4.13BondingbetweenMoleculesIntermolecularforces:the molecules ofcompounds are attracted to each other byforces whicharealways present but are much weaker than those which connect the atoms in covalent bonds. The larger themass of themolecules, thegreaterarethose intermolecularforces.Boiling points: CH4<SiH4<GeH4<SnH4HydrogenBond:Thecompounds HF,HOand NH,all contain molecules withverypolarH-F,H-OandH-N bonds.Furthermore,theF,O,and N atoms in thesebonds all have one or more nonbonding electronpairs.The positive Hend ofa bond in one ofthese molecules canfom a bridge to the F,OandN atom ofa neighboringmolecule.Thisbridgeiscalledahydrogenbond.Hydrogen bonding among H2O, NHs and HF moleculesHHCH?Strengthof Hbonds comparedwithtypical ionicand covalentbondsBondStrength,kcal/bond30 x 10-23lonic13 x 10-23Covalent1 x 10-23HydrogenAssignment21)Writeareciperegardingthecookingprocessofoneofyourfavoritedishes.15
15 4.13 Bonding between Molecules Intermolecular forces: the molecules of compounds are attracted to each other by forces which are always present but are much weaker than those which connect the atoms in covalent bonds. The larger the mass of the molecules, the greater are those intermolecular forces. Boiling points: CH4 < SiH4 < GeH4 < SnH4. Hydrogen Bond: The compounds HF, H2O and NH3 all contain molecules with very polar H-F, H-O and H-N bonds. Furthermore, the F, O, and N atoms in these bonds all have one or more nonbonding electron pairs. The positive H end of a bond in one of these molecules can form a bridge to the F, O and N atom of a neighboring molecule. This bridge is called a hydrogen bond. Hydrogen bonding among H2O, NH3 and HF molecules F H F H O H O H H H N H H H N H H H Strength of H bonds compared with typical ionic and covalent bonds Bond Strength, kcal/bond Ionic 30 x 10-23 Covalent 13 x 10-23 Hydrogen 1 x 10-23 Assignment 2 1) Write a recipe regarding the cooking process of one of your favorite dishes
Chapter 5Gases and Atmosphere5.1 IntroductionMany important substances exist normally as gases at room temperature and sea-levelpressure, including life-sustaining O2, as well as N2, F2, Clz, H, and the noble gases He, Ne, Ar,Kr, Xe, and Rn.A large number of low-molecular-weight covalently bonded compounds are gases,including carbon dioxide (CO2),a waste product of animal metabolism (代谢), nitrousoxide(N2O), used as a general anesthetic (麻醉剂), methane(CH4), a major component of naturalgas, and a variety of others.Except those which are naturally gaseous, gases can also be produced when liquidsevaporate to become gases. Such gaseous substances, which are liquids under normal conditions,are called vapors.5.2Kinetic-Molecular Theory of GasesFirst we present some of the important assumptions of the theory, showing how they fitwiththeknownbehaviorofgases1) Gas molecules are far apartand so the forces of attraction and repulsion are negligibleThus gases can be easily compressed, since there is enough distance between theirmolecules to move them closer together.2) Gas molecules are in constant, rapid motion. The movement of gas molecules causescollisions with the walls of their container, giving rise to gas pressure.3) The speed with which gas molecules move depends upon their temperature. A gasconfinedinarigidcontainerexertsmorepressureasthetemperaturegoesupbecauseitsmoleculesmovefasterandthuscollidemorefrequentlywitheachotherandthecontainerwalls. Hence, heating an aerosol can may cause it to explode. A gas confined in a flexiblecontainer (such as a balloon) will increase in volume if the temperature increases becauseitsmolecules movefasterand hitthewall withgreaterforce,causingthemtoexpandThus we see that the kinetic-molecular theory of gases explains many familiar properties ofgases.Sofar our discussionabout how gasesrespond to changes intemperature,volume,andpressure has been purely qualitative. In order to make quantitative predictions about gasbehaviorwemust presentequationsthat relate the important variablesneeded todescribeasample of gas -- the temperature, the volume, and the pressure.5.3GasPressureThe pressure exerted by a gas is defined as the amount of force per unit area. Gaspressures are often measured in terms of the height of a column of liquid with the gas willsupport. The device used to measure the pressure of a gas in this way is called a barometer. (seein Figure 5.1)Units of pressure:Many different units are used to express gas pressure. One of these isatmosphere (atm), which is approximately equal to the pressure exerted by air in the areas16
