Principles of Chemistry I
Principles of Chemistry I CHEM 1307
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This 31 page Class Notes was uploaded by Kara Dibbert on Thursday October 22, 2015. The Class Notes belongs to CHEM 1307 at Texas Tech University taught by Tamara Hanna in Fall. Since its upload, it has received 40 views. For similar materials see /class/226511/chem-1307-texas-tech-university in Chemistry at Texas Tech University.
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Date Created: 10/22/15
Intermolecular Forces amp Liquids Nitrogen gas Liquid benzene Solid benzene Liquid nitrogen Intermolecular Forces Intramolecular Forces The forces holding atoms together to form molecules Intermolecular Forces The forces between molecules between ions or between molecules and ions Learning about intermolecular forces will help us to explain why ice floats on water why 12 is a solid while Cl2 is a gas why water boils at such a high temperature why your body is cooled when you sweat Intermolecular Forces Iondipole Forces The force of attraction between a positive or negative ion and a polar molecule When a polar molecule is mixed with an ionic compound forces of attraction occur Let s consider what happens when NaCI is placed into water Water is a polar molecule which breaks apart the salt into ions amp surrounds the ions where positive and negative charges attract 9 g 99 9 a d Water surrounding Water Surrounding a cation an amon Intermolecular Forces The formation of hydrated ions is one of the most important iondipole interactions An enthalpy change is associated with the solvation or hydration of ions The magnitude of the enthalpy depends on the distance between the center of the ion and the dipole 5 5 5 5 Li Mgz i J 5 5 d d 78 p d Lir prn Mgzr79 m Ki I r 133 pm AH 7515 kJmol AH 71922 kJmoi AH 321kClmol Increasing force of attraction more exothermic enthalpy of hydration As d increases the enthalpy becomes less exothermic Intermolecular Forces Iondipole forces are the strongest intermolecular force between molecules whether polar or nonpolar Attractive Forces Depends On 1 The distance between the ion and the dipole The closer the ion and dipole the stronger the attraction 2 The charge on the ion The higher the charge the stronger the attraction 3 The magnitude of the dipole The greater the magnitude the stronger the attraction Intermolecular Forces Dipoledipole Forces The force of attraction between opposite poles of two interacting polar molecules 5 5 Intermolecular Forces Let s consider the energy required to vaporize a liquid TABLE 122 Molar Masses Boiling Points and AVBPHquot of Nonpolar and Polar Substances Nonpolar Polar M BF AVZPH M BP AVEPHquot gmot c knmot Elquot101 0c knmot N2 28 i196 557 CD 28 i192 604 51H 32 11Z 1210 PH 34 88 1406 39H 77 90 1406 ASH 78 62 1669 Brg 160 59 2996 ICl 162 97 7 N2 and C0 are nearly the same size and have the same mass However CO is polar while N2 is nonpolar Due to dipoledipole interactions it requires more energy to vaporize CO than N2 Intermolecular Forces Boiling Point When the bp is reached molecules have enough energy to overcome the force of attraction of neighboring molecules For molecules with similar masses the greater the polarity the higher the boiling point Nonpolar Polar M BP AvapHquot M BP AVEPH gmol C kJmol gmol C kJmol Brz 160 59 2996 ICl 162 97 Solubility Like dissolves likequot so polar molecules are likely to dissolve other polar molecules and nonpolar molecules dissolve other nonpolar molecules Intermolecular Forces Hydrogen Bonding An unusually strong dipoledipole attraction between a hydrogen atom on one molecule and an electronegative atom on another molecule usually N O or F X H Y H The hydrogen atom acts as a bridge between two electronegative atoms and a dashed line is used to represent the hydrogen bond Types of Hydrogen Bonds X H Y N 0 HN F HN N H N H0 0 H0 F H0 N H F 0 HF F HF Hydrogen Bonding in Water Water behaves differently than any other substance Due to the highly polar OH bond in water added to the two lone pairs of electrons on the oxygen atom each water molecule can hydrogen bond