Exam 4 Study Guide Chapter 9 December 11 2014 1 Comparing Solids Liquids and Gases a Two main reasons for differences between liquids solids and gases i Molecules are much closer to one another in liquids and solids 1 Molecules touch one another why solids and liquids have higher densities than gases 2 Solids and liquids are less compressible than gases ii Intermolecular forces which are essentially negligible in gases play a much more important role in liquids and solids 1 Surface tension is shown in liquids molecules at surface are attracted inward by unbalanced intermolecular forces liquids form spherical drops surface area to volume ratio is as small as possible 2 Liquid Vapor equilibrium a Introduction b Vapor Pressure i ii In an open container evaporation occurs until there is no liquid left in the container If the container is closed the molecules primarily move fro liquid to vapor 1 Some of them collide with the surface and re enter as liquid 2 As time passes the concentration of molecules in the vapor increases so does the rate of condensation a When the rate of condensation becomes equal to the rate of vaporization the liquid and vapor are in a state of dynamic equilibrium i Once equilibrium between liquid and vapor is reached the number of molecules per unit volume in the vapor does not change the pressure exerted by the vapor over the liquid remains constant ii Vapor pressure the pressure of vapor in equilibrium with a liquid 1 This is a characteristic property of a given liquid at a particular temperature iii So long as both liquid and vapor are present the pressure exerted by the vapor is independent of the volume in the container 1 If a small amount of liquid is introduced into a closed container some of it will vaporize establishes its equilibrium vapor pressure 2 The greater volume of the container the greater will be the amount of liquid that vaporizes to establish the pressure 3 The ratio n V stays constant so P nRT V doesn t change 4 Only if all the liquid vaporizes will the pressure drop below the equilibrium c Vapor Pressure versus Temperature value i Vapor pressure of a liquid always increases as temperature rises ex Water evaporates faster on a hot dry day ii The slope of a graph plotted temperature vs vapor pressure is not linear slope steadily increases as temperature rises shows the fact that more molecules vaporize at higher temperatures 1 Want a linear line because they are easier to interpret and plot a Can do this by plotting the natural log of the vapor pressure on the y axis versus 1 T on the x axis 2 Equation ln P2 P1 a Hvap i R 8 31 J mol K 1 T 2 R T 1 1 d Boiling Point i For a pure liquid the temperature remains constant throughout the boiling process ii The temperature at which a liquid boils depends on the pressure above it 1 For a vapor bubble to form the pressure within it P1 must be at least equal to the pressure above it P2 2 P1 is simply vapor pressure of the liquid it follows that a liquid boils at a temperature at which its vapor pressure is equal to the pressure above its surface a Normal boiling point if this pressure is 1 atm It is called the normal boiling point iii The boiling point of a liquid can be reduced by lowering the pressure above it iv At high attitudes it takes longer to cook food in boiling water in an open container the reduced pressure lowers the temperature at which water boils in an open container and slows down the physical and chemical changes that take place to cook 1 Use a pressure cooker to solve the pressure that develops is high enough e Critical Temperature and Pressure to raise the boiling point of water above 100C i Critical temperature temperature that above which the liquid phase of a pure substance can t exist ii Critical pressure the pressure that must be applied to cause condensation at that iii temperature the vapor pressure of the liquid at the critical temperature Permanent gas critical temperatures below 25C applying pressure at room temperature will not condense a permanent gas it must be cooled as well iv Above the critical temperature and pressure a substance is referred to as a supercritical fluid 1 They can diffuse through a solid like a gas and dissolve materials like a liquid 2 Carbon dioxide and water are the most commonly used ones 3 Phase diagrams a Introduction i Phase diagram graph that shows the temperature and pressures at which different phases are in equilibrium with each other ii b Sublimation i Sublimation the process by which a solid changes directly to vapor without passing though the liquid phase ii Deposition opposite of sublimation phase transition from the vapor phase to solid without passing through the liquid phase iii The pressure of the solid in equilibrium with the gas is called the vapor pressure of the solid iv A solid can only sublime at temperature below the triple point because above that point they will melt into a liquid c Melting point i For a pure substance the melting point is identical to the freezing point 1 Represents the temperature at which solid and liquid phases are in equilibrium ii Usually measured in an open container at atmospheric pressure iii An increase in pressure favors the formation of the more dense phase 1 The solid is the more dense phase when inclined to the right smaller angel between liquid and solid line the solid liquid equilibrium line is a At higher pressures the solid becomes stable at temperature above the normal melting point the melting point is raised by an increase in pressure 2 The liquid is the more dense phase when the left wider angel between solid and liquid line the liquid solid line is inclined to a Increase in pressure favors the formation of liquid the melting point is decreased by raising the pressure b Water is one of the few substances that does this 4 Molecular Substances Intermolecular Forces a Introduction i Molecular substances have the following characteristics 1 Nonconductors of electricity when pure molecules are uncharged so they 2 can t carry an electric current Insoluble in water but soluble in nonpolar solvents such as CCl4 or benzene iodine is typical of most molecular substances it is only slightly soluble in water and much more soluble in benzene 3 Low melting and boiling a Reflect the fact that the intermolecular forces are weak b Boiling points of different molecular substances are directly related to the strength of the intermolecular forces involved i The stronger the IMF the higher the boiling