Intermolecular forces Intermolecular Forces van der Waals forces Ion-dipole forces Dipole-dipole forces

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1 Intermlecular frces frces that exist between mlecules determines many f the physical prperties f mlecular liquids and slids lead t deviatins frm ideal gas behavir as well Mlecular Cmparisns f Liquids and Slids Liquids IMF are strng enugh t hld mlecules clse tgether Liquids are much denser and far less cmpressible than gases Have a definite vlume Attractive frces are nt strng enugh t keep mlecules frm mving past ne anther, s they can be pured Slids IMF are s strng that mlecules are virtually lcked int place Slids are nt very cmpressible b/c there is little space between them Often mlecules take f psitins in a highly regular pattern CRYSTALLINE SOLIDS Slids are rigid Units f slid vibrate in place b/c they have thermal energy Can change substances frm ne state t anther by heating r cling them This changes the avg. KE f the particles Increasing pressure will frce mlecules clser tgether This increases the strength f the IMF Prpane vs. liquid prpane at RT (increased pressure) Intermlecular Frces Much weaker than inic r cvalent bnds Less E is required t vaprize a liquid r melt a slid than t break cvalent bnds in mlecules Example vaprize HCl requires 16 kj/ml, Break H-Cl bnd requires 431 kj/ml Biling pints and melting pints reflect strengths f intermlecular frces Biling = bubbles f its vapr frm in the liquid Strnger IMF between mlecules = higher BP and MP IMF between neutral mlecules: Diple-diple frces, Lndn dispersin frces, hydrgen bnding Are called van der Waals frces (as a grup) All are less than 15% as strng as a cvalent bnd In-diple frces Between an in and a partial charge n the end f a plar mlecule Psitive ins attract t the negative end f a diple Magnitude f attractin increases as charge f in r magnitude f diple mment increases Imprtant fr slutins f inic substances in plar liquids Diple-diple frces The psitive end f ne diple is near the negative end f anther Only wrks when plar mlecules are clse tgether Mlecules are free t mve with respect t ne anther Smetimes in repulsive rientatin, smetimes attractive Spend mre time in attractive rientatin

2 Fr mlecules f apprximately equal mass and size, strengths f IMF increase with increasing plarity Different diple mments affect strength f IMF Smaller mlecules can get clser tgether, s have higher diplediple frces between them Lndn Dispersin Frces Nnplar mlecules can t have diple-diple interactins Mtin f electrns in an atm r mlecule can create an instantaneus diple mment Instantaneus distributin f electrns f electrns can be different frm the average distributin Because e- repel ne anther, mtins f e- influences mtin f e- n neighbring mlecules The temprary diple n ne mlecule can induce a similar diple n a neighbring atm Significant nly when mlecules are clse tgether Plarizability the ease with which the charge distributin in a mlecule can be distrted by an external electric field Larger mlecules tend t have greater plarizabilites They have a greater # f e- and their e- are farther frm the nuclei Lndn dispersin frces increase with increasing MW Shapes f mlecules influence as well Mre surface area fr attractins t ccur result in strnger attractin Dispersin frces perate between all mlecules Can accunt fr mst f the attractin between plar mlecules like HCl When mlecules have similar weights and shapes, then diplediple interactins will determine which attractin is strnger When mlecules have very different weights, dispersin frces decide mst massive mlecule will have the strngest attractins Hydrgen bnding A special type f IMF between the hydrgen atm in a plar bnd (usually with F, O r N) and an unshared electrn pair n a nearby small electrnegative in r atm (usually an F, O r N atm n anther mlecule) Are unique diple-diple attractins because F, N and O are s electrnegative, the bnd with H is quite plar (H has n inner e-) H has a nearly bare p+ that can attract the negative charge f the EN atm in a nearby mlecule H is als very small, s it can apprach the mlecule mre clsely H bnds are weaker than rdinary chemical bnds, but are strnger than ther IMF Stabilize structures f prteins and DNA Respnsible fr the lw density f ice When water freezes, the mlecules assume a very rdered arrangement t ptimize H bnding arrangements between mlecules Lwers the density f ice (ccupies mre vlume)

3 Cmparing IMF Dispersin frces fund in all substances Strength increases with increasing MW Diple-diple frces add t effect f DF and are fund in plar mlecules Hydrgen bnds add t effect f DF and are the strngest type f IMF N IMF is as strng as rdinary inic r cvalent bnds Nice summary chart pcpy fr students Sme prperties f liquids Viscsity the resistance f a liquid t flw Greater viscsity, the mre slwly it flws Unit = pise (P) = 1 g/cms Reprted in centipises (cp) mst ften Depends n attractive frces between mlecules and structural features Increases with MW Decreases with increasing temperature Surface tensin Mlecules at surface experience a net inward frce (pulled inward) Reduces surface area (such as in a sphere) Mlecules at surface behave like a skin Measured in energy required t increase the surface area f a liquid by a unit amunt (J/m 2 ) Chesive frces IMF that bind similar mlecules t ne anther Adhesive frces IMF that bind a substance t a surface Meniscus frms b/c f this! IMF between water and glass is greater Mercury meniscus pints up! Capillary actin the rise f a liquid up a narrw tube Adhesive frces between liquid and wall increase the SA f the liquid, surface tensin f liquid reduces the area, pulls liquid up tube Helps plants get water and nutrients