16 Chapter 5 Gases and Atmosphere 5.1 Introduction Many important substances exist normally as gases at room temperature and sea-level pressure, including life-sustaining O2, as well as N2, F2, Cl2, H2 and the noble gases He, Ne, Ar, Kr, Xe, and Rn. A large number of low-molecular-weight covalently bonded compounds are gases, including carbon dioxide (CO2), a waste product of animal metabolism (代谢), nitrous oxide(N2O), used as a general anesthetic (麻醉剂), methane(CH4), a major component of natural gas, and a variety of others. Except those which are naturally gaseous, gases can also be produced when liquids evaporate to become gases. Such gaseous substances, which are liquids under normal conditions, are called vapors. 5.2 Kinetic-Molecular Theory of Gases First we present some of the important assumptions of the theory, showing how they fit with the known behavior of gases. 1) Gas molecules are far apart and so the forces of attraction and repulsion are negligible. Thus gases can be easily compressed, since there is enough distance between their molecules to move them closer together. 2) Gas molecules are in constant, rapid motion. The movement of gas molecules causes collisions with the walls of their container, giving rise to gas pressure. 3) The speed with which gas molecules move depends upon their temperature. A gas confined in a rigid container exerts more pressure as the temperature goes up because its molecules move faster and thus collide more frequently with each other and the container walls. Hence, heating an aerosol can may cause it to explode. A gas confined in a flexible container (such as a balloon) will increase in volume if the temperature increases because its molecules move faster and hit the wall with greater force, causing them to expand. Thus we see that the kinetic-molecular theory of gases explains many familiar properties of gases. So far our discussion about how gases respond to changes in temperature, volume, and pressure has been purely qualitative. In order to make quantitative predictions about gas behavior we must present equations that relate the important variables needed to describe a sample of gas - the temperature, the volume, and the pressure. 5.3 Gas Pressure The pressure exerted by a gas is defined as the amount of force per unit area. Gas pressures are often measured in terms of the height of a column of liquid with the gas will support. The device used to measure the pressure of a gas in this way is called a barometer. (see in Figure 5.1) Units of pressure: Many different units are used to express gas pressure. One of these is atmosphere (atm), which is approximately equal to the pressure exerted by air in the areas
around sea level and is exactly equal to 760mmHg.The unit millimeter of mercury (mmHg) isalso referred to as the torr.In the SI system the unit of pressure is called the pascal (Pa),1 Pa = 1 kg s-2ml=1 N m-1.1 atm = 760 mmHg = 101 kPaPatmP=0Barometer:TheApparatustoDeterminePressureofGas(a) The pressure of the atmosphere Pam is exerted onthe mercury in the dish and on the mercury in theHg?PatrDtube.Thus themercurydoes notrise.(b)the760mmpressure of the atmosphere is exerted on the mercuryin thedish, but nopressure is exerted on themercuryin the evacuated tube. Thus the mercury rises in thetube.Theheight of themercurv is a measure of the(a)(b)5.4Boyle's LawAs a fixed sample of gas is compressed to a smaller volume at constant temperature, itspressure increases. This happens because forcing the same number of molecules into a smallervolume makes for more collisions withtheir container, thus exerting more pressureRobert Boyle (1627-1691) turned this qualitative observation into a gas law by compressingand expanding a gas and recording the pressure that correspond ing to each volume.Boyle's law says that at constant temperature, the volume of a gas is inversely proportionalto pressure. That isPV= constant.5.5Charles'LawMore than 100 years after Boyle's discovery, Jacques Charles (1746-1823) found that thevolume of a gas divided by its absolute (Kelvin) temperature remained constant:V,/T/=V2/T2=-V3/T3ands0onor, in general,V/T=constantCharles'Law: At constant pressure, the volume of a gas is directly proportional to itstemperature.5.6Combined Gas LawAnothervery useful relationship comes from thecombination of Boyle's and Charles'laws.According to the combined gas law, the pressure times the volume of fixed sample of gasdivided by its absolute temperature is constant.P,V/T,=P2V2/T2=P,V3/T3and so onor, in general, PV/T =constant.17
17 around sea level and is exactly equal to 760mmHg. The unit millimeter of mercury (mmHg) is also referred to as the torr. In the SI system the unit of pressure is called the pascal (Pa), 1 Pa = 1 kg s-2m-1 = 1 N m-1. 1 atm = 760 mmHg = 101 kPa 5.4 Boyle’s Law As a fixed sample of gas is compressed to a smaller volume at constant temperature, its pressure increases. This happens because forcing the same number of molecules into a smaller volume makes for more collisions with their container, thus exerting more pressure. Robert Boyle (1627-1691) turned this qualitative observation into a gas law by compressing and expanding a gas and recording the pressure that corresponding to each volume. Boyle’s law says that at constant temperature, the volume of a gas is inversely proportional to pressure. That is PV = constant. 