Why? Intermolecular Forces. Intermolecular Forces. Chapter 12 IM Forces and Liquids. Covalent Bonding Forces for Comparison of Magnitude

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1 1 Why? Chapter 1 Intermolecular Forces and Liquids Why is water usually a liquid and not a gas? Why does liquid water boil at such a high temperature for such a small molecule? Why does ice float on water? Why do snowflakes have 6 sides? Why is I a solid whereas Cl is a gas? Why are NaCl crystals little cubes? Jeffrey Mack California State University, Sacramento All of these questions may be answered by Intermolecular Forces Intermolecular Forces The forces holding solids and liquids together are called intermolecular forces. Intermolecular Forces are the attractions and repulsions between molecules. They are NOT chemical bonds. The intermolecular forces of a substance may exhibit are a function of: 1. charge (ions vs. neutrals). polarity (molecular shape, dipoles) 3. molar mass Intermolecular Forces Intermolecular forces influence chemistry in many ways: They are directly related to properties such as melting point, boiling point, and the energy needed to convert a solid to a liquid or a liquid to a vapor. They are important in determining the solubility of gases, liquids, and solids in various solvents. They are crucial in determining the structures of biologically important molecules such as DNA and proteins. Covalent Bonding Forces for Comparison of Magnitude Covalent Bonding Forces for Comparison of Magnitude 0 to 30 kj/mol C=C (610 kj/mol) C C (346 kj/mol) D (H-Cl) = 43 kj/mol C H (413 kj/mol) CN (887 kj/mol) Intermolecular forces are much weaker than the bonds that make up compounds.

2 Ion-Ion Forces: Formal Charges The forces that govern charged particles are defined by Coulomb s law. Q Q F k r Q = the charges on the cation and anion r = the distance between k = a constant Greater charge = stronger attraction These are the strong forces that lead to salts with high melting temperatures. H O, mp = 0 C Greater distance = weaker attraction NaCl, mp = 800 C MgO, mp = 800 C Attractions Between Ions & Permanent Dipoles The polar nature of water provides for attractive forces between ions and water. Solvation of Ions Enthalpies of Hydration: A Measure of Ion-Dipole Forces + When a cation exists in solution, it is surrounded by the negative dipole ends of water molecules. When as anion exists in solution, it is surrounded by the positive dipole ends of water molecules. As the size of the ion increases, the exothermicity of the process decreases. This is due to the weaker ion-dipole forces. Enthalpies of Hydration: A Measure of Ion-Dipole Forces Attraction Between Ions & Permanent Dipoles As the size of the ion increases, the exothermicity of the process decreases. This is due to the weaker ion-dipole forces. Water is highly polar and can interact with positive ions to give hydrated ions in water.

3 3 Molecular Polarity Dipole-Dipole Forces Molecular Geometry Linear Trigonal Planar Tetrahedral Trigonal bipyramidal Octahedral Non-Polar Molecule Both atoms the same (outer the same for linear tri-atomic) All bonding groups the same All bonding groups the same All bonding groups the same or both axial groups the same and all three equatorial groups the same, All bonding groups the same or all groups trans to one another the same. Dipole-dipole forces bind molecules having permanent dipoles to one another. Any deviations of symmetry yield a polar molecule. Dipole-Dipole Forces Hydrogen Bonding As the polarity for a given set of molecules with similar molar masses increases, the boiling point increases. Compound Molar Mass (amu) Dipole Moment (D) BP (K) CH 3 CH CH CH 3 OCH CH 3 Cl CH 3 CN A special form of dipole-dipole attraction, which enhances dipole-dipole attractions Boiling point Molar Mass 3 4 BP Dipole molar Mass Dipole Moment H-bonding is strongest when X and Y are N, O, or F Hydrogen Bonding The water molecules network with one another. H bonding in water brings about a network of interactions which explain phenomena such as: capillary action surface tension why ice floats Surface Tension Molecules at surface behave differently than those in the interior. Molecules at surface experience net INWARD force of attraction. This leads to SURFACE TENSION the energy reqired to break the surface.

4 4 Surface Tension Capillary Action IMF s also lead to CAPILLARY action and to the existence of a concave meniscus for a water column. concave meniscus ATTRACTIVE FORCES between water and glass SURFACE TENSION also leads to spherical liquid droplets. H O in glass tube COHESIVE FORCES between water molecules Capillary Action Ice, H O(s) floats because it is less dense than water, H O(l). The H bonds allow the molecules in the liquid phase to to approach closer than normal for non H bonding liquids. This is why water has its maximum density at 4 C. Movement of water up a piece of paper is a result of H-bonds between H O and the OH groups of the cellulose in the paper. Hydrogen Bonding in H O Ice has open lattice-like structure. Ice density is < liquid and so solid floats on water. The Consequences of Hydrogen Bonding One of the VERY few substances where solid is LESS DENSE than the liquid.

5 5 18 g/mol Boiling Points of Simple Hydrogen- Containing Compounds Notice that water has an unusually high bp for its M wt... Hydrogen Bonding 0 g/mol 17 g/mol H-bonding leads to abnormally high boiling point of water. 16 g/mol This is a result of hydrogen bonding! Forces Involving Induced Dipoles How can non-polar molecules such as O and I dissolve in water? The water dipole INDUCES a dipole in the O electron cloud. Induced Dipole Forces How can non-polar molecules such as O and I dissolve in water? The water dipole INDUCES a dipole in the O electron cloud. Dipole-induced dipole Once polarized, the O is attracted to additional water molecules. Induced Dipole Forces Forces Involving Induced Dipoles Formation of a dipole in two nonpolar I molecules. Induced dipoleinduced dipole The degree to which electron cloud of an atom or molecule can be distorted is measured by its polarizability. The larger the molecule, the more easily it is polarized. As the electrons in a molecule become more loosely held and more spread out, the greater the degree of polarizibility in the molecule. The explains the trend we see in solubility.

