Thermodynamic study of CdCl 2 in 2-propanol (5 mass %) + water mixture using potentiometry

Transcription

1 Thermdyamic study f CdCl 2 i 2-prpal (5 mass %) + water mixture usig ptetimetry Reat Tmaš, Ađelka Vrdljak UDC: Uiversity f Split, Faculty f Chemistry ad Techlgy, Teslia 10/V, HR Split, Cratia Summary The ptetial differece (pd) measuremets (E) are reprted fr galvaic cell withut liquid jucti: Cd(Hg) (l, satd.)cdcl 2 () i ZAgCl(s)Ag(s) at differet temperatures ad varius CdCl 2 mlalities () i aqueus mixture f 2-prpal (ctaiig 5 mass % 2-prpal), Z. Frm these values ad usig literature data fr staility cstats f the chlrcadmium cmplexes, the values f the stadard pd f the cell were taied at each temperature. These values served t calculate the stadard thermdyamic quatities fr the cell reacti, ad als mea mlal activity cefficiets f CdCl 2. The crrespdig thermdyamic results are discussed ad cmpared with literature data. Keywrds: thermdyamic prperties, cadmium chlride, 2-prpal + water mixed slvet, ptetimetry. Itrducti Ptetimetric methd usig a galvaic cell withut liquid jucti was fud t e a attractig experimetal techique fr studyig the thermdyamic prperties f electrlyte slutis i aqua-rgaic mixed slvets. This paper is a extesi f ur systematic ivestigati thermdyamic prperties f CdCl 2 i varius aqua-rgaic slvets usig ptetimetry. Specifically, i previus wrks y ur grup, the ahaviur f the CdCl 2 has ee studied i water mixtures with alchl c-slvet: ctaiig 10, 30, ad 50 mass % 2-prpal (Višić Mekjavić, 1993) r 2-methylprpa-2-l (tert. utal) (Tmaš et al., 2000), ad als with 5, 10, ad 15 mass % 2-utal (Tmaš et al., 2005). We have recetly pulished data the thermdyamic prperties f CdCl 2 i 2- methylprpa-2-l (5 mass %) + water mixture (Z) usig a flw ptetimetric methd (Tmaš et al., 2011). Namely, the ptetial differece (pd) measuremets have ee carried ut the fllwig galvaic cell: Cd(Hg) (l, satd.)cdcl 2 () i ZAgCl(s)Ag(s) (1) Crrespdig authr: rtmas@ktf-split.hr 41

2 i the temperature rage ( K t K) at 5 K itervals, fr differet CdCl 2 mlalities (). I this wrk, aalgus ivestigatis were perfrmed i 2-prpal + water mixture f the same ctet (ie., Z = 5 mass % 2-prpal), i rder t determie the thermdyamic quatities f the cell reacti, ad the stichimetric mea mlal activity cefficiets f CdCl 2. The results frm this study are cmpared with similar systems. Withi the ptetimetric data aalysis, the ccetratis f all iic species was calculated (Višić Mekjavić, 1989) usig data fr the thermdyamic staility cstats f chlrcadmium cmplexes i water medium, ad fr 10 mass % 2-prpal (Višić et al., 1993). Materials ad methds CdCl 2 H 2 O ad 2-prpal were p.a. purity (Merck). Befre use, 2-prpal ad water were distilled. Slvet mixture (Z) ad ccetrated electrlyte sluti i Z (stck sluti) were prepared y weight usig CdCl 2 H 2 O, water ad 2-prpal. The mlality ( max. ) f the stck sluti f CdCl 2 i Z was ml kg 1, with salt mass fracti, w = Desity f Z was measured usig scillatig U-tue desimeter (At Paar, mdel DMA 4500 M) with precisi f g cm 3 (Bald et al., 2013). I rder t ctiuusly tai the pd (E i ) f the cell (1) at differet mlalities ( i ) f CdCl 2, we chage electrlyte ccetratis y addig a stck sluti i Z (mass, m i ) t a cell ctaiig a apprpriate mass f slvet mixture (m Z ). The mlality f CdCl 2 i the cell (1) after i th additi ( i ) is give y the expressi (Zhag et al., 1993): where M is the mlar mass f cadmium chlride. m w M i i / i m m 1 (2) w The preparati f electrdes, descripti f the cell, ad f the equipmet fr pd measuremets, ad the measurig prcedure itself were explaied earlier (Tmaš et al., 2011). Results ad discussi The ptetimetric results (E i ) fr the cell (1) i 5 mass % 2-prpal (Z) at the differet CdCl 2 mlalities ( i ) ad at all peratig temperatures (T) are cmpiled i Tale 1. Z i 42

