Properties of Pure Substances

Transcription

1 ure Substance roperties o ure Substances A substance that has a ixed cheical coposition throuhout is called a pure substance such as water, air, and nitroen. A pure substance does not hae to be o a sinle eleent or copound. A ixture o two or ore phases o a pure substance is still a pure substance as lon as the cheical coposition o all phases is the sae. hases o a ure Substance A pure substance ay exist in dierent phases. here are three principal phases solid, liquid, and as. A phase: is deined as hain a distinct olecular arraneent that is hooenous throuhout and separated ro others (i any) by easily identiiable boundary suraces. A substance ay hae seeral phases within a principal phase, each with a dierent olecular structure. For exaple, carbon ay exist as raphite or diaond in the solid phase, and ice ay exist in seen dierent phases at hih pressure. Molecular bonds are the stronest in solids and the weakest in ases. Solid: the olecules are arraned in a three diensional pattern (lattice) throuhout the solid. he olecules cannot oe relatie to each other; howeer, they continually oscillate about their equilibriu position. Liquid: the olecular spacin in liquid phase is not uch dierent ro that o the solid phase (enerally slihtly hiher), except the olecules are no loner at ixed positions relatie to each other. Gas: the olecules are ar apart ro each other, and a olecular order does not exist. Gas olecules oe randoly, and continually collide with each other and the walls o the container they are in. Molecules in the as phase are at a considerably hiher enery leel than they are in liquids or solid phases. hase Chane rocesses o ure Substances Consider a process where a pure substance starts as a solid and is heated up at constant pressure until it all becoes as. Dependin on the preailin pressure, the atter will pass throuh arious phase transorations. At 0 :. Solid. Mixed phase o liquid and solid M. Bahrai ENSC 88 (F09) roperties o ure Substances

2 . Sub cooled or copressed liquid (eans it is not about to aporize) 4. Wet apor or saturated liquid apor ixture, the teperature will stop risin until the liquid is copletely aporized. 5. Superheated apor (a apor that is not about to condense). Fi. : diara or the heatin process o a pure substance. At a ien pressure, the teperature at which a pure substance starts boilin is called the saturation teperature, sat. Likewise, at a ien teperature, the pressure at which a pure substance starts boilin is called the saturation pressure, sat. M. Bahrai ENSC 88 (F09) roperties o ure Substances

3 Durin a phase chane process, pressure and teperature are dependent properties, sat = ( sat ). he critical point is the point at which the liquid and apor phases are not distinuishable he triple point is the point at which the liquid, solid, and apor phases can exist toether. On or diaras, these triple phase states or a line called the triple line. able : Critical and triple point or water and oxyen. Critical oint riple oint (at) (K / C) (at) (K / C) H O /(74.4) (0.0) O /( 8.6) /( 9) Vapor Doe he eneral shape o a diara or a pure substance is ery siilar to that o a diara. critical point sat. apor line SUERHEAED VAOR REGION COMRESSED LIQUID REGION = const. > sat. liquid line SAURAED LIQUID VAOR REGION = const Fi. : diara o a pure substance. M. Bahrai ENSC 88 (F09) roperties o ure Substances

4 he or hase Chane Diara his is called phase diara since all three phases are separated ro each other by three lines. Most pure substances exhibit the sae behaior. One exception is water. Water expands upon reezin. Fi. : phase diara o pure substances. here are two ways that a substance can pass ro solid phase to apor phase i) it elts irst into a liquid and subsequently eaporates, ii) it eaporates directly without eltin (subliation). the subliation line separates the solid and the apor. the aporization line separates the liquid and apor reions the eltin or usion line separates the solid and liquid. these three lines eet at the triple point. i <, the solid phase can chane directly to a apor phase at < the pure substance cannot exist in the liquid phase. Norally (> ) the substance elts into a liquid and then eaporates. atter (like CO ) which has a triple point aboe at subliate under atospheric conditions (dry ice) M. Bahrai ENSC 88 (F09) roperties o ure Substances 4

