How Many Pennies? Fill a cup of water so that water is level with the top of cup. Carefully add a penny to the cup of water. Continue until the water spills over the edge. –Predict the number of pennies it will take. –Actual number of pennies required. –Sketch a picture of this phenomena. –Provide an explanation of what you have observed.

Ch Intermolecular Forces Intermolecular forces are attractive forces between molecules. –Become significant when particles are close together. –Influence properties such as boiling point. –Intermolecular forces vary in strength but are generally weaker than ionic and covalent bonds.

Dipole- Dipole Forces One type of intermolecular force exists between polar molecules. Polar molecules are created when the polar bonds in a molecule are asymmetrically arranged. This creates partially positive and negative regions in the molecule. Polar molecules are also called dipoles. The negative region in one polar molecule attracts the positive region in adjacent molecules. Such forces of attraction between polar molecules are known as dipole-dipole forces.

Dipole – Dipole Interactions Occur when polar molecules are attracted to each other.

Hydrogen Bonding Some hydrogen-containing compounds have unusually high boiling points. This is explained by a particularly strong type of intermolecular force. Hydrogen bonding – intermolecular force that exists between a hydrogen atom covalently bonded to a highly electronegative atom (N, O, or F) in one molecule and highly electronegative atom (N, O, or F) in another nearby molecule. Hydrogen bonds are usually represented by dotted lines connecting the hydrogen-bonded hydrogen to the electronegative atom to which it is attracted.

Hydrogen Bonding In Water

London Dispersion Forces Even noble gas atoms and nonpolar molecules can experience weak intermolecular attraction. In any atom or molecule—polar or nonpolar—the electrons are in continuous motion. As a result, at any instant the electron distribution may be uneven. A momentary uneven charge can create a positive pole at one end of an atom or molecule and a negative pole at the other.

London Dispersion Forces

This temporary dipole can then induce a dipole in an adjacent atom or molecule. The two are held together for an instant by the weak attraction between temporary dipoles. The intermolecular attractions resulting from the constant motion of electrons and the creation of instantaneous dipoles are called London dispersion forces. London forces tend to increase with molar mass.

We can identify the types of intermolecular forces present in a substance by considering its composition and structure. London dispersion forces are found in ALL substances. Strength of these attractions varies directly with molar mass. Dipole-dipole forces and hydrogen bonding can also add to the force of attraction. Hydrogen bonding is the strongest of the intermolecular forces.

What Do You Think? In each of the following select the substance with the higher boiling point. Justify your answer. (a) CH 4 or CCl 4 (b) NH 3 or PH 3