CHEM 116 Concentrations and Colligative Properties

Transcription

1 UMass stn, Chem 116 CHEM 116 Cncentratins and Clligative Prperties FSG is Mndays 11:00 am Lecture 10 Prf. Sevian and Tuesdays 3:30 pm bth in S-1-89 Tday s agenda Ways f expressing cncentratin Clligative prperties Vlatility Decrease when a slute is present Cmpsitin f vapr when bth cmpnents are vlatile (Rault s law fr ideal slutins) iling pint elevatin Freezing pint depressin Osmtic pressure Henry s law fr gas slubility in a liquid slvent Return Exam #1 and g ver it H. Sevian 1

2 UMass stn, Chem 116 Clicker questin abut slutins In a saline slutin (fr cntact lenses), which f the fllwing crrectly identifies the cmpnents f the slutin? Slute Slvent () Na + and Cl ins Water () Water Na + and Cl ins (C) NaCl (s) Water (D) Water NaCl (s) Knw the difference between mass and mles Three inert gases are placed in the same cntainer: 3.00 g f He, 3.00 g f Ne, and 3.00 g f r. The ttal pressure in the cntainer is 22.2 atm. (a) What is the partial pressure f He in the cntainer? (b) What is the weight % f He? (c) What is the mle fractin f He? 3.00g He mles f He 4.00 g mle fractin f He= = ml as a decimal = ttal mles 3.00g He 3.00g Ne 3.00g r g g g all mixed tgether 3.00 g He 3.00 g Ne 3.00 g r ml ml ml mass f He 3.00g % wt. f He= = as a % = 33.3% ttal mass 9.00g What causes pressure? Cllisins f gas particles with the sides f the cntainer. If the gases are ideal, what des this mean? The identities f the gases d nt matter. What des the partial pressure f He mean? The prtin f the ttal pressure that is caused by He. H. Sevian 2

3 UMass stn, Chem 116 Hw des chemistry explain the ways in which slutins are different than pure slvents? Example: iling pint f a slutin vs. pure slvent In the lab, we find that slutins always bil at higher temperatures than the pure slvent (we call this iling Pint Elevatin) We knw that biling is the physical phenmenn that happens when the vapr pressure f the liquid (slvent) reaches atmspheric pressure Therefre, the cause fr the PE must have t d with the presence f the slute causing a change in the slvent s vapr pressure What is ging n at the particle level that culd be causing this vapr pressure difference? Once we understand the basic mdel, we will discuss nnideal behavir that is, hw the mdel desn t quite fit reality Cmpare biling pints: pure slvent vs. a slutin 1.0 kg pure water T = ºC T = 100 ºC T = 90 ºC T = 80 ºC T = 70 ºC T = 60 ºC T = 50 ºC T = 40 ºC T = 30 ºC T = 20 ºC 1.0 kg pure water ml CuCl 2 ils at ºC ils at ºC ΔT bp = ºC H. Sevian 3

4 UMass stn, Chem 116 What des the biling pint elevatin depend n? T make sense f this, we need t picture what is happening at the particle level Cmparing slutin t slvent at the same temperature: fewer slvent particle in the vapr phase, because slute particles are in the way Think abut vlatility f the slvent pure slvent has a vapr pressure ssume that slute des nt have vapr pressure Particle level mdel f biling pint elevatin Which has lwer vapr pressure at the same temperature? Why? vapr phase surface f liquid Cu 2+ Pure Slvent liquid phase Slutin H. Sevian 4

5 UMass stn, Chem 116 Vapr pressure f slvent ver slutin vs. vapr pressure f pure slvent In this case, benzene is the slvent Red line is the L-G prtin f the phase diagram fr pure benzene (vapr pressure curve) Cmpare curves at the same temperature (60ºC) Slutin vapr pressure is than slvent vapr pressure. Why? Cmpare curves at the same pressure (760 trr) iling pint f slutin is than biling pint f slvent. Why? Frm Chemistry & Chemical Reactivity 5 th editin by Ktz / Treichel. C Reprinted with permissin f rks/cle, a divisin f Thmsn Learning: Fax What des the biling pint elevatin depend n? T make sense f this, we need t picture what is happening at the particle level Cmparing slutin t slvent at the same temperature: fewer slvent particles in the vapr phase, because slute particles are in the way and/r hlding the slvent in the liquid phase ls wuld depend n what slvent it is, because different slvents have different vapr pressures slvent factr bp ( effective mlality) ( slvent factr) i mslutekbp ΔT = = effective mlality H. Sevian 5