4 Phase Changes Changes f state fr a chemical (frm slid t liquid t gas, r vice versa) Every phase change is accmpanied by a change in the energy f the system As temperature f slid increases, units f slid vibrate with increasingly energetic mtin When it melts, they are freed t mve with respect t ne anther (their average separatins increase) Melting prcess is called FUSION Heat f fusin (enthalpy f fusin) - H fus the enthalpy change required t melt a slid As temperature f liquid phase increases, the mlecules f the liquid mve abut with increasing energy Cncentratin f gas mlecules ver the liquid increases with temperature These mlecules exert vapr pressure (increases with increasing temperature) Once vapr increases t external pressure, the liquid bils The mlecules f the liquid mve int the gaseus state when they are widely separated Heat f vaprizatin - H vap the energy required t cause the transitin frm the liquid state t gaseus state (the enthalpy change fr vaprizing a liquid) H vap values tend t be larger because in the transitin frm liquid t vapr state, the mlecules must essentially sever all f their intermlecular attractive frces H sub the sum f Hfus and Hvap Practical applicatins heat f fusin f ice cls the liquid in which the ice is immersed heat f vaprizatin is drawn frm ur bdies as the water evaprates frm ur skin (sweat/stepping ut f a pl) refrigeratr has an enclsed gas that can be liquefied under pressure, gas absrbs heat as it expands t a chamber where it is evaprated, and this cls the interir f the refrigeratr, then vapr is recycled thrugh a cmpressr what abut when the vapr is cndensed? Heat f cndensatin is equal in magnitude but with ppsite sign frm the heat f vaprizatin The heat is dissipated thrugh cling cils in the back f the refrigeratr Heat f cndensatin ppsite f heat f vaprizatin (exthermic) Heat f freezing ppsite f heat f fusin (exthermic) Heat f depsitin ppsite f heat f sublimatin (exthermic) MELTING/VAPORIZING/SUBLIMING = ENDOTHERMIC FREEZING/CONDENSING/DEPOSITING = EXOTHERMIC

5 Heating curve a graph f the temperature f the system versus the amunt f head added heat ice frm -25 C t 0 C heat added, temperature changes melt ice heat added, n temperature change heat water frm 0 C t 100 C heat added, temperature changes vaprize water heat added, n temperature change heat steam frm 100 C t 125 C heat added, temperature changes Fr heating a single phase frm ne temperature t anther, use q = mct *specific heat f water is greater than that f ice T cnvert frm ne phase t anther, use H fus r H vap Specific heats fr H 2 O Ice = 2.09 J/g C Water = 4.18 J/g C Steam = 1.84 J/g C Phase change cnstants fr water H fus = 6.01 kj/ml H vap = kj/ml Critical Temperature and Pressure teach IMF first A gas liquefies at sme pint when pressure is applied t it If we increase the pressure n water vapr at 55 C, then it liquefies when the pressure equals 118 trr, and an equilibrium between the gaseus and liquid phases exists If the temperature is 110 C, then the liquid phase des nt frm until the pressure is 1075 trr. At 374 C, the liquid phase will nly frm at x 10 5 trr (215.7 atm) Critical temperature the highest temperature at which a distinct liquid phase can frm Critical pressure the pressure required t bring abut liquefactin at this critical temperature Nnplar, lw mlecular weight substances have lwer critical temperatures and pressures than plar r heavier substances The transitin frm gaseus t liquid state is determined by IMF Fr every gas, a temperature can be reached at which the mtinal energies f the mlecules are sufficient t vercme the attractive frces that lead t the liquid state, regardless f increasing pressure Water and ammnia have high critical temperatures and pressures, due t strng hydrgen bnding frces Imprtant t engineers/gas wrkers, b/c they give infrmatin abut cnditins at which gases liquefy Smetimes we want this, ther times we want t avid this O 2 is harder t liquefy than ammnia

