VAPOUR PRESSURE. Ranges of Vapour Pressures for some Important Classes of Organic Compounds at 25 o C

Transcription

1 VAPOUR PRESSURE The r pressure (P ) is the pressure f the r f a cmpund in equilibrium with its pure cndensed phase (slid r liquid). Vapur pressures depend strngly n the temperature and vary widely with different cmpunds due t differences in mlecule mlecule interactins. The nrmal biling pint f a liquid is defined as the temperature at which the r pressure f the liquid is 1 atmsphere (P = 1 atm). The ur pressure f a substance is an intrinsic physical prperty that plays a crucial rle in determining it s distributin t and frm gaseus envirnmental phases (the atmsphere, marsh bubble gas). The ur pressure is als crucial fr the predictin f equilibrium distributin cefficients t and frm natural waters, such as Henry s Law cnstant (K H ). Ranges f Vapur Pressures fr sme Imprtant Classes f Organic Cmpunds at 25 C

2 Phase Diagrams Simplified phase diagram f rganic liquids and slids shwing hypthetical subcled and superheated liquid bundaries. Biling pint: temperature at which the liquids ur pressure equals the external pressure. Nrmal biling pint: temperature at which the liquids ur pressure equals 1 atm. Triple pint: the temperature and pressure at which the slid, liquid and gas are at equil. Vapurizatin curve: at any pint n this line (specified by a temperature and pressure) the liquid and gas are at equilibrium. Fusin curve: at any pint n this line (specified by a temperature and pressure) the slid and liquid are at equilibrium. Sublimatin curve: at any pint n this line (specified by a temperature and pressure) the slid and gas are at equilibrium. Critical pint: at any pint with a higher temperature and pressure than the critical temperature and pressure, the densities f the gas and liquid are equal. The gas and the liquid have becme ne phase described as a supercritical fluid.

3 Kinetic Mlecular Thery: At ambient temperatures, the r pressure f a cmpund can vary by an rder f magnitude, s the dependence n temperature is imprtant. It is illustrative t imagine what the mlecules f a substance achieve t establish an equilibrium r pressure by examining a kinetic-mlecular descriptin. The mlecules in the liquid will be thermally jstling each ther and will be cntinuusly exchanging energies. At a given temperature, the energies f the mlecules in the liquid will have a distributin f energies accrding t the Maxwell-Bltzmann distributin. Sme f these mlecules will have sufficient energy t vercme the intermlecular frces f attractin. Thse mlecules n the surface f the liquid with energies greater than the intermlecular frces will escape the liquid. The cncentratin (partial pressure r r pressure) f the mlecules abve the liquid will steadily increase and the rate f cllisins will als increase. A fractin f these clliding mlecules will lse sufficient energy s that if they cme in cntact with the liquid surface, they will becme trapped again. When the rate f escape (eratin) frm the liquid equals the rate f entrapment (cndensatin), the partial pressure f the gas abve the pure liquid is called the equilibrium r pressure, P. The fractin f mlecules in the liquid with energy greater than the intermlecular frces is given by energy the Maxwell-Bltzmann distributin, e, and the r pressure will be prprtinal t this term: and s P e energy energy ln P = + cnstant Vapur pressure at 25 C f n-alkanes as a functin f chain length. Nte: Larger n-alkane mlecules have lwer P due t greater plarizability and hence increased strength f Lndn Dispersin intermlecular frces.

4 Thermdyanmics: It can be shwn that the temperature dependence f pressure is related t the change in mlar enthalpy and the mlar vlume upn urizatin. dp ΔH = dt TΔV Fr rizatin and sublimatin, this equatin may be simplified by assuming the r beys the perfect gas law and by neglecting the mlar vlume f the liquid. Substituting /P fr the change in mlar vlume yields; dp P ΔH = 2 dt ΔH (heat f rizatin) r ΔH sub (heat f sublimatin) is the enthalpy required t cnvert ne mle f liquid r slid int a r. Assuming ΔH is cnstant, integratin yields: r ΔH ln P = + cnstant ΔH sub ln P = + cnstant fr the rizatin f a liquid fr the rizatin f a slid With the exceptin f a few classes f cmpunds (e.g., lw mlar mass hydrcarbns and sme halgenated methanes such as frens), the majrity f envirnmentally relevant rganic chemicals have biling pints well abve ambient temperatures and these cmpunds exist primarily as slids r liquids at ambient temperatures. Over the range f ambient temperatures, ΔH can be assumed cnstant. The figure belw shw the graphs f lnp versus 1/T fr sme representative cmpunds, nte that the slpes are ΔH/R. The change in slpe ccurs at the phase transfrmatin, liquid t slid and the ΔH term switches frm ΔH t ΔH sub. Anther useful frm f this equatin, fr the r pressures ( P 1 and P 2 ) abve a liquid at temperatures T1 and T 2 is: P ln P ΔH = R T1 T2 where ΔH sub is substituted fr ΔH when cnsidering the r pressure abve a slid.

5 Temperature Dependence f Vapr Pressure fr sme Representative Cmpunds If there is a phase change (liquid t slid) within the temperature range cnsidered, the r pressure curve shws a break at this phase transitin temperature (naphthalene in the figure abve). Belw the melting pint, T m the slid sublimes (slid gas), ΔH sub can be cnsidered t be: ΔH sub = ΔH (L) + ΔH melt In ther wrds, the energy required t transfrm a slid t a gas is the sum f the energies t cnvert the slid t the liquid, ΔH melt, plus the energy t cnvert the liquid t the gas, ΔH. ΔH (L) is the heat f rizatin f the hypthetical liquid at a temperature belw the melting pint, i.e., the imaginary liquid that is cled belw its melting pint withut allwing it t crystallize. This hypthetical state is cmmnly referred t as the subcled liquid state and we will dente it with the symbl (L). This cncept is imprtant as we are using the pure liquid as ur reference state fr phase transfer prcesses and we represent the r pressure f the pure subcled liquid as P (L) and the r pressure f the pure slid as P (s). The r pressure f the subcled liquid can be btained by extraplatin f r pressure data abve the melting pint (see naphthalene abve). At temperatures belw the melting pint we have: ( ) ΔH 1 ΔH ( L) + ΔH sub melt 1 ln P ( S) = + cnstant = + cnstant R T R T

6 Vapur Pressure Estimatin Methds Empirical Apprach: Experimentally measured ur pressures at different temperatures used t fit the Antine Equatin: ln P B = - + A T where T = temperature (Kelvin) and A & B are cnstants (pressure in atmspheres). Pltting ln P vs 1/T, with slpe = B and intercept = A. Values f A and B fr many rganic cmpunds are tabulated and available in CRC Handbk f Physics and Chemistry and a number f nline resurces. Use tabulated values fr a given cmpund t calculate P at any given temperature, T. Semi-empirical Apprach: Fr nn-hydrgen bnding mlecules in liquid (r sub-cled liquid) state, use; Tb ln P (L) T Tb ln T Where T is the Kelvin temperature and T b = nrmal biling pint (Kelvin), (pressure in atmspheres). Fr nn-hydrgen bnded mlecules in slid state use; P (S) T ln - 1 P (L) T m [ (n - 5) ] where T m = nrmal melting pint (Kelvin) and n = # flexing chain atms and n = 5 fr # flexing chain atms < 5, (pressure in atmspheres). Mlecular Mdeling Apprach: Input chemical structure Mlecular mechanics determine 3D gemetry Calculate surface area, diple mments etc. Estimate intermlecular frces Predict physical prperties (T b, P etc).