Intermolecular Forces PDF

Intermolecular forces Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions (bonds) that hold compounds together. Many physical properties reflect intermolecular forces, like boiling points, melting points,...

Intermolecular forces Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions (bonds) that hold compounds together. Many physical properties reflect intermolecular forces, like boiling points, melting points, viscosity, surface tension, and capillary action. States of Matter The fundamental difference between states of matter is the strength of the intermolecular forces of attraction. Stronger forces bring molecules closer together. Kinetic energy keeps them apart and moving. Remember that average kinetic energy is related to temperature! Properties and State of Matter Table 11.1 Characteristic Properties of the States of Matter Gas Liquid Solid Assumes both volume and Assumes shape of portion of Retains own shape and shape of its container container it occupies volume Expands to fill its container Does not expand to fill its Does not expand to fill its container container Is compressible Is virtually incompressible Is virtually incompressible Flows readily Flows readily Does not flow Diffusion within a gas Diffusion within a liquid Diffusion within a solid occurs rapidly occurs slowly occurs extremely slowly *The atoms in a solid are able to vibrate in place. As the temperature of the solid increases, the vibrational motion increases. Relative Strength of Attractions Table 11.2 Melting and Boiling Points of Representative Substances Force Holding Particles Melting Boiling Point Together Substance Point (K) (K) Chemical bonds blank blank blank Ionic bonds Lithium fluoride (L i F) 1118 1949 Metallic bonds Beryllium (B e) 1560 2742 Covalent bonds Diamond (C) 3800 4300 Intermolecular forces blank blank blank Dispersion forces Nitrogen (N2) 63 77 Dipole–dipole interactions Hydrogen chloride (H 158 188 C l) Hydrogen bonding Hydrogen fluoride (H 190 293 Intermolecular attractions are weaker than bonds. F) Hydrogen bonds are NOT chemical bonds. Types of Intermolecular Force between Neutral Molecules Weakest to strongest forces: – Dispersion forces (or London dispersion forces or induced dipole-induced dipole interactions) – Dipole–dipole forces – Hydrogen bonding (a special dipole–dipole force) Note: The first two types are also referred to collectively as van der Waals forces. Dispersion Forces A nonpolar particle (helium atoms below) can be temporarily polarized to create dispersion force. The tendency of an electron cloud to distort is called its polarizability. Factors That Affect Amount of Dispersion Force in a Molecule Number of electrons in an atom (more electrons, more dispersion force) Size of atom or molecule/molecular weight Shape of molecules with similar masses (more compact, less dispersion force) Polarizability and Boiling Point If something is easier to polarize, it has a lower boiling point. Remember: This means less intermolecular force (smaller molecule or atom: lower molecular weight, smaller size, fewer electrons). Dipole–Dipole Interactions (1 of 2) Polar molecules have a more positive and a more negative end—a dipole (two poles, + and   ). The oppositely charged ends attract each other. Dipole–Dipole Interactions (2 of 2) For molecules of approximately equal mass and size, the more polar the molecule, the higher its boiling point.  Which Have a Greater Effect: Dipole–Dipole Interactions or Dispersion Forces? If two molecules are of comparable size and shape, dipole– dipole interactions will likely be the dominating force. If one molecule is much larger than another, dispersion forces will likely determine its physical properties. What Does This Graph Show Us? In a group, the period 3/4/5 elements have higher boiling points as the group member gets larger. What happens with the period 2 elements? For group 4A, the trend is continued. What about for the other groups? Hydrogen Bonding The dipole–dipole interactions experienced when H is bonded to N, O, or F are unusually strong. We call these interactions hydrogen bonds. A hydrogen bond is an attraction between a hydrogen atom attached to a highly electronegative atom and a nearby small electronegative atom in another molecule or chemical group. What Forms Hydrogen Bonds? Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. These atoms interact with a nearly bare hydrogen nucleus (which contains one proton but NO inner electrons). Ice Compared to Liquid Water Hydrogen bonding makes the molecules farther apart in ice than in liquid water. Ion–Dipole Interactions Ion–dipole interactions are found in solutions of ions. The strength of these forces is what makes it possible for ionic substances to dissolve in polar solvents, like water. Determining Intermolecular Forces in a Substance Polar Polar molecules molecules containing O Nonpolar without O H, H, Ionic solids Atoms molecule N H, N H, or H F dissolved in Exampl s or H F groups groups polar liquids Type of Intermolecular e: N e, A Example: Examples: Examples: Examples: interaction r B F 3 , C H4 H C l, C H3C N H2O, N H3 N aC l in H2O Dispersion forces (0.1–30 ✔ ✔ ✔ ✔ ✔ kJ/mol) Dipole–dipole interactions blank blank ✔ ✔ blank (2–15 kJ/mol) Hydrogen bonding (10–40 blank blank blank ✔ blank kJ/mol) Ion–dipole interactions Note that ALLblank (>50 kJ/mol) blank blank blank chemicals exhibit dispersion ✔ forces. The strongest force dictates the extent of attractions between molecules. Generalizations about Relative Strengths of Intermolecular Forces 1) When two molecules have comparable molar masses and shapes, dispersion forces are roughly equal. 2) When two molecules have very different molar masses and there is no H-bonding, dispersion force determines the substance with stronger attractions.  Liquid Properties Affected by Intermolecular Forces Boiling point (previously discussed) and melting point Viscosity Surface tension Capillary action