6 UMass stn, Chem 116 n example: biling pint elevatin in a slutin Example (frm p. 552) ntifreeze, ethylene glycl, HOCH 2 CH 2 OH, is added t water t raise its biling pint (and als lwer its freezing pint). What wuld be the biling pint temperature f a 25.0 mass % slutin f ethylene glycl in water? Key pints s far abut slutins Clligative prperties are prperties f a slutin cmpared t the pure slvent Clligative prperties are caused by the presence f the slute When slute is present, the biling pint is elevated The equatin t describe this is ΔT bp = (i m slute ) K bp ecause clligative prperties depend n the cncentratin f slute disslved in a slvent, clligative prperties can be used t determine mlecular weights f slutes H. Sevian 6

7 UMass stn, Chem 116 Vapr pressure Vapr pressure f the slvent decreases as mre slute disslves (as cmpared t pure slvent) Rault s law describes the behavir fr ideal behavir p slvent = χ slvent p slvent Simplest versin f Rault s law: nly the slvent is vlatile (i.e., slute is nt vlatile) Rault s law can be extended t situatin where slute is vlatile (e.g., ethanl and water mixed tgether) t a given temperature, hw much lwer is the vapr pressure f a slutin than the pure slvent? Same T Slute has n vapr pressure (nnvlatile) Fractin f surface ccupied by slvent particles is 1.0 Vapr pressure f slvent is P slvent Fractin f surface ccupied by slvent particles is χ slvent Vapr pressure f slvent is χ slvent P slvent H. Sevian 7

8 UMass stn, Chem 116 The mathematics f slutin in pictures One (nnvlatile) slute unit = One slvent unit = mle fractin f slvent = ttal mles f mles f + units units in entire slutin Mle fractin must be a number between 0 and 1. χ = mles f mles f + mles f χ = mles f mles f + mles f ll the mle fractins must sum t 1. χ + χ =1, and generally χi = 1 i n example: vapr pressure in a slutin Practice Exercise, p. 550 The vapr pressure f pure water at 110ºC is 1070 trr. slutin f ethylene glycl and water has a vapr pressure f 1.00 atm at 110ºC. ssuming that Rault s law is beyed, what is the mle fractin f ethylene glycl in the slutin? H. Sevian 8

9 UMass stn, Chem 116 Vapr pressure when tw cmpnents are bth vlatile t 20ºC the vapr pressure f pure benzene is 75 mmhg and the vapr pressure f pure tluene is 22 mmhg. If equal mles f benzene and tluene are mixed and then allwed t achieve equilibrium vapr pressure in a cntainer at 20ºC, hw wuld the mle fractins f the tw cmpnents in the vapr phase cmpare? Pure benzene Vapr pressure 75 mmhg + = Pure tluene Vapr pressure 22 mmhg 50/50 mixture f benzene & tluene What is the cmpsitin f the vapr? Rault s law (ideal behavir) when tw cmpnents are vlatile Cnsider water and ethanl as an example Fr water p = χ p with p = vapr pressure f pure water at that temperature water water water water Fr ethanl (EtOH fr brevity) p = χ p with p = vapr pressure f pure ethanl at that temperature EtOH EtOH EtOH EtOH ut mle fractins must sum t 1, s χ water χ =1 + EtOH Therefre, ethanl vapr pressure can be expressed in terms f water mle fractin p ( ) EtOH = 1 χ water petoh Generalize t tw cmpnents and p p = χ p = with p = vapr pressure f at that temperature ( 1 χ ) p with p = vapr pressure f at that temperature H. Sevian 9