with 4 other molecules This leads to a tetrahedral arrangement of hydrogen atoms around each oxygen 7 our Two hydrogen atoms are attached H J 1 V covalently while two hydrogen atoms are hydrogen bonded Hydrogen Bonding in Water The regular arrangement of water molecules leads ice to have a structure with lots of empty space which explains its lower density than liquid water which is why ice floats Hydrogen bonding also explains why water has such a high heat capacity V 39 When ice melts the hydrogen bonds break down and a large increase in density occurs Temperature C Hydrogen Bonding in Water Due to hydrogen bonding water has a much higher boiling point than expected 100 Intermolecular Forces DipoleInduced Dipole Forces Polar molecules like water create a dipole in molecules that are nonpolar This explains how nonpolar O2 is soluble in water The dipole of water induces a dipole in 02 by distorting a the 02 electron cloud Polarizability The degree of electron cloud distortion in a molecule Larger molecules like I2 with large electron clouds can distort more readily than molecules like He which hold its valence electrons very close to the nucleus Intermolecular Forces The higher the molar mass the more polarizable The more polarizable the stronger the dipoleinduced dipole interaction The stronger the interaction the more soluble TABLE 123 The Solubility of Some Gases in Water Molar Mass Solubility at 20 C gmol g gas100 g waterT Hz 201 0000160 N2 280 000190 02 320 000434 Intermolecular Forces Induced DipoleInduced Dipole Forces The formation of a dipole in two nonpolar molecules The ONLY intermolecular forces that allow nonpolar molecules to interact Electrons are in a constant state of motion and as nonpolar molecules approach each other attractions and repulsions can distort their electron clouds leading to intermolecular attraction i s g v i A lja39 39 39 4 Induced dipoleinduced dipole forces also called London dispersion forces are weak but exist in all covalent molecules Summary of Intermolecular Forces Intermolecular forces involve molecules that are polar or those where polarity can be induced TABLE 125 Summary of Intermolecular Forces Approximate Type of Interaction Factors Responsible for inlerarrierr Energy klmo Example Icnadipole Idn change magnitude eidrpple 40 500 Na H20 Dipoledipole Dipole moment depends an atom elerlronegaliriries and molecular 2020 H20 mow slruclrrre Hydrogen bonding Very polar xiii bond wirere x F N o and atom v mlir one pair seam H20 H20 X H V of Electrons Dipoleinduced dipule Dipole moment of polar molecule and polarizability of nonpolar 2710 H10 lZ molecule Induced dipoleinduced dipole Polarizability 005740 12 lz londun dispersion forces You should be able to predict what intermolecular forces occur between different molecules and which are the most important Summary of Intermolecular Forces Decide which intermolecular forces occur in each molecule and then rank the strength of the interactions methane CH4 induced dipoleinduced dipole water and CH3OH dipoledipole hydrogen bonding induced dipoleinduced dipole 8 H a 5 5 5H 5 6 5i OH O and OH 0 6quot H I H c I water and Br2 dipoleinduced dipole CH4 lt H20 and Br2 lt H20 and CH3OH Liquids Liquids are the most difficult state of matter to describe o a O a 0 39 0000 5 g 0 cocoonco q 00000000 0 0 W 393 0320375 mquot o a 39 ocean 003 0 a I 000II0000 39939 o s 00000000 quot quot Solid Liquid Gas Molecules are in constant motion There are appreciable intermolecular forces Molecules are close together Liquids are essentially noncompressible Liquids do not fill their container Vaporization The process of changing a substance from the liquid to gas phase Molecules in a liquid have a range of energies where the average energy depends only on the temperature Higher temperature leads to higher kinetic energy which leads to more molecules that can overcome intermolecular forces and enter the gas phase T2 gt T1 D Number of molecules having enough energy to evaporate at lower temperature T1 gt Rebtive number of molecules m Number of molecules having enough energy I to evaporate at higher temperature T2 Energy