Intermolecular Forces of Attraction (PDF)

SH1726 Intermolecular Forces of Attraction (IMFA) I. Introduction to Intermolecular Forces Intermolecular forces (IMF, collectively known as van der Waals forces) exist between molecules and influence physical properties. For example, we can think of H2O in its three...

SH1726 Intermolecular Forces of Attraction (IMFA) I. Introduction to Intermolecular Forces Intermolecular forces (IMF, collectively known as van der Waals forces) exist between molecules and influence physical properties. For example, we can think of H2O in its three forms, ice, water, and steam. In all three cases, the bond angles are the same, the dipole moment is the same, the molecular shape is the same and the hybridization of the oxygen is the same. However, what properties do these forms vary? The physical properties of H2O are very different in the three states. Let’s take a look at the structure of water in solid form as ice. These crystal lattices are hexagonally bonded solid ice, H2O possesses a definite shape and volume. It is incompressible. Liquid water possesses a definite volume but will assume the shape of its container. It is slightly compressible. Steam will assume both the shape and volume of its container and is extremely compressible. Intermolecular forces (IMF) are the forces which cause real gases to deviate from ideal gas behavior. The IMF govern the motion of molecules as well. In the gaseous phase, molecules are in random and constant motion. Each gas molecule moves independently of the others. In liquids, the molecules slide past each other freely. In solids, the molecules vibrate about fixed positions. So what are the different types of IMFA? II. Types of Intermolecular Forces The London dispersion force is the weakest of the intermolecular forces. This is the force between two nonpolar molecules. It tends to be stronger the larger the atom or the molecule. BC Open Textbooks Fig 1. Dispersion forces result from the formation of temporary dipoles, as illustrated here for two nonpolar diatomic molecules. Dipole-dipole interaction occurs whenever two polar molecules get near each other. The positively charged portion of one molecule is attracted to the negatively charged portion of another molecule. Example interaction between two sulfur dioxide (SO2) molecules, hydrogen bonding is considered a specific example of a dipole-dipole interaction always involving hydrogen. BC Open Textbooks Fig 2 This. image shows two arrangements of polar molecules, such as HCl, that allow an attraction between the partial negative end of one molecule and the partial positive end of another. 01 Handout 1 *Property of STI Page 1 of 2  SH1726 Hydrogen bonding refers to the electrostatic interaction between a hydrogen and a lone electronegative pair of atom. Usually, the electronegative atom is fluorine, nitrogen or oxygen, which has a partial negative charge. The hydrogen atom then has the partial positive charge. BC Open Textbooks Fig. 3. Water molecules participate in multiple hydrogen-bonding interaction with nearby water molecules. Ion-dipole interaction occurs when an ion encounters a polar molecule. In this case, the charge of the ion determines which part of the molecule attracts and which repels. III. Trends in Intermolecular Forces Intermolecular forces vary in their strength, and given the same molecular size, their magnitudes are as follows: 1. All else being equal, stronger intermolecular forces result in higher boiling points and melting points.  With stronger intermolecular forces, more energy is required to separate the liquid-state molecules and convert them to gas-state molecules.  The same is true for molecules in the solid-state that gets converted to liquid-state molecules. 2. At a constant temperature, vapor decreases with the increasing strength of IMF. Between two samples, the substance with the stronger IMF has the lower concentration of molecules in the gaseous state, and therefore have lower vapor pressure. References: Chang, Raymond & Goldsby Ken (2016). Chemistry, 12th ed. Penn Plaza, New York, NY: McGraw- Hill Education. University of Wisconsin-Madison. (n.d.). Intermolecular forces. Retrieved on July 8, 2017, from https://www.chem.wisc.edu/deptfiles/genchem/netorial/rottosen/tutorial/modules/intermolecular_ forces/02imf/imf1.htm 01 Handout 1 *Property of STI Page 2 of 2