CHEM 1212 1st Edition Lecture 2 Outline of Last Lecture I States of Matter and Intermolecular Forces II Interactions between Ions and Molecules with a Permanent Dipole III Interactions between Molecules with a Dipole a Dipole Dipole Forces b Hydrogen Bonding c Hydrogen Bonding and the Unusual Properties of Water IV Intermolecular Forces Involving Nonpolar Molecules a Dipole Induced Dipole Forces b London Dispersion Forces Induced Dipole Induced Dipole V A Summary of van der Waals Intermolecular Forces Outline of Current Lecture I Properties of Liquids a Vaporization and Condensation b Vapor Pressure c Vapor Pressure Enthalpy of Vaporization and the Clausius Clapeyron Equation II Crystal Lattices and Unit Cells Current Lecture These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute I Properties of Liquids Vaporization and Condensation a Vaporization Evaporation the process in which a substance in the liquid state becomes a gas b As molecules gain kinetic energy through raising temp they are able to break the interactions with the neighboring molecules and escape the liquid as a gas c Vaporization is an endothermic process because it requires the input of energy to overcome the intermolecular forces d The energy required to vaporize a sample is often given as the standard molar enthalpy of vaporization i Liquid vaporization energy absorbed gas e As gas molecules lose their kinetic energy they are then able to reenter the liquid phase called condensation think of it as the gas condensing back into a liquid f Condensation is the reverse process of vaporization it is exothermic because energy is transferred to the surroundings g The enthalpy change for condensation is equal but opposite in sign to the enthalpy of vaporization i Ex The enthalpy of vaporization for 1 mol of water at 100 degrees Celsius is 40 7 kJ 1 The enthalpy of condensation of 1 mol of water vapor at 100 degrees Celsius is 40 7 kJ ii Vapor condensation energy released liquid h The boiling points of nonpolar liquids increase with increasing molecular mass i In the heavier hydrogen halide H and Cl Br or I the boiling points and enthalpies increase with increasing mass i In these three molecules London forces and dipole dipole forces are responsible for the intermolecular attractions j Among the hydrogen halides HF is the exception i HF has a much higher boiling point and enthalpy of vaporization due to the extensive hydrogen bonding in the molecule k The process of evaporation is what is happening when we work out our energy heats up the water molecules in our body and they turn to vapor sweat l Rain is just the condensation of water vapors in the sky Vapor Pressure m Water in an open container with eventually evaporate completely because the escaping water vapor molecule will leave the container n Water in a closed container will remain all in the container because the water vapor molecules can not escape and some will re condense to again form liquid i Eventually the masses of liquid and vapor in the container will remain constant meaning that they are at a dynamic equilibrium ii At equilibrium the rate at which molecules change from liquid to vapor is equivalent to the rate at which they change from vapor to liquid this results no net change in the masses of the two phases o Equilibrium Vapor pressure aka vapor pressure i The vapor pressure can be measured once the two phases reach the liquidvapor equilibrium ii Vapor pressure is defined as the pressure exerted by the vapor in equilibrium with the liquid phase the tendency of molecules to escape from liquid to vapor phase at a given temperature Vapor Pressure Enthalpy of Vaporization and the Clausius Clapeyron Equation p Clausius Clapeyron equation provides a method for obtaining values of the enthalpy of vaporization i ln P enthalpy of vaporization RT C ln P vapor pressure R ideal gas constant 8 314472 J K x mol T temp C constant characteristic of liquid in question q Another way to calculate enthalpy of vaporization can be used if you have the vapor pressure of a liquid at two different temperatures ln P2 P1 enthalpy of vaporization R 1 T2 1 T1 Boiling Point r Boiling point is the temperature at which a liquids vapor pressure is equal to the external pressure allowing bubbles of water vapor to float to the surface s Normal Boiling Point the temperature at which that external pressure is 760 mm Hg i Normal boiling point of water is 100 degrees Celsius Critical Temperature and Pressure t Critical point when a high enough temperature is reached and the interface between the liquid and the vapor disappears i Tc the temperature at which critical point is reached ii Pc the pressure at which the critical point is reached u Supercritical fluid a fluid that exists under these conditions it is like a gas under such high pressure that its density resembles that of a liquid but its viscosity remains close to that of a gas i The molecules are pushed closely together like they are in a liquid but each molecule has just enough energy to overcome the intermolecular forces Surface Tension Capillary Action and Viscosity v Unlike molecules in the middle of a liquid which interact with other molecules all around them molecules on the surface of a liquid only interact with molecules at or below the surface i Leads to an inward force of attraction contracting the surface molecules and making them behave as a sort of skin for the liquid w Surface Tension the energy required to break through the surface or to disrupt a liquid drop i This is what causes water droplets to be contained in little spheres x Capillary action demonstrated by a glass tube in water water will rise inside the tube just as water would climb up a piece of paper if it was placed in water i Adhesive forces create an attracting between the polar water molecules and the polar molecules on the surface of the glass tube ii Cohesive forces between the water and the glass cause the water level inside the tube to rise until the attractive forces are balanced by the force of gravity pulling down on the water column 1 This leads to the concave meniscus seen with water in a test tube iii There are rare exceptions such as mercury where the surface tension is much stronger than the adhesive forces between the water and glass 1 In the case of Mercury it a convex upward curving meniscus y Viscosity the resistance of liquids to flow i