gt Vaporization Vaporization is an endothermic process because E must be added to overcome the intermolecular forces holding molecules together If molecules lose enough E and come in contact with the surface of a liquid condensation occurs Condensation is an exothermic process where the enthalpy change is equal but opposite in sign to AHvap AH vap VAPOR addE 0 0 Z 0 o o 0 release E O AIcond 39AHvap Q mpnund Nonpolar ampaundi cm methane mi butane Va porization Boiling points and AHyap increase with increasing molar mass Maiar Mass gmol Amm Boiling Point l Vayor idmom pressure 760 mm Hg 82 161 5 157 is 5 190 742 1 224 70 5 When looking at the hydrogen halide series HF has higher values than expected due to extensive hydrogen bonding Compound Polar unwound HF HCl HBr HI Malar Mass SJquotWU AWH Boiling Point c Vapor lamam pres m Hg sure 760 m 252 197 162 7848 193 664 198 455 Vapor Pressure The pressure of the vapor of a substance in contact with its liquid or solid phase in a sealed container A measure of the ability of molecules to escape from the liquid phase and enter the gas phase at a given temperature called volatility Molecules move continuously from the liquid e 0 phase to the vapor phase so there is no net a change in the masses of the two phases Equilibrium vapor pressure is the pressure exerted by the vapor in o 5 3 equilibrium with the liquid phase Vapor Pressure A plot of pressure versus temperature show all the conditions where the liquid and gas are in equilibrium The vapor pressure increases with increasing temperature The liquid boils when vapor pressure equals 1 atm normal bp 1000 NormalBP NormalBP NormalBP 783 DC 10 C 3 i 600 I M I J L Diethyt J E J Ethanol i H 0 t l 800 760 mm Hg 346 0C ether 400 Pressure mm Hg 20quot 0 20 40 60 80 100 120 Temperature C Vapor Pressure The boiling points of liquids change at different altitudes If you live high above sea level the barometric pressure is lower than 1 atm so water will boil at a lower temperature Torr Celsius Vapor Pressure Practice Time The vapor pressure of diethyl ether is 400 mm Hg at 180 C What is the temperature at which the vapor pressure of heptane is 400 mm Hg 77 C Vapor pressure mm Hg Is the normal bp of diethyl ether higher or lower than heptane lower Are the intermolecular forces of diethyl ether stronger or weaker weaker weaker forces lead to higher vapor pressures in P Pin mm Hg Vapor Pressure Vapor pressure increases nonlinearly with temperature The ClausiusClapeyron Equation relates the natural log of vapor pressure to the reciprocal temperature o 00025 00030 00035 00040 1TK 1 Plotting In P vs lfl39 allows for determination of the enthalpy of vaporization slope vap Molar Heat of Vaporization Using the following vapor pressure data for iodomethane determine the molar heat of vaporization kJmol P mmHg T K 400 249 100 266 Molar Heat of Vaporization The normal bp of liquid methanol is 338 K Assuming that the molar heat of vaporization is 366 kJmol what is the boiling point of methanol when the pressure is 137 atm At the normal boiling point the pressure is 1 atm P1 R T2 T1 In 2 1 In 137 i z 1 z 1 1 000289K1 T2 AHvap T1 366 338 R 0008314 Therefore T2 346 K Surface Tension Molecules in the interior of a liquid interact with molecules all around them while molecules on the surface are only affected by those directly at or below the surface There is a net inward force of attraction which makes the liquid act like it has a skin gim i tw quotif aim is the energy required to break through the surface and disrupt the attractions Capillary Action The movement of liquids in glass tubes Surface tension pulls a liquid up a column until there is a high enough mass of the liquid that gravity overcomes the intermolecular forces and pulls down on the liquid We see this as a meniscus 5 Meniscus Viscosity The resistance of liquids to flow When you pour out a cup of water the water flows readily In contrast a cup of oil takes much longer to empty out Oil has a higher viscosity because the long chains of carbon atoms have greater intermolecular forces between them
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