10 UMass stn, Chem 116 Vapr pressure when tw cmpnents are vlatile Ideal behavir predicted by Rault s law Pure water T bil = 100 ºC P water = 289 trr at 75 ºC Pure ethanl T bil = 78 ºC P water = 665 trr at 75 ºC trr Vapr pressures f tw cmpnents sum ethanl Water mle fractin is ppsite ethanl mle fractin Extremes ll water ll ethanl water In between water χ Rault s law applied t tw cmpnents The mixture cntains bth and mlecules. Therefre, there are three kinds f interactins t vercme when mlecules change frm liquid t vapr phase: -, - and -. ssumptin in ideal (pretend) case: intermlecular attractins are predictable - attractins may be different frm - attractins, leading t different pure vapr pressures at the same temperature (which results in the tw cmpnents having different biling pints) - attractins assumed t be the average f - and - interactins When this assumptin is nt true (reality) - attractins are greater than the average f - and - results in mlecules entering vapr phase less easily than Rault s law predicts - attractins are smaller than the average f - and - results in mlecules entering vapr phase mre easily than Rault s law predicts H. Sevian 10

11 UMass stn, Chem 116 Nn-ideal behavir with tw vlatile cmpnents When - attractins are strnger than the average f - and - When - attractins are weaker than the average f - and - Surce f graphs: R. Carter (UMass stn) Summary f equatins fr PE, FPD and Osmtic pressure iling pint elevatin (PE) Δ = i m K T bp slute slvent, bp Freezing pint depressin (FPD) ΔT fp = i mslutekslvent, fp Osmtic pressure ( smtic pressure, π ) V = nrt n π = RT = effective mlarity in V ml π = i mlarity in RT L ml L RT Nte: text bk uses M t represent mlarity, p. 554 H. Sevian 11

12 UMass stn, Chem 116 What is smtic pressure? Prcess f smsis thrugh a semipermeable membrane separating slutins f different cncentratins Osmsis cntinues until dwnward frce exerted by air n slutin equals upward frce exerted by smsis Initially Frce exerted by atmsphere Frce (air) exerted by smsis Frm Chemistry & Chemical Reactivity 5 th editin by Ktz / Treichel. C Reprinted with permissin f rks/cle, a divisin f Thmsn Learning: Fax fter reaching equilibrium Frce exerted by smsis Frce exerted by atmsphere (air) Nte: lengths f arrws here are intended t represent strengths f frces Where smtic pressure cmes frm Slutin n the inside f the bag Pure water n the utside f the bag Frm Chemistry & Chemical Reactivity 5 th editin by Ktz / Treichel. C Reprinted with permissin f rks/cle, a divisin f Thmsn Learning: Fax Net directin f flw f water H. Sevian 12

13 UMass stn, Chem 116 Many (mst) slutes are nt ideal Ideal behavir f slutes (pretend) Inic cmpunds dissciate cmpletely in water; mlecular cmpunds d nt dissciate in water van t Hff factr fr a mlecular slute equals 1 van t Hff factr fr an inic slute equals number f ins that an inic unit theretically breaks int Nn-ideal behavir f slutes (reality) Sme inic cmpunds are nly partially dissciated in water; sme mlecular cmpunds dissciate a tiny bit van t Hff factr fr a mlecular slute can be > 1 van t Hff factr fr an inic slute < number f ins that an inic unit theretically breaks int Ideal vs. Nn-ideal behavir in clligative prperties Ideal behavir Strng electrlytes dissciate cmpletely in water, especially if they are at very lw cncentratins Examples (slutins at very lw cncentratins) NaCl i = 2 K 2 SO 4 i = 3 (NH 3 ) 3 PO 4 i = 4 Nn-ideal behavir Real slutins f inic cmpunds have van t Hff factrs that are nt integers because f in-in interactins while disslved in slutin s the cncentratin f inic slute increases, ins interact mre with each ther, and i- value decreases Examples K 2 SO 4 at m i = 2.84 K 2 SO 4 at m i = 2.69 K 2 SO 4 at m i = 2.32 H. Sevian 13