6 Vapr Pressure The pressure exerted by a vapr in equilibrium with its liquid r slid phase In a clsed cntainer, sme liquid will begin t evaprate int the gaseus phase After a shrt time, the pressure will attain a cnstant value At any instant, the mlecules n the surface f the liquid pssess enugh KE t vercme attractive frces and escape int the gas phase At any temperature, mvement frm liquid t gas phase ccurs cntinuusly As number f gas-phase mlecules increases, the prbability increases that a mlecule in the gas phase will strike the surface f the liquid and be recaptured Eventually the rate at which mlecules return t the liquid will exactly equal the rate at which they escape The number f mlecules will reach a steady value, and the pressure f the vapr at this stage becmes cnstant Called a dynamic equilibrium evapratin and cndensatin ccur at equal rates Appears that nthing is happening! A great deal is happening Vlatile liquids that evaprate readily In an pen cntainer, vapr spreads away frm the liquid, and there is little chance t recapture it Equilibrium never ccurs, and the vapr cntinues t frm until the liquid cmpletely evaprate Substances with high vapr pressure evaprate mre quickly and are called vlatile liquids Nrmal biling pint the biling pint f a liquid at 1 atm pressure Bubbles f vapr frm in the interir f the liquid Temperature f biling increases with increasing temperature Maximum temperature f cking fd is biling pint f water Pressure cker causes water t bil at a higher temperature, fd cks faster High altitudes water bils at a lwer temperature, s much ck fd fr lnger Clausius-Clapeyrn Equatin ln P = -H vap + C RT Graph ln P vs. 1/T, and H vap = -slpe x R

7 Phase Diagrams Phase diagram a graphic way t summarize the cnditins under which equilibria exist between the different states f matter 2D graph with pressure and temperature as the axes Has 3 curves each represents the cnditins f temperature and pressure at which the phases can exist at equilbrium T-C = vapr-pressure curve f the liquid (eq. between liquid and gas phases) Nrmal biling pint = T at 1 atm vapr pressure Ends at critical pint (at critical pressure and temperature f substance) Beynd this pint liquid and gas phases are indistinguishable, called a supercritical fluid Slid-gas separatr = change in vapr pressure f slid as it sublimes at different temperatures Slid-liquid separatr = change in melting pint f slid with increasing pressure Melting pint = freezing pint *at 1 atm, this is the nrmal melting pint Slpes right as pressure increases, b/c slid frm is denser than liquid frm Increase in pressure favrs slid phase (mre cmpact Triple pint = the temperature and pressure where all three phases are in equilibrium Regins n graph = areas where that phase is stable H 2 O vs. CO 2 phase diagrams Water is strange melting pint decreases with increasing pressure (liquid frm is mre cmpact than slid!) CO 2 des nt have a nrmal biling pint has a nrmal SUBLIMATION pint makes dry ice a great clant Structures f Slids Crystalline slid atms, ins r mlecules are rdered in well-defined 3D arrangements Flat surfaces r faces that make definite angles with ne anther Examples: pyrite, flurite, amethyst, quartz Amrphus slid particles have n rderly structure Lack well-defined faces and shapes Mixtures f particles that d nt stack tgether well Examples: rubber, glass (melt silicn dixide and cl quickly) Bnding in Slids Mlecular slids atms r mlecules held tgether by intermlecular frces (diple-diple, Lndn dispersin, r hydrgen bnds) Relatively lw melting pints Sft Examples: Ar, H 2 O, CO 2

8 Benzene = symmetrical and planar, s can pack efficiently, s has higher MP than tluene, which has a CH3 n the tp (lw symmetry prevents efficient packing) BUT BP f tluene is higher than benzene, s attractive frces are larger in liquid tluene than benzene (b/c it is heavier) Cvalent-netwrk slids atms held tgether in large netwrks r chains by cvalent bnds Much harder, have higher MP Examples: diamnd, graphite, silicn, germanium, quartz, silicn carbide, brn nitride Graphite gd cnductr f electricity, b/c f delcalized pi electrns ver the layers, sheets are held tgether w/ weak dispersin frces, s they can slide past ne anther (lubricant) Inic slids held tgether by inic bnds Depends n charge f ins higher charge = strnger attractin Can have different crystalline structures Metallic slids simple metals entirely metal atms, packed structures Bnding is t strng t be just Lndn dispersin frces Strength is due t delcalized e- thrugh slid (why they can cnduct heat and electricity) Packing f spheres Metallic slids In general, strength f bnd increases as # f e- available fr bnding increases Best way t maximize attractive frces between spheres each sphere is surrunded by 6 thers in the layer A secnd layer is added abve the first in the depressins Third layer n depressins in secnd Hexagnal clse packing - Spheres in 3 rd are in line with thse f first ABAB Cubic clse packing Spheres in 3 rd are nt abve 1 st. 4 th is abve 1 st ABCA Each sphere has 12 equidistant nearest neighbrs called crdinatin number f 12 Spheres ccupy 74% f space, 26% empty space in bth types f clsepacking Other pssibilities are large anin/small catin taking up the spaces