Ch13_Liqs - 1 Liquids Section 13.3 Section 13.3 In a liquid...

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Unformatted text preview: 1 Liquids Section 13.3 Section 13.3 In a liquid • molecules are in constant motion • there are appreciable intermolec. forces intermolec. • molecules close together • Liquids are almost incompressible • Liquids do not fill the container Liquids Liquids Section 13.3 Section 13.3 2 Liquids Liquids Section 13.3 Section 13.3 3 The two key properties we need to describe are EVAPORATION and its EVAPORATION opposite—CONDENSATION opposite—CONDENSATION To evaporate, molecules must have sufficient energy to break IM forces. Breaking IM forces requires energy. The process of evaporation is endothermic. endothermic. evaporation---> LIQUID Add energy VAPOR break IM bonds make IM bonds Remove energy <---condensation Liquids Liquids lower T Number of molecules higher T 4 5 6 Liquids Liquids lower T higher T 0 Molecular energy Minimum energy req’d to break IM forces and evaporate Distribution of molecular energies in a liquid. KE is proporproportional to T. 0 Molecular energy minimum energy needed to break IM forces and evaporate At higher T a much larger number of molecules has high enough energy to break IM forces and move from liquid to vapor state. High E molecules carry away E. You cool down when sweating or after swimming. When molecules of liquid are in the vapor state, they exert a VAPOR VAPOR Liquids Liquids Section 13.3 Section 13.3 Number of molecules PRESSURE EQUILIBRIUM VAPOR PRESSURE is is the pressure exerted by a vapor over a liquid in a closed container when the rate of evaporation = the rate of condensation. See Fig. 13.18 See Figure 13.17 Page 1 Vapor Pressure CD, Screen 13.9 7 Liquids Liquids FIGURE 13.19 shows VP as a function of T. 1. The curves show all conditions of P and T where LIQ and VAP are in EQUILIBRIUM 2. The VP rises with T. 3. When VP = external P, the liquid boils. This means that BP’s of liquids change with altitude. 8 9 Boiling Liquids Boiling Liquids Liquid boils when its Liquid boils when its vapor pressure vapor pressure equals atmospheric equals atmospheric pressure. pressure. Boiling Point Boiling Point at Lower Pressure at Lower Pressure 10 11 Consequences of Vapor Consequences of Vapor Pressure Changes Pressure Changes Liquids Liquids 12 Section 13.3 Section 13.3 FIGURE 13.19 shows VP as a function of T. 4. If external P = 760 mm Hg, T of boiling is the NORMAL BOILING POINT 5. VP of a given molecule at a given T depends on IM forces. Here the VP’s are in the order ether O When pressure is lowered, the vapor pressure can equal the external pressure at a lower temperature. When can cools, vp of water drops. Pressure in the can is less than that of atmosphere, so can is crushed. C2H5 H5C2 dipoledipole alcohol O H5C2 H H-bonds water O H H extensive H-bonds increasing strength of IM interactions Page 2 Liquids Liquids 13 Section 13.3 Section 13.3 Liquids Liquids 14 15 Section 13.3 Section 13.3 Surface Tension Surface Tension Section 13.3 Section 13.3 HEAT OF VAPORIZATION is the heat is req’d (at constant P) to vaporize the liquid. req’d (at constant P) to vaporize the liquid. LIQ + heat ---> VAP Compd. Compd. H2O 2 SO2 2 Xe Hvap (kJ/mol) mol) kJ/mol) vap (kJ/ oC) 40.7 (100 oC) oC) 26.8 (-47 oC) oC) 12.6 (-107 oC) Molecules at surface behave differently than those in the interior. IM Force H-bonds dipole induced dipole Molecules at surface experience net INWARD force of attraction. This leads to SURFACE TENSION — the energy req’d to break the surface. SURFACE TENSION also leads to spherical liquid droplets. Liquids Liquids 16 17 Section 13.3 Section 13.3 Capillary Action Capillary Action Intermolec. forces also lead to CAPILLARY Intermolec. action and to the existence of a concave meniscus for a water column. concave meniscus H2O in glass tube ADHESIVE FORCES between water and glass COHESIVE FORCES between water molecules Movement of water up a piece of paper depends on H-bonds between H 2O and the OH groups of the cellulose in the paper. Page 3 ...
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This note was uploaded on 01/11/2011 for the course ENGINEERIN MAE 107 taught by Professor Pozikrizdis during the Fall '08 term at San Diego.

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