5 or water (as the ost coon workin luid) we are ainly interested in the liquid and apor reions. Hence, we are ostly interested in boilin and condensation. roperty ables For ost substances, the relationships aon therodynaic properties are too coplex to be expressed by siple equations. hus, properties are requently presented in the or o tables, see able A 4. he subscript is used to denote properties o a saturated liquid and or saturated apor. Another subscript,, denotes the dierence between the saturated apor and saturated liquid alues o the sae property. For exaple: = speciic olue o saturated liquid = speciic olue o saturated apor = dierence between and ( = ) Enthalpy: is a property deined as H = U + V (kj) or h = u + (kj/k) (per ass unit). Enthalpy o aporization (or latent heat): represents the aount o enery needed to aporize a unit ass o saturated liquid at a ien teperature or pressure. It decreases as the teperature or pressure increase, and becoes zero at the critical point. Saturated Liquid Vapor Mixture Durin aporization, a ixture o part liquid part apor exists. o analyze this ixture, we need to know the proportions o the liquid and apor in the ixture. he ratio o the ass o apor to the ass o the total ixture is called quality, x: x apor total total liquid apor Saturated liquid apor ixture is treated as a cobination o two sub systes (two phases). he properties o the ixture are the aerae properties o the saturated liquid apor ixture. M. Bahrai ENSC 88 (F09) roperties o ure Substances 5

6 V V t ae ae ae x ae V t diidin by or, x x t x t ae / k t and x / t or critical point sat. liquid states sat. apor sat. apor states sat. liquid Fi. 4: he relatie aounts o liquid and apor phases (quality x) are used to calculate the ixture properties. Siilarly, u h ae ae u h xu xh Or in eneral, it can be suarized as y ae = y +x.y. Note that: 0 x y y ae y Note: pressure and teperature are dependent in the saturated ixture reion. M. Bahrai ENSC 88 (F09) roperties o ure Substances 6

7 Fi. 5: Quality deined only or saturated liquid apor ixture. Exaple : Saturated liquid apor ixture A closed, riid container o olue 0.5 is placed on a hot plate. Initially the container holds a two phase ixture o saturated liquid water and saturated water apor at = bar with a quality o 0.5. Ater heatin, the pressure in the container is =.5 bar. Indicate the initial and inal states on a diara, and deterine: a) the teperature, in C, at each state. b) the ass o apor present at each state, in k. c) i heatin continues, deterine the pressure, in bar, when the container holds only saturated apor. Solution: Assuptions:. Water in the container is a closed syste.. States,, and are equilibriu states.. he olue o container reains constant. wo independent properties are required to ix state and. At the initial state, the pressure and quality are known. hus state is known, as entioned in the proble. he speciic olue at state is ound usin the ien quality: x Fro able A - 5 at bar 00 ka ( ) At state, the pressure is known. Volue and ass reain constant durin the heatin process within the container, so =. For = 0.5 Ma, able A 5 ies = and =.59 /k. Since / k M. Bahrai ENSC 88 (F09) roperties o ure Substances 7

8 < < State ust be in the two phase reion as well. Since state and are in the two phase liquid apor reion, the teperatures correspond to the saturation teperatures or the ien. able A 5: = 99.6 C and =.4 C o ind the ass o water apor present, we irst ind the total ass,. V k / k 0.59k 0. k x =.5 bar = bar he ass o apor at state is ound siilarly usin quality x. Fro able A 5, or =.5 bar, we hae: x x k 0.4 k I heatin continued, state would be on the saturated apor line, as shown in on the diara aboe. hus, the pressure would be the correspondin saturation pressure. Interpolatin in able A 5 at = /k, we et =. bar. M. Bahrai ENSC 88 (F09) roperties o ure Substances 8