5.5 Charles’ Law More than 100 years after Boyle’s discovery, Jacques Charles (1746-1823) found that the volume of a gas divided by its absolute (Kelvin) temperature remained constant: V1/T1 = V2/T2 =V3/T3 and so on or, in general, V/T =constant. Charles’Law: At constant pressure, the volume of a gas is directly proportional to its temperature. 5.6 Combined Gas Law Another very useful relationship comes from the combination of Boyle’s and Charles’ laws. According to the combined gas law, the pressure times the volume of fixed sample of gas divided by its absolute temperature is constant: P1V1/T1 = P2V2/T2 =P3V3/T3 and so on or, in general, PV/T =constant. 760mm (a) (b) Hg Patm Patm Patm P=0 Barometer: The Apparatus to Determine Pressure of Gas • (a) The pressure of the atmosphere Patm is exerted on the mercury in the dish and on the mercury in the tube. Thus the mercury does not rise. (b) the pressure of the atmosphere is exerted on the mercury in the dish, but no pressure is exerted on the mercury in the evacuated tube. Thus the mercury rises in the tube. The height of the mercury is a measure of the atmospheric pressure, which is 760mmHg in this diagram
We can use the combined gas law equation to make calculations about gas samples in whichthe pressure, temperature, and volume are undergoing change.5.7Ideal GasLawBy measuring the pressure, volume, and temperature of a given amount of gas in thelaboratory,we can determine a value for the constant in thecombined gas law equation.When the quantity of gas used is exactly I mol, the constant is called the ideal gas constantR:PV/T=R (for 1 mol)For 2 mol of gas R is doubled, for 3 mol it is tripled, and so on. If n is used to represent thenumber of moles of gas, the combined gas law equation can be writtenPV/T =nR(fornmol)Thisequation is called theidealgas law.5.8Dalton's Law of Partial PressuresMany experiments done on gases involve mixtures of gases than pure gaseous substancesTherefore, we need some way to relate the pressure exerted by the components of a mixture tothe pressure exerted by themixture as a wholeTo find the pressure exerted by by a gaseous mixture Dalton's partial pressures is applied.The partial pressure of a gas is the pressure which that gas would exert if it occupied a containeby itself.According to Dalton's Law, the total pressure P of a mixture of gases is the sum of the partialpressures p of each component gases.Ptotal=pi + p2 + p3 and so on.We can show that the partial pressure of a gas is directly related to the number of moldes ofthat gas present in a gaseous mixture.P=n RT/V= n ×constant.p1/p2 =ni/n218
18 We can use the combined gas law equation to make calculations about gas samples in which the pressure, temperature, and volume are undergoing change. 5.7 Ideal Gas Law By measuring the pressure, volume, and temperature of a given amount of gas in the laboratory, we can determine a value for the constant in the combined gas law equation. When the quantity of gas used is exactly 1 mol, the constant is called the ideal gas constant R: PV/T = R (for 1 mol) For 2 mol of gas R is doubled, for 3 mol it is tripled, and so on. If n is used to represent the number of moles of gas, the combined gas law equation can be written PV/T = n R (for n mol) This equation is called the ideal gas law. 5.8 Dalton’s Law of Partial Pressures Many experiments done on gases involve mixtures of gases than pure gaseous substances. Therefore, we need some way to relate the pressure exerted by the components of a mixture to the pressure exerted by the mixture as a whole. To find the pressure exerted by by a gaseous mixture Dalton’s partial pressures is applied. The partial pressure of a gas is the pressure which that gas would exert if it occupied a container by itself. According to Dalton’s Law , the total pressure P of a mixture of gases is the sum of the partial pressures p of each component gases. Ptotal = p1 + p2 + p3 and so on. We can show that the partial pressure of a gas is directly related to the number of moldes of that gas present in a gaseous mixture. P = n RT/V = n constant. p1 / p2 = n1/n2
Chapter 6Liguids and Solids6.1IntroductionExcept for air, most of the substances that you encountered are in a liquid or solid state.Themathematical treatment of solids and liquids, unlike that of gases, is complicated by the fact thattheir molecules (or ions or atoms) are close together and thus the forces of attraction orrepulsionamongthemcannotbeignored.Because of the order in solids, it is possible to evaluate the forces operating among theirconstituent particles. The ions, atoms, or molecules of solids are present in a regular, unchangingarrangement.The study of liquids is complicated by the fact that they do not have the order present insolids,and although theirmolecules arein constant motion,themotion is not random as it is ingas. Thus theories to describe the behavior of liquids are particularly difficult to