6 6 London Dispersion Forces London dispersion forces exist between all molecules. London dispersion forces are a function of molecular polarizability. The Polarizability of a molecule is measured by the ease with which an electron cloud can be distorted. The larger the molecule (the greater the number of electrons) the greater polarizability. The greater the surface area available for contact, the greater the dispersion forces. London dispersion forces therefore increase as molecular weight increases. London Dispersion Forces For molecules with the same relative polarizability, the forces scale with molar mass: Higher M wt. = Note the linear relation between bp and molar mass. larger induced dipoles. Molecule BP ( o C) CH 4 (methane) C H 6 (ethane) C 3 H 8 (propane) C 4 H 10 (butane) CH 4 C H 6 C 3 H 8 C 4 H 10 Forces Involving Induced Dipoles Intermolecular Forces Summary The induced forces between I molecules are very weak, so solid I sublimes (goes from a solid to gaseous molecules). Intermolecular Forces Properties of Liquids Of the three states of matter, liquids are the most difficult to describe precisely. Under ideal conditions the molecules in a gas are far apart and are considered to be independent of one another. The structures of solids can be described easily because the particles that make up solids are usually in an orderly arrangement. The particles of a liquid interact with their neighbors, like the particles in a solid, but, unlike in solids, there is little long-range order.

7 7 Properties of Liquids Liquids Particles are in constant motion. Particles are in close contact. Liquids are almost incompressible Liquids do not fill the container. Intermolecular forces are relevant. Liquids: Vaporization In order for a liquid to vaporize, sufficient energy must be available to overcome the intermolecular forces. Breaking IM forces requires energy. The process of vaporization is therefore endothermic. Liquids: Enthalpy of Vaporization The HEAT OF VAPORIZATION is the heat required to vaporize the liquid at constant P. vap H Liquid + energy = Vapor Notice how the types of forces greatly affects the H vap and boiling point. Compound IMF vap H (kj/mol) BP H O SO Xe H-bonds Dipole London C 47 C 107 C Liquids: Enthalpy of Vaporization When molecules of liquid are in the vapor state, they exert a VAPOR PRESSURE. The EQUILIBRIUM VAPOR PRESSURE is the pressure exerted by a vapor over a liquid in a closed container. At equilibrium, rate of evaporation = the rate of condensation. Vapor Pressure When molecules of liquid are in the vapor state, they exert a VAPOR PRESSURE EQUILIBRIUM VAPOR PRESSURE is the pressure exerted by a vapor over a liquid in a closed container when the rate of evaporation = the rate of condensation. Vapor Pressure Recall from kinetic molecular theory As Temp increases, so does the average KE of the particles. This means that there are more particles that can escape into the gas phase!

8 8 Boiling Point Boiling Point at Reduced Pressure Liquid boil when P vap = P atm (Vapor pressure equals atmospheric pressure. As the external pressure is lowered, the vapor pressure equals the external pressure at a lower temperature. Boiling therefore occurs at a reduced temperature. Consequences of Vapor Pressure Changes Equilibrium Vapor Pressure When can cools, vapor pressure of water drops. Pressure inside of the can is less than that of atmosphere, which collapses the can. The vapor pressure of a liquid is seen to increase exponentially with temperature. Measuring Equilibrium Vapor Pressure The Temperature Dependence of Vapor Pressure Goes As: lnp vap vaph C RT slope : DvapH - R Liquid in flask evaporates and exerts pressure on manometer. 1 A plot of lnp vap vs. yields a T slope of: vap H is related to T and P by the Clausius-Clapeyron equation y-intercept = C

9 9 Rather than plot the data, it is convenient to arrange the equation in terms of two temperatures: lnp vap lnp(t ) lnp(t 1 ) = vaph C RT vaph vaph + C RT + C RT 1 Problem: Determine the vapor pressure of water at 50.0 C given that the H vap = 40.7 kj/mol and the vapor pressure at 0.0 C is torr. æ P(T ) ö ç H = vap è ø R T1 T ë û ln P(T 1 ) æ P(T ) ö ç = è ø ln P(T 1 ) vaph RT vap RT H 1 solving: æ P(T ) ö = ç P(T 1) è ø exp H vap R T1 T ë û æ P(T ) ö ç = è ø ln P(T 1 ) H vap R T1 T ë û Where P = vapor pressure, T = temp (K) J R = mol K P(T ) = P(T 1) exp H vap R T1 T ë û Problem: Determine the vapor pressure of water at 50.0 C given that the H vap = 40.7 kj/mol and the vapor pressure at 0.0 C is torr. Liquids: IMF s Summary P(T ) = P(T 1) exp H vap R T1 T ë û Molecules in the Liquid State vap H Volatility Equilibrium Vapor Pressure Boiling Point P(50.0 C) = torr exp 3 kj 10 J 40.7 mol 1kJ é 1 1 ù ê - J ë93.15k 33.15K û mol K Strong IMF s Weak IMF s More Endothermic Low Low High Less Endothermic High High Low = 8.7 Torr