9 Superheated Vapor Superheated reion is a sinle phase reion (apor only), teperature and pressure are no loner dependent. See able A 6 or superheated apor properties. I >> critical or << critical, then the apor can be approxiated as an ideal as. Copressed (or Sub cooled) Liquid he properties o a liquid are relatiely independent o pressure (incopressible). A eneral approxiation is to treat copressed liquid as saturated liquid at the ien saturation teperature. y he property ost aected by pressure is enthalpy. For enthalpy use the ollowin approxiation: he Ideal Gas Equation o State h M. Bahrai ENSC 88 (F09) roperties o ure Substances Any equation that relates the pressure, teperature, and speciic olue o a substance is called an equation o state. he siplest and best known equation o state or substances in the as phase is the ideal as equation o state. Gas and apor are oten used as synonyous words. he apor phase o a substance is called a as when it is aboe the critical teperature. Vapor usually iplies a as that is not ar ro a state o condensation. It is experientally obsered that at a low pressure the olue o a as is proportional to its teperature: R Where R is the as constant. he aboe equation is called the ideal as equation o state (ideal as relation). Since R is a constant or a as, one can write: R where and denote two states o an ideal as. he constant R is dierent or each as; see able in Cenel book. R u = 8.4 kj / (kol. K) is the uniersal as constant, R = R u /M. he olar ass, M (k/kol): is deined as the ass o one ole o a substance. he ass o a syste is equal to the product o its olar ass M and the ole nuber N: sat MN (k) 9

10 See able A or R and M or seeral substances. An ideal as is an iainary substance that obeys the relation = R. It is experientally obsered that the ideal as closely approxiate the behaior o real ases at low densities. In the rane o practical interest, any ailiar ases such as air, nitroen, oxyen, hydroen, heliu, aron, neon, and CO can be treated as ideal ases with neliible error. Water apor (in eneral see Fi Cenel book), rerierant apor in rerierators should not be treated as ideal ases. Water apor at pressures below 0 ka can be treated as an ideal as, reardless o teperature. M. Bahrai ENSC 88 (F09) roperties o ure Substances 0

11 Copressibility Factor he assuption o ideal as relation iplies that: the as particles take up neliible olue the interolecular potential enery between particles is sall particles act independent o one another Howeer, real ases deiate ro ideal as behaior. his deiation at ien teperature and pressure can be accurately accounted or by introduction o a correction actor called the copressibility actor Z. Z or R or Z = actual / ideal. Obiously, Z= or ideal ases. ZR Gases behae ery uch the sae at teperatures and pressures noralized with respect to their critical teperatures and pressures. R cr and Here R and R are called the reduced pressure and teperature, respectiely. By cure ittin all the data, the eneral copressibility chart is obtained which can be used or all ases. R cr Fi. 6: Z actor, eneral copressibility chart. M. Bahrai ENSC 88 (F09) roperties o ure Substances

12 Fro the Z chart, one can conclude: at ery low pressure ( R <<), the ases behae as an ideal as reardless o teperature at hih teperatures ( R >), ideal as behaior can be assued. the deiation is hihest in the icinity o the critical point. Exaple : Ideal Gas Deterine the speciic olue o R 4a at Ma and 50 C, usin (a) ideal as equation (b) the eneralized copressibility chart. Copare the alues obtained with the actual alue o 0.07 /k. Solution: Fro able A, or R 4a, R = ka. /(k.k), cr = Ma, and cr = 74. K (a) Ideal as equation o state.085 ka. / k. K K 000 ka 0 R 0.06 k / Coparin with the tabulated alue, usin ideal as equation one would et an error o ( )/0.07=0. or.%. (b) o deterine the correction actor Z, R R cr cr Ma Ma K 74.K 0.86 Fro Fi. A 8, Z= hus, = Z ideal = 0.84 (0.06 /k) =0.0 /k he error is less than %. hereore, in the absence o exact tabulated data, the eneralized copressibility chart can be used with conidence. M. Bahrai ENSC 88 (F09) roperties o ure Substances