Ch13_Liqs - 1 Liquids Liquids In a liquid Liquids Secctoon...

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Unformatted text preview: 1 Liquids Liquids In a liquid Liquids Secctoon 13..3 Se ti i n 13 3 The two key properties we need to describe are EVAPORATION and its are EVAPORATION and EVAPORATION opposite—CONDENSATION opposite—CONDENSATION To evaporate, molecules must have sufficient energy to break IM forces. evaporation---> LIQUID 3 Liquids Liquids Secctoon 13..3 Se ti i n 13 3 • molecules are in constant motion • there are appreciable intermolec . forces intermolec. • molecules close together • Liquids are almost incompressible • Liquids do not fill the container Sectiion 13.3 Sect on 13.3 2 Breaking IM forces requires energy. The process of evaporation is endothermic . is endothermic Add energy VAPOR break IM bonds make IM bonds Remove energy <---condensation 4 Liquids Liquids 0 Liquids Liquids higher T Molecular energy Minimum energy req’d to break IM forces and evaporate lower T Distribution of molecular energies in a liquid. KE is proporproportional to T. to Number of molecules Number of molecules lower T 5 0 higher T 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. See Figure 13.17 Page 1 When molecules of liquid are in the vapor state, they exert a VAPOR they VAPOR PRESSURE EQUILIBRIUM VAPOR PRESSURE 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 6 Liquids Liquids Secctoon 13..3 Se ti i n 13 3 Vapor Pressure 7 CD, Screen 13.9 Liquids Liquids 8 9 Boiling Liquids Boiling Liquids FIGURE 13.19 shows VP as a function of T. shows 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. Liquid boils when its Liquid boils when its vapor pressure vapor pressure equals atmospheric equals atmospheric pressure. pressure. This means that BP’s of liquids change This with altitude. Boiling Point Boiling Point at Lower Pressure at Lower Pressure 10 Liquids Liquids 11 Consequences of Vapor Consequences of Vapor Pressure Changes Pressure Changes 12 Secttion 13..3 Sec ion 13 3 FIGURE 13.19 shows VP as a function of T. shows 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 Secttion 13..3 Sec ion 13 3 Hvap (kJ/mol) vap kJ/mol) oC) 40.7 (100 oC) oC) 26.8 (-47 oC) Xe oC) 12.6 (-107 oC) Molecules at surface experience net INWARD force of attraction. This leads to SURFACE TENSION — the This SURFACE energy req’d to break the surface. dipole induced dipole Secttion 13..3 Sec ion 13 3 Intermolec . forces also lead to CAPILLARY CAPILLARY Intermolec. forces action and to the existence of a concave meniscus for a water column. H 2 O in glass t ube Secttion 13..3 Sec ion 13 3 IM Force IM H-bonds Liquids Liquids conca v e m e n is c u s 15 Surface Tension Surface Tension Molecules at surface behave differently than those in the interior. req’d (at constant P) to vaporize the liquid. req’d (at constant P) to vaporize the liquid. LIQ + heat ---> VAP H2O 2 SO2 2 14 Secttion 13..3 Sec ion 13 3 HEAT OF VAPORIZATION is the heat Compd. Compd. Liquids Liquids 16 17 Capillary Action Capillary Action ADHESIVE FORCES between water and glass COHESIVE FORCES between water molecules SURFACE TENSION also leads to also spherical liquid droplets. 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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