3 Tale 1. Experimetal data fr cadmium chlride i Z at differet temperatures 10 3 / ml kg 1 E / V 10 3 / ml kg 1 E / V T = K T = K T = K T = K

4 Tale 1. (Ctiued) 10 3 / ml kg 1 E / V T = K The values f E frm Tale 1 were used t calculate the stadard mlal pd ( E ) fr the cell reacti: Cd(s) + 2 AgCl(s) 2 Ag(s) + Cd 2+ (Z) + 2Cl (Z) (3) accrdig t the relati, RT 2 2 3RT 1/2 1/2 E E l( (Cd )/ )( (Cl )/ ) l(10) A ( I/ ) /(1 a0b ( I/ ) ) 2F F 3RT 3RT l1 M Z Σx(X) E l(10) C I / (4) 2F 2F This relati was taied usig a cmiati f the Nerst equati ad the Deye- Hückel equati fr the mea activity cefficiet. Here I detes iic stregth, ad = 1 ml kg 1. The fllwig data are eeded t slve Eq. (4): 44

5 a) a value fr the i-size parameter (a 0 ); a 0 = 0.45 m (Višić et al., 1993), ) the mlar mass ad mass fracti f water ad f 2-prpal; these data are used t determie the mea mlar mass (M) f ivestigated mixed slvet Z, c) Deye-Hückel cstats (A, B ); these cstats were calculated usig literature data fr the relative permittivity, r (Åkerlöf, 1932) ad the desity, d (frm the preset study) f the slvet Z: the prperties ( r, d) at differet temperatures are give i Tale 2. d) the equilirium mlalities f all iic species, give y the term x (X), as well as the cell pd (E) fr each cadmium chlride mlality. Determiati f the E ca e perfrmed either y extraplati f E' frm Eq. (4) t zer iic stregth r y the least-squares methd. T calculate (Cd 2+ ), (Cl ), ad (f the remaiig iic species) fr each mlality f CdCl 2, it is ecessary t csider the cmplexati reacti i ivestigated mixed slvet Z (Višić Mekjavić, 1989): Cd 2+ + Cl (2 ) CdCl ( = 1, 2 i 3) (5) Iasmuch as the iterative prcedure has ee descried i detail i ur previus papers (Tmaš et al., 2004 ad 2011), ly a shrt ulie will e prvided here. Namely, fr a give cadmium chlride mlality, the stichimetric iic stregth f the sluti is first calculated as I = 3 d, ad the ccetrati staility cstats, K, fr the cmplex frmig reactis (5) are estimated at this iic stregth usig ext relati: lk 2 1/2 1/2 Δz Ac ( I/ c ) /(1 Bca0 ( I/ c ) ) lk (l10)δ CI/ c (6) These staility cstats are the served t calculate the ccetrati f each iic species. Iitial ccetratis thus taied are used t calculate the ew iic stregth ad staility cstat values. The treatmet is repeated util a satisfactry cstacy f K values is taied. I Eq. (6) C is a empirical cstat, c = 1 ml dm 3, ad 2 Δz = (2 ) 2 4, where z is the charge f each iic species. The parameters C ad (thermdyamic staility cstat) were determied experimetally fr water medium, ad fr 10 mass % 2-prpal (Višić et al., 1993). Fr the preset study, C ad K, are iterplated frm literature data; values at differet temperatures fr mixed slvet Z are reprted i Tale 2. K 45

