding since there are no Hs.

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