Chap 11B - Chem 210 WANG Chapter 11B Liquids Phase Changes...

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Unformatted text preview: Chem 210 / WANG / Chapter 11B Liquids, Phase Changes and Phase Diagram Chapter 11.3 through 11.6 Liquids Liquids 11.1 A Molecular Comparison of Gases, Liquids, Solids. 11.2 Intermolecular Forces 11.3 11.3 Some Properties of Liquids 11.4 11.4 Phase Changes 11.5 11.5 Vapor Pressure 11.6 11.6 Phase Diagrams The molecular basis of high surface tension in H2O. hydrogen bonding occurs across the surface and below the surface hydrogen bonding occurs in three dimensions the net vector for attractive forces is downward Properties Properties of the Liquid State Intermolecular forces in liquids result in these properties. Surface Tension - Attraction of molecules at the surface; measured in J/m2 Viscosity - Resistance to flow; measured in N⋅s/m2 Vapor Pressure - Molecules escaping the liquid & exert pressure over it; measured in mmHg mmHg or torr. torr. Boiling Point – Occurs when vapor pressure equals atmospheric pressure. Freezing Point – equals melting point. melting Surface Tension Surface tension in mercury (4.6x10-1 J/m2) being very strong, results in its meniscus rounded meniscus. Surface tension in water (7.29x10-2 J/m2) is weaker than adhesive the adhesive force between water and glass, resulting in its U-shaped meniscus. meniscus • Surface molecules are attracted only to those below them. • Internal molecules are attracted in all directions. • S.T. = Energy Energy required to increase increase the surface per unit area (J/m2). Surface Tension H2O and Hg meniscus differs in glass (capillarity). Adhesive forces between water and glass are stronger than hydrogen bonding in water. stronger cohesive (~intermolecul ar) forces H 2O Mosquito Mosquito larva take advantage of the strong surface tension of water by suspending themselves from the surface. Adult mosquito later escapes into air. Viscosity Hg Question: Describe how dipole moment relates to boiling point. Explain why this relationship is reasonable. Viscosity = ease with which molecules can move with respect to others. 1. temperature, viscosity. temperature, viscosity. 2. intermolecular force, viscosity. intermolecular viscosity. Adding soap to water breaks up surface tension and causes mosquito larvae to sink thereby dooming the adult from ever making it to the air. Fall 2009 UM-SJTU JI Page 1 Chem 210 / WANG / Chapter 11B Liquids, Phase Changes and Phase Diagram Question: Describe how dipole moment relates to boiling point. Explain why this relationship is reasonable. Question: (a) How is hydrogen bonding in H2O depicted in this picture? By By the dotted lines. Generally speaking, for molecules with similar molar mass (as are the ones shown in the greater leads previous figure), greater dipole moment leads to higher higher boiling point. This is reasonable because the intermolecular intermolecular dipoledipole-dipole attraction becomes stronger with greater dipole moment, and higher intermolecular attraction makes it harder for molecules in a liquid to escape into the gaseous state, and the liquid boils at a higher temperature. SAMPLE PROBLEM 2 ANSWRR Name the intermolecular force in each compound; explain the difference in B.P. dispersion DipoleDipole-dipole The dispersion forces in each compound are similar, since they have the same molar mass. Acetone (on the right), being polar, has also the dipole-dipole force, dipolewhich is a stronger intermolecular force than dispersion. Its boiling point is therefore higher. The highest number of hydrogen bonds for a H2O molecule = 4, each of which is shared with another H2O molecule. (b) Are there more H-O covalent bonds per H2O than there are hydrogen bonds per H2O? hydrogen ? Based on this data table, describe how surface tension relates to intermolecular forces. Explain why this relationship is reasonable. Intermolecular forces hold surface molecules