Evaporation Vaporization = conversion of a liquid to a gas or vapor Evaporation = transition from a liquid to a gas BELOW a substances boiling point Some of the liquid particles have enough kinetic energy to overcome the forces of attraction around them and escape into the gas phase

Vapor Pressure Amount of pressure gas particles are exerting on the surface of the liquid (to escape to the air)

Temperature and Vapor Pressure The higher the temperature the faster the particles are moving The faster the particles are moving the higher the vapor pressure Low Temp High Temp

Liquid/Vapor Equilibrium The point at which (in a closed container) the rate of evaporation is equal to the rate of condensation

STP Standard Temperature Standard Pressure STP = Standard Temperature and Pressure Standard Temperature 0 °C or 273 K Standard Pressure 1 atmosphere (atm) 101.3 kilopascals (kPa) 760 Torr (mmHg)

Boiling Point Atmospheric Pressure Temperature at which vapor pressure  = atmospheric pressure  Normal boiling point is at standard pressure (Water = 100 °C) Boiling point is lower at higher elevations (there is less gas above them) Vapor Pressure V.P. = A.P.

Boiling Point Atm Pressure Atm Pressure Atm Pressure Vapor Pressure 70 ºC 100 ºC 25 ºC Atm Pressure Atm Pressure Atm Pressure Vapor Pressure Vapor Pressure Vapor Pressure

How would a change in elevation affect boiling point How would a change in elevation affect boiling point? Would water boil at 100 ºC on a mountain? Boiling point is lower at higher elevations (there is less gas above them; lower atmospheric pressure)

Altitude and Boiling Point

As the temperature of a liquid increases, its vapor pressure iRespond Question F Multiple Choice A.) decreases B.) increases C.) remains the same D.) E.)

As the pressure on the surface of a liquid decreases (atmospheric pressure), the temperature at which the liquid will boil iRespond Question F Multiple Choice A.) decreases B.) increases C.) remains the same The atmospheric pressure is lower, meaning that the vapor pressure will be lower, and will cause boiling to occur at a lower temperature D.) E.)

Thermochemistry & Phases Test Phase Changes Energy transformations Conservation of Energy Endothermic/Exothermic reactions Temperature conversions (memorize K = ºC+ 273) Heat & Temperature Heat Flow Calorimetry problems Interpret potential energy diagrams Thermochemical Stoichiometry (Mole ↔ Energy) Properties of 3 Phases Names of Phase Changes Endothermic/Exothermic Phases Changes Energy Changes During Phase Changes Heat of Fusion/Vaporization Calculations Dynamic Equilibrium During Phase Changes Vapor Pressure Boiling (Changes Using Elevation or Pressure) Interpreting Heating Curves

Relationship between intermolecular forces and boiling point The stronger the intermolecular forces the less vapor pressure; higher boiling point (it requires more heat energy to overcome the attraction between the molecules) Water has strong intermolecular forces and has a high boiling point (low V.P.) Gasoline has weak intermolecular forces and has a low boiling point (high V.P.)

What is the boiling point of water at 70 kPa? What do all of the curves have in common? Which substance has the weakest intermolecular forces? What is the vapor pressure of propanone at 25 C? What is the normal boiling point of water? What pressure is needed for ethanoic acid to boil at 110 C?

Phase Diagrams A way to represent the various phases of a substance and the conditions under which each phase exists. A phase diagram is a plot of pressure (P) vs temperature (T). Lines on the diagram represent conditions (T,P) under which a phase change is at equilibrium. That is, at a point on a line, it is possible for two (or three) phases to coexist at equilibrium. Other regions of the plot, only one phase exists at equilibrium

Triple Point – temperature and pressure at which all 3 phases exist in equilibrium Critical Point - At a temperature above the critical temperature, Tc, and a pressure above the critical pressure, Pc, it is no longer possible to distinguish between the gas and liquid phases. At T > Tc and P > Pc the substance is referred to as a super-critical fluid.