14 UMass stn, Chem 116 Mre n nn-ideal van t Hff factrs Sme substances that appear (frm their chemical cmpsitin) t be mlecular actually dissciate partially when disslved in water this is mst imprtant fr rganic acids s with inic cmpunds, the higher the cncentratin f the substance, the less dissciated it will be Example: acetic acid reacts with water t a small extent CH 3 COOH (aq) + H 2 O (l) CH 3 COO - (aq) + H 3 O + (aq) s yu will learn t calculate when we get t chapter 16, when the cncentratin f the acid decreases, the % dissciatin increases m CH 3 COOH slutin i = 1.04 (4% dissciated) m CH 3 COOH slutin i = 1.13 (13% dissciated) m CH 3 COOH slutin i = 1.34 (34% dissciated) (The main pint here is that the van t Hff factr fr acetic acid can be greater than 1, but nly at very lw cncentratins.) Hw d pressure f gas and temperature f the slutin affect slubility? Slids and liquids Increasing the gas pressure ver the liquid has n effect n hw much slid r liquid can disslve in a liquid slvent Increasing the temperature usually has the effect f causing greater slubility, but there are sme exceptins Gases that d nt react with a slvent ( ideal situatin) Increasing the gas pressure results in greater slubility f the gas in the liquid slvent Increasing the temperature f the slutin results in lwer slubility f the gas in the liquid slvent Gases that react with slvent (e.g., CO 2 reacts with water) If sme f the gas mlecules react with water, mre mlecules mve int the liquid frm the gas phase, t cmpensate and re-establish equilibrium H. Sevian 14

15 UMass stn, Chem 116 Gases disslved in liquids: Henry s law Hw much gas can disslve in a liquid depends n the partial pressure f the gas abve the liquid gas slubility = k H P gas Maximum cncentratin f gas that can disslve (in mles f gas per liter f slvent) Partial pressure f the gas abve the liquid slvent Henry s law gas cnstant (dependent n strength f slute-slvent attractins) Different gases can disslve t different extents, wing t the diple (water) induced diple (slute) interactins Gas Mlecular mass Henry s law cnstant (k H ) at 25ºC N g/ml M/mmHg O g/ml M/mmHg CO g/ml M/mmHg The mst useful use f clligative prperties: identifying unknwn substances by their mlar mass asic strategy: ll clligative prperties depend n the cncentratin f slute in a slvent Make a slutin and measure a clligative prperty, then calculate cncentratin f slute that must be in the slvent Frm cncentratin f slute, if yu knw hw much slvent there is, yu can figure ut the mles f slute If yu independently knw the mass f slute that was used t create the slutin, then yu can get the mlar mass f the slute mass f slute ( g) M w = mles f slute ( ml) H. Sevian 15

16 UMass stn, Chem 116 Example: Mlecular frmula determinatin by biling pint elevatin Similar t FPD Practice Exercise, p g f azulene are disslved in 99.0 g f benzene. The biling pint f the slutin is ºC. If the empirical frmula f azulene is C 5 H 4, what is its mlecular frmula? The nrmal biling pint f pure benzene is ºC, and the biling pint cnstant, K bp = 2.53 ºC/m. Key pints frm tday Clligative prperties are prperties f a slutin cmpared t the pure slvent Clligative prperties are caused by the presence f the slute When slute is present, the vapr pressure is lwer, biling pint is elevated and freezing pint is depressed (the liquid phase f a slutin is extended at bth ends ver the pure slvent) Real slutins have van t Hff factrs that are larger when less slute is disslved. This is because at lwer cncentratins, disslved + and slute particles cannt pair as easily. ecause clligative prperties depend n the cncentratin f slute disslved in a slvent, clligative prperties can be used t determine mlecular weights f slutes H. Sevian 16

17 UMass stn, Chem 116 Clicker questin abut slutins Which slutin shuld have the highest biling pint? () 1 gram f CaCO 3 (f.w g/ml) in 100 ml f slutin () 2 mles f CaCO 3 in 1 kg f water (C) 2.0 wt% aqueus slutin f CaCO 3 (D) m aqueus slutin f CaCO 3 Only ne ther majr difficulty n Exam 1 besides the tw prblems already addressed in tday s lecture Cmpund #1 #2 #3 Lattice energy (kj/ml) () MgO CaO SrO (+2) (-2) where +2 s are increasingly larger in radius () KCl K 2 S CaS same perid s apprx. same radius, cmparing (+1)(-1) (+1)(-2) (+2)(-2) (C) Lir NaCl KF (+1 in Pd 2)(-1 in Pd 4). (+1 in Pd 3)(-1 in Pd 3). (+1 in Pd 4)(-1 in Pd 2) (D) Nar NaCl NaF (+1)(-1) where -1 s are decreasing in radius H. Sevian 17