develop.6.2VaporPressureThe term Vapor is used instead of gas to refer to the gaseous state of substances which arenot gases under ordinary conditions.Liquids produce vapor when energetic molecules at theliquid surface escape into the gas phas. This is evaporation (or vaporization).The vapor pressure of a liquid is the pressure exerted by the gas molecules above a liquid,molecules that are produced from evaporation of the liquid. Liquids that evaporate easily haverelativelyhighvaporpressures and are said tobevolatile.It is also possible for solids to exert vapor pressures when solid molecules become energeticenough to escape from the solid phase to the gas phase. This process is called sublimation.The vapor pressure of a solid can be used predict whether of not it could have a detectableodor.Forsolidstohaveodorstheymustbevolatile.6.3DistillationDistillation is the process of first vaporizing and then condensing (liquefying) the liquidcomponents of a mixture to separate them from each other or from the solid components whichmaybepresent.It is possible to separate or to partially separate two or more liquid components from eachother provided that the boiling points of the two substances are sufficiently far apart.6.4Properties of liquidsLiquid Pressure:Liquids exert pressure equally in all directions in the same way that gasesdo. The pressure exerted at any particular point in a container of liquid depends upon the heightof the liquid above that point: the greater the height of the liquid, the greater the pressure.Viscosityis the resistance of liquids to flow. In general, viscosity depends upon the densityof the liquid and the strength of its intermolecular attractions: the higher these are, the moredifficult it is for molecules tomove over oneanother, thereby increasing the resistance toflowand hence the the viscosity.Surfacetension isa characteristic property of liquids whichcanbe observed wheneverliquids are in contact with a gas. The molecules at the very surface of the liquid are attracted to19
19 Chapter 6 Liquids and Solids 6.1 Introduction Except for air, most of the substances that you encountered are in a liquid or solid state. The mathematical treatment of solids and liquids, unlike that of gases, is complicated by the fact that their molecules (or ions or atoms) are close together and thus the forces of attraction or repulsion among them cannot be ignored. Because of the order in solids, it is possible to evaluate the forces operating among their constituent particles. The ions, atoms, or molecules of solids are present in a regular, unchanging arrangement. The study of liquids is complicated by the fact that they do not have the order present in solids, and although their molecules are in constant motion, the motion is not random as it is in gas. Thus theories to describe the behavior of liquids are particularly difficult to develop. 6.2 Vapor Pressure The term Vapor is used instead of gas to refer to the gaseous state of substances which are not gases under ordinary conditions. Liquids produce vapor when energetic molecules at the liquid surface escape into the gas phas. This is evaporation (or vaporization). The vapor pressure of a liquid is the pressure exerted by the gas molecules above a liquid, molecules that are produced from evaporation of the liquid. Liquids that evaporate easily have relatively high vapor pressures and are said to be volatile. It is also possible for solids to exert vapor pressures when solid molecules become energetic enough to escape from the solid phase to the gas phase. This process is called sublimation. The vapor pressure of a solid can be used predict whether of not it could have a detectable odor. For solids to have odors they must be volatile. 6.3 Distillation Distillation is the process of first vaporizing and then condensing (liquefying) the liquid components of a mixture to separate them from each other or from the solid components which may be present. It is possible to separate or to partially separate two or more liquid components from each other provided that the boiling points of the two substances are sufficiently far apart. 6.4 Properties of liquids Liquid Pressure: Liquids exert pressure equally in all directions in the same way that gases do. The pressure exerted at any particular point in a container of liquid depends upon the height of the liquid above that point: the greater the height of the liquid, the greater the pressure. Viscosity is the resistance of liquids to flow. In general, viscosity depends upon the density of the liquid and the strength of its intermolecular attractions: the higher these are, the more difficult it is for molecules to move over one another, thereby increasing the resistance to flow and hence the the viscosity. Surface tension is a characteristic property of liquids which can be observed whenever liquids are in contact with a gas. The molecules at the very surface of the liquid are attracted to