Gases Liquids Solids 1 2 3 4 1 2 3 4 1 2 3 4 a b Chapter 12 Intermolecular Forces and the Physical Properties of Liquids and Solids A The States of Matter and their Properties Molecules are far apart with large distances between them Zxc There are no appreciable attractive or repulsive forces between molecules Can be compressed Have low densities Molecules are close to each other but can slide past one another freely Molecules are held together by several types of intermolecular forces Only slightly compressible Have higher densities than gases Particles are in fixed position can vibrate but cannot move from one location to another in the crystal lattice Particles are held strongly in a three dimensional configuration Virtually incompressible Have high densities Liquids and solids are known collectively as the condensed phases and the forces that hold particles together in the condensed phase are called intermolecular forces B Intermolecular Forces collectively are known as Van der Waals forces These are attractive forces between atoms or molecules and include dipole dipole interactions hydrogen bonding and dispersion forces These attractive forces are much weaker than chemical bonds 1 Dipole dipole Interactions are attractive forces between polar molecules The strength of the attractive forces depend on the magnitude of the molecule s dipole The larger the dipole the stronger the attraction between molecules The strength of the dipole dipole attractions is reflected in the physical properties of liquids such as the boiling point vapor pressure surface tension and viscosity The stronger the attraction between particles the higher the boiling point because more energy is required to disrupt these attractions 1 2 a b c d 4 3 a b Hydrogen Bonding is a special type of dipole dipole interaction that occurs between molecules that contain hydrogen and in which the hydrogen is bonded to a small highly electronegative atom The three atoms that qualify are N O and F The hydrogen bond is stronger than other type of dipole dipole force Is responsible for some unusual structures and properties Hydrogen bonded species have higher boiling points than substances of similar molar mass Hydrogen bonding is responsible for the unusual properties of water1 1 Water is a liquid at room temperature Based on its molar mass it should be a gas 2 Water has an unusually high boiling point 3 Water is less dense in the solid state than in the liquid state while the opposite is true for other substances Hydrogen bonding in HF Boiling point generally increases with increasing molar mass Note in the figure that the lightest molecule in each group has the highest boiling point This is due to hydrogen bonding Dispersion Forces are attractive forces between nonpolar molecules These attractions are due to the of movement of electrons Electrons in nonpolar molecules are generally distributed uniformly however because electrons are free to move at any given time the molecule may have a nonuniform distribution of electron density that results in a temporary dipole known as an instantaneous dipole An instantaneous dipole can induce dipoles in neighboring molecules and the polarized molecules attract one another by opposite poles 2 c d e f g 4 C 1 2 3 The attractive forces that result from instantaneous dipoles are known as London dispersion forces or simply dispersion forces The magnitude of the dispersion forces depends on how mobile the electrons are in a molecule a property known as the polarizability Electrons in small molecules are close to the nucleus and their movement is restricted by the attraction of the nucleus and are not easily polarized The electrons in large molecules are far from the nucleus their movement is less restricted and the electron density is more easily distorted Large molecules are more polarizable than small molecules Dispersion forces are stronger between large molecules than between small molecules Dispersion forces are present in all substances but are the only intermolecular force present in nonpolar substances What type s of intermolecular force s is are present in the following molecules H2S C6H6 CH3OH N2 O3 HF H2O H2CO Ion Dipole Interactions are Coulombic attractions between ions and polar molecules The magnitude of the ion dipole interaction depends on the charge of the ion and the dipole moment of the polar molecule Cations interact more strongly with polar molecules than anions of the same charge because cations tend to be smaller than anions and can approach the polar molecule more closely Generally the higher the charge and smaller the size the stronger the interaction between an ion and a polar molecule 3 4 Based on your knowledge of intermolecular forces explain why methane CH4 is a gas while hexane C6H14 is a liquid II Properties of Liquids The physical properties of liquids largely depend on the magnitude of the intermolecular forces that hold the molecules in the liquid state Surface Tension Surface tension is the quantitative measure of the elastic force on the surface of a liquid It is the energy required to increase the surface of a liquid by a unit area 1 cm2 Responsible for capillary action the movement of liquid in a narrow tube Strong intermolecular forces result in high surface tension B Viscosity is the measure of the liquid s resistance to flow Viscosity depends on the magnitude of the intermolecular forces Stronger the intermolecular forces higher the viscosity of a liquid Viscosity decreases with increasing temperature As molecules gain thermal energy they are able to break the intermolecular forces and flow more freely 4 A 1 2 3 1 2 3 Vapor Pressure Vapor pressure is defined as the pressure exerted by a vapor above a liquid at equilibrium at a specific temperature Equilibrium refers to a condition at which vaporization and condensation occur at the same rate liquid vapor Vapor pressure is related to the strength of the intermolecular forces The stronger the intermolecular forces the lower the vapor pressure Vaporization is the conversion of a liquid to a vapor at its boiling point Requires the input of energy How much energy is required depends on the strength of the intermolecular forces Molar enthalpy of vaporization Hvap kJ mol is the energy required to convert 1 mole of liquid to vapor at its boiling point Condensation is the conversion of vapor to liquid at the boiling point Has the same magnitude as the enthalpy of vaporization but is of opposite sign