6 The taied equlirium ccetratis f all iic species were the expressed as mlalities, ad usig Eq. (4) the stadard mlal pd, E, ad its stadard deviati were determied usig the least-squares methd. These values are listed i Tale 3. Regressi aalysis shwed that depedece f E T ca e adequately fitted y the secd-rder plymial. E ( T) ct 2 a T (7) Tale 2. Parameters f Eq. (6) i Z at differet temperatures T / K K K K 3 ΔC ΔC ΔC r d / g cm Tale 3. Stadard ptetial differece ( E ) f cell (1) i Z at differet temperatures T / K E ± (E ) / V ± ± ± ± ± The plymial cefficiets a, ad c, taied y fittig Eq. (7) t the experimetal results are preseted i Tale 4, tgether with their stadard deviatis. Tale 4. Adjustale cefficiets a, ad c f Eq. (7) i system CdCl 2 Z a / V ± / V K ± c / V K ±

7 Frm the first derivate f Eq. (7), accrdig t relati r S = d E /dt = z F ( + 2 ct), (z = 2) (8) the stadard etrpy ( r S ) f the cell reacti (3) i Z is taied. The stadard Gis eergy ( r G ) is calculated accrdig t the expressi, Δ rg zfe (9) while the stadard ethalpy ( r H ) ca e calculated frm the relatiship, r H = r G + T r S (10) The stadard thermdyamic quatities fr the cell reacti (3) i Z at K are give i Tale 5. The data fr water medium are, fr cmparisi, listed i the same tale. The deviatis were calculated frm the stadard deviati fr E. Tale 5. Stadard thermdyamic quatities fr the cell reacti (3) i water medium ad slvet Z at K mass % r G / kj ml 1 r H / kj ml 1 r S / J K 1 ml 1 0 (Višić Mekjavić, 1993) ± ± 5 78 ± 10 5 (preset study, Z) ± ± ± 25 As see frm Tale 5 the stadard Gis eergy fr th slvets has the egative sig, which prves that the cell reacti (3) is sptaeus. The values fr the stadard ethalpy ad etrpy chage are als egative. It ca e see that the cell reacti (3) is exthermic ad lead t decrease i etrpy. It ca als e see that the sptaeity ( r G ) f reacti (3) decreaces with addig 2-prpal i water. The same was reprted fr the aqueus mixtures with 5 mass % 2-uta, 2-utal, r t-utal (Tmaš et al., 2004, 2005, ad 2011). I this wrk, the stichimetric mea mlal activity cefficiet ( ) f cadmium chlride i Z was calculated usig the data fr E ad E f the cell (see Tales 1 ad 3), accrdig t the Nerst equati fr the cell reacti (3): RT/2F) l 4( / ) E E ( γ (11) 3 47

8 prvided that chlrcadmium cmplexes were t frmed i the sluti. Tale 6 shws the taied values f CdCl 2 i 5 mass % 2-prpal (Z) at each mlality ad at differet temperatures. Values f fr cadmium chlride i 5 mass% 2-prpal (Z) were t fud i literature. Tale 6. Stichimetric mea mlal activity cefficiets ( ± ) f CdCl 2 i Z at differet temperatures (CdCl2) / ml kg 1 T / K Accrdig t the values give i Tale 6, activity cefficiets decrease with icreasig CdCl 2 mlality, as well as decrease with icreasig tempertaure. This result is expected frm Deye-Hückel thery. A crrespdig ehaviur has ee fud i ur earlier studies f the same electrlyte i aqueus kete (Tmaš et al., 2004) ad aqueus alchls (Tmaš et al., 2005