BONDING & INTERMOLECULAR FORCES Page 1
INTERMOLECULAR FORCES Intermolecular forces (van der Waals forces) relative weak interactions that occur between molecules. Most of the physical properties of gases, liquids, and solids are controlled by the strengths of these attractions. In liquids and solids, the intermolecular attractions are much stronger than in gases. It is important to realize that attractions between molecules (intermolecular) are always much weaker than the attractions within molecules (intramolecular i.e. bonding) which determines the chemical properties of the substance. 1) Dipole-dipole forces molecules with dipole moments (polar molecules) tend to line up so that the (δ + )(δ ) interactions are maximized and the weak (δ + )(δ + ) or (δ )(δ ) interactions are minimized. Dipole-dipole attractions are relatively weak since the charges associated with dipoles are only partial charges. The farther apart the molecules are, the weaker these dipole forces become. 2) Hydrogen bonding an important and very strong type of dipole-dipole attraction. Occurs when hydrogen is covalently bonded to a small, highly electronegatively atom (F, O, N). This bond is very polar, and the hydrogen atoms are very small allowing for the close approach of the dipoles. This accounts for the unusual strength of the interactions. Hydrogen bonding also accounts for some unusual properties of compounds. For example, ice is less dense than liquid water since hydrogen bonding forces a more expanded or open arrangement of molecules in ice. Also, extensive hydrogen bonding can dramatically increase the boiling point of even very small molecules. 3) Dispersion forces attractive forces occur even between the particles of nonpolar substances such as the atoms of noble gases and molecules such as CH4 and Cl2. These nonpolar substances can also be condensed to liquids and even solids if cooled to a low enough temperature. Therefore, attractions between their particles must exist, although they are weak when compared to polar substances of about the same size. As electrons move about the nucleus, at any given moment the electron density of a particle can be unsymmetrical. For that instant, the particle is a weak dipole called an instantaneous dipole. This dipole can then induce a similar dipole in a neighboring atom. This momentary dipole-dipole attraction is called instantaneous dipole-induced dipole attraction or dispersion forces. This attraction is weak and short-lived but can be significant for large molecules and atoms. For example, as the number of carbon atoms in a hydrocarbon chain increases, the chance for dispersion forces to occur increases, and the boiling point increases (methane = -161 C, ethane = -88.6 C, pentane = 36.1 C). Also, the importance of dispersion forces increases as the size of an individual atom increases. This is because there are more electrons present, increasing the probability of unsymmetrical electron density and making it more polarizable. All atoms and molecules, anything with electrons, experience some level of dispersion forces, though it is generally not relevant in ionic and metallic solids where it is the absolute weakest attractive force present. Regardless of the types of IMFs present in a sample, use data provided to ultimately determine which molecule s IMFs are stronger or weaker overall. Practice: 1) Which is expected to have a higher boiling point, C8H18 or C4H10? Explain. 2) What kinds of IMFs are present in the following? a. HF b. CS2 d. SF6 e. SO2 Page 2 c. PCl3
IMFs in Mixtures When substances with different types of bonding are mixed, new intermolecular attractions can form. 1) Ion-dipole interaction between a cation or anion from an ionic compound and a polar molecule. Most commonly seen when salts are dissolved in water. 2) Ion-induced dipole interaction between a cation or anion from an ionic compound and a nonpolar molecule. The charge of the ion induces an instantaneous dipole in the nonpolar molecule. 3) Dipole-induced dipole interaction between a polar molecule and a nonpolar molecule. The partial charges on the polar molecule induces an instantaneous dipole in the nonpolar molecule. The other IMFs may also exist in mixtures (dispersion, dipole-dipole, H-bonding) provided that the two particles in questions are of different identities. For example, a mixture of benzene and pentane (both nonpolar) would experience benzene-benzene, pentane-pentane, and benzene-pentane dispersion forces. The strength of each of these three forces ultimately determines if the two will for a homogenous mixtures (solution). * * * * * * * * * * * * * * Regarding van der Waal Forces: Whether a substance exists as a solid, liquid, or gas depends on the relationship between kinetic energy and attractive forces and which one dominates. 1. Long skinny molecules tend to have stronger dispersion forces than short fat ones due to increased surface area. n-pentane bp 309.4K neo-pentane bp 282.7K 2,2-dimethylpropane 2. Larger atoms have larger electron clouds and are easier to polarize (form an instantaneous or induced dipole of greater strength). Which one has the highest boiling point? He, Ne, Ar, Kr, Rn 3. For two molecules of similar size and shape, dipole-dipole interactions have a greater effect than LDF on physical properties. For the two molecules below, which one has the lower boiling point? OR 4. If one molecule is much larger than another, LDF will be more important in determining physical properties. For example, C 25 H 52 is a formula for candle wax, a solid at room temperature. H 2 O, water, is a liquid at room temperature. 5. Increased strength of intermolecular forces causes lower equilibrium vapor pressure, increased boiling point, increased melting point, and increased heats of vaporization and fusion. Page 3
IMF PRACTICE PROBLEMS 1) List the substances BaCl 2, H 2, CO, HF, and Ne in order of increasing boiling point based on both intra- and intermolecular forces present. 2) List the kinds of attractive forces present in each of the following systems: a. NaCl(s) b. Xe(l) c. KBr(aq) d. H 2 O(s) 3) Which member of each of the following pairs has the higher boiling point and why? a. N 2, O 2 b. CH 4, SiH 4 c. NaCl, CH 3 Cl d. C 2 H 5 Cl, C 2 H 5 OH 4) At room temperature, neon and xenon are gases, but both become liquids if the temperature is low enough. Which substance has a lower boiling point and why? 5) Predict whether intermolecular forces become more or less significant when the following changes are made: a. A gas is expanded to a larger volume at constant temperature b. More gas is forced into the same volume at constant temperature c. The temperature of the gas is lowered at constant volume 6) Arrange the following in order of increasing boiling point: Ar, He, Ne, Xe 7) Arrange the following in order of ease of liquefaction (condensation): CCl 4, CH 4, CF 4 8) List ethanol (C 2 H 5 OH), propane (C 3 H 8 ), and n-pentane (C 5 H 12 ) in order of increasing boiling point and explain what features determine this order. Page 4
IMF FRQ PRACTICE Page 5
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