to each other, causing surface tension. That is why compounds with stronger intermolecular forces have higher surface tension. Sample Problem 3 Substance diethyl ether Formula Surface Tension (J/m2) at 200C (a) C2H6 has no H bonding sites. H H COH H H HOC H H (c) H H O H O HN NH H N CH3C CH3C N H Changes of State movie O O CH3C CH3C H Three States of Matter and changes of State Major Force(s) CH3CH2OCH2CH3 1.7x10-2 dipole-dipole; dispersion CH3CH2OH 2.3x10-2 H bonding butanol CH3CH2CH2CH2OH 2.5x10-2 H bonding; dispersion water H2O 7.3x10-2 H bonding mercury ‘Phase Changes’ refers to Which of the following substances exhibits H bonding? For those that do, draw two molecules of the substance with the H bonds between them. (b) ethanol • the transformation from one physical state to physical to another, also called state changes. state • Six changes shown below: 3 are endothermic and endothermic 3 are exothermic. exothermic SAMPLE PROBLEM 1 ANSWER Hg 48x10-2 metallic bonding The The energy involved in phase changes. Why does it require more heat to vaporize a liquid than to melt a solid? solid? Is it true that energy needed for sublimation equals the sum of fusion + vaporization? vaporization? Why? Heat Heat of vaporization, ∆Hvap Energy required to vaporize 1 mole of a vaporize liquid at its normal boiling point. H2O(l) H2O(g) ∆Hvap= + 40.67 kJ/mole liquefaction How much heat is required to completely boil away 1 cup (about 250 mL) of water? evaporation fusion crystallization Fall 2009 UM-SJTU JI For water, 250 mL ~ 250 g (250 g ÷ 18.0 g/mole) x 40.67 kJ/mole = 565 kJ Page 2 Chem 210 / WANG / Chapter 11B Liquids, Phase Changes and Phase Diagram Heat of fusion, ∆Hfus Energy required to completely convert a substance convert from a solid into a liquid at its normal melting point. at H2O(s) H2O(l) ∆Hfus = + 6.008 kJ/mole How many Calories are required to melt an ice cube of 31.5 g? Can one lose weight by chewing ice cubes all day? (31.5 g ÷ 18.0 g/mole) x 6.008 kJ/mole =10.5 kJ ÷ 4.18 J/cal =2.52 kcal or 2.52 Cal (big cal). ☺ Lose 2.52 Cal for melting one ice cube (+ 1.17 Cal more if including calories used to warm melted ice water to body temperature)! Heating Curve of H2O T remains constant during phase changes Length of line B-C Length of line D-E ∆Hfus • Because there is no change in average kinetic energy during phase changes. ∆Hvap Slope of line A-B relates to • Heat used to overcome Specific heat of ice attractive forces in the Slope of line C-D relates to Specific heat of water solid (melting) or liquid Slope of line E-F relates to Specific heat of steam (vaporizing) phases. A cooling curve (heating curve reversed) for the conversion of gaseous water to ice. Specific heat or heat capacity Substance Aluminum, solid Copper, solid Hydrogen, gas Mercury, liquid J/goC 1.0 0.4 14.2 0.1 Substance Ice Water Steam Heat involved in converting ice to steam Estimate the total energy required to completely convert convert 31.5 grams of ice at -20.0o C to steam at 122oC. g mole mole first: 31.5 g ÷ 18.0 g/mole = 1.75 mole Line A-B -20oC ice Aice 0oC ice q = n x Cice x ∆T 1.75 mole x 37.6 J/mol⋅oC x 20.0oC = 1320 J 37.6 J/mol ice 0oC water Need ∆Hfus Line B-C 0oC ice B1.75 mole x 6.008 kJ/mole =10.5 kJ 6.008 =10.5 Line CLine C-D 0oC water 100 100oC water q = n x Cwater x ∆T 1.75 mole x 75.4J/mol⋅oC x 100oC = 13200 J 75.4J/mol Line D-E 100oC water D- 100 Shows how Temperature Temperature changes as J/goC J/moleoC 2.09 37.6 4.19 75.4 1.84 33.1 100 100oC steam Need ∆Hvap 1.75 mole x 40.67 kJ/mole =71.2 kJ 40.67 =71.2 Line E-F 100oC steam E- 100 122 122oC steam q = n x Csteam x ∆T 1.75 mole x 33.1J/mol⋅oC x 22oC = 1270J 33.1J/mol Figure 12.1 Heats of vaporization and fusion for common substances. heat heat is added Heat involved in converting ice to steam Estimate the total energy required to completely convert convert 31.5 grams of ice at -20.0o C to steam at 122oC. -20oC ice ice Convert J to kJ before adding all together! 0oC ice = 1320 J 1.32 1.32 kJ 0oC ice ice 0oC water 100 100oC water 100oC water 100oC steam 0oC water =10.5 kJ = 13200 J 13.2 13.2 kJ 100 100oC steam =71.2 kJ 122 122oC steam = 1270 J 1.27 1.27 kJ Total in kJ = 1.32 + 10.5 + 13.2 + 71.2 + 1.27 1.32 10.5 13.2 71.2 1.27 = 97.5 kJ 3 Important aspects of phase changes Vapor Pressure (VP) - Molecules escaping the liquid and exert pressure over it; measured in mmHg mmHg or torr. torr. What factors control the value of ∆Hvap for each substance? Do these same factors control the value of ∆Hfus ? Answer by considering the three states of matter on the molecular level. Strength of intermolecular forces controls ∆Hvap . This factor does controls not control ∆Hfus as much (see the dip of blue lines) since molecular molecular shape impacts solid structure more so than liquid, so shape impacts ∆Hfus more than it impacts ∆Hvap . Fall 2009 UM-SJTU JI Heating Curve - Plots Temp versus Heat Temp Heat Specific heat - Energy needed to increase temperature of 1 g of a substance by 1oC or 1 Kelvin. Molar heat capacity - Energy needed to increase mole temperature of 1 mole of a substance by 1oC or 1 K. Both depend on • physical state (l, s, or g) of the substance • type of substance Boiling Point (BP) – Occurs when vapor pressure equals atmospheric pressure. “Normal” BP recorded at standard recorded atmospheric pressure of 1 atm. = 760 mmHg = 760 torr Freezing Point (FP) of the liquid – equals melting of melting point (MP)of the solid. “Normal” MP recorded at standard atmospheric pressure of 1 atm. = 760 mmHg = 760 torr. torr. Page 3 Chem 210 / WANG / Chapter 11B Liquids, Phase Changes and Phase Diagram Vapor Pressure At At any temperature, a portion of the liquid molecules acquire the minimum minimum kinetic energy needed to escape (vaporize). needed (vaporize). Vapor Pressure is the pressure exerted by the vapor when a liquid and its vapor are in dynamic dynamic equilibrium. Initial condition: the liquid placed in a vacuum chamber Vapor Vapor pressure is measured when liquid and gaseous states are in dynamic equilibrium. • When T↑ , more molecules can escape, vapor vapor pressure ↑ Final condition = “dynamic equilibrium” molecules leave and return to the liquid at the same rate. Ref to Figure 11.23 Vapor Pressure and Boiling Point Vapor pressure ↑ as T ↑ Normal BP is the temperature temperature at which vapor vapor pressure equals standard standard atmospheric pressure = 760 mmHg = 760 torr = 1 atm. Figure 11.24 Pressure cooking is a method of cooking in a sealed vessel that does not permit air or liquids to escape below a preset pressure. Because the boiling point of boiling water increases as the increases pressure pressure increases, the pressure built up inside the cooker allows the liquid in the pot to rise to a higher rise temperature before boiling. boiling. The higher temperature allows food to cook to a greater tenderness in a shorter time. Pressure cooker - The weight on the nozzle next to the handle on the lid regulates the preset pressure. Fall 2009 UM-SJTU JI Atmospheric pressure Boiling Point depends on depends (1) (1) atmospheric pressure & (2) vapor pressure (2) liquid Atmospheric pressure = Vapor pressure liquid Bubbles form Vapor pressure can not sustain the bubble Question on Boiling Point Boiling point of water at various elevations City San Fransisco Salt Lake City Denver La Paz, Bolivia Mount Everest Elevation Sea level 4,390 5,280 12,795 20,028 BP (oC) 100.0 95.6 95.0 91.4 76.5 Why / How does “Pressure cooker” work? (Food cooks fast when using a tightly sealed pot.) “Boiling” !! Melting point (MP) Melting point The temperature at which a solid rapidly and completely rapidly converts into a liquid. Relatively independent of external pressure. • liquids & solids have low compressibility • MP of most solids ↑ slightly when P ↑ • Ice MP ↓ when P ↑ (facilitates ice skating!) Normal MP recorded at 1 atm. atm. What do these data tell about the relationship between between bond type /intermolecular forces and MP MP /BP? Chemical Bond N2 Nonpolar O2 Nonpolar Polar NH3 H 2O Polar NaCl Ionic Mp -210 -219 -78 0 804 Bp -196 -183 -33 100 ? Data in this table show that molecules with nonpolar bonds would melt and boil at a lower temp, consistent with the weaker dispersion weaker force in them. Ionic bond is much stronger, so Ionic is NaCl has exceptionally high mp compared to the others, which only have intermolecular forces. intermolecular Page 4 Chem 210 / WANG / Chapter 11B Liquids, Phase Changes and Phase Diagram Phase Diagram: graph of Vapor Pressure vs. Temperature Vapor Temperature supercritical fluid B liquid solid Normal P melting point Figure 11.26 1 atm 1. Which substance(s) cannot be pressurized into a liquid at room temperature (298 K)? Ar, N2, O2 2. Polar compounds have Polar higher higher critical temperatures and higher critical higher pressures than nonpolar compounds. nonpolar A triple point gas T Phase diagrams are unique to each substance. Point out 3 differences between CO2 and H2O. A state of matter existing beyond the critical T beyond and critical P. • A condition where clear distinctions between “liquid” and “gas” states disappear – The substance behaves as a gas: it takes on gas shape the shape and size of the container. – But its density approaching that of a liquid; density liquid can dissolve other substances as liquid solvents do. • Example: supercritical CO2 is used to extract supercritical – Caffeine to produce “decaffeinated“ coffee or tea. – Dirt from clothing in dry cleaning. Phase diagrams are unique to each substance. Point out 3 differences between CO2 and H2O. CO2 H2O CO2 CO Figure 12.9 H2O Figure 12.9 1) Solid ↔ Liquid border slanted differently. RightRight-slant of CO2 is common for most liquids: mp↑ as P↑ is LeftLeft-slant of H2O is rare: mp↓ as P↑ (b) TP at P > 1.0 atm for CO2; at P ~ 0 atm for H2O at (c) (c) Solid CO2 gas / sublimes at 1 atm, no normal m.p. gas Solid H2O liquid liquid / melts at 1 atm, has normal m.p. m.p. How does Critical Temperature & critical Pressure relate to intermolecular forces? Label each Phase Change exothermic Critical temperature is the highest temperature at highest which a substance can exist as a liquid. sublimation vaporizing melting solid endothermic Critical Critical pressure is the lowest pressure required lowest for the liquid to exist at critical temperature. liquid freezing Fall 2009 UM-SJTU JI Critical point Normal boiling point Triple point: When all three states exist in equilibrium. Critical Critical Point: Beyond which P (critical pressure) & T (critical indistinguishable temperature) liquid and gas are indistinguishable. In a typical phase diagram, the boundary between gas phase and liquid runs from the triple point to the critical point. triple critical Red Red arrow shows how supercritical fluid extraction supercritical works by going around the critical point. critical Green Green arrows are ordinary drying which directly converts water as a liquid to a gas by boiling. Blue Blue arrow shows how freeze freeze drying works by bringing the system around the triple point: 1st triple freezing the material, then reducing reducing pressure and adding just enough heat to allow the frozen water in the material to sublime sublime directly from solid to gas. Supercritical fluid Mark important features of a phase diagram. phase gas condensing Critical Critical point is defined as: the point of critical T and critical P for a substance. The greater the intermolecular force of a intermolecular substance, the more readily a gas is liquefied, the higher the critical temperature Page 5 ...
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This note was uploaded on 07/30/2011 for the course CHEM 210 taught by Professor Zhang during the Spring '09 term at Shanghai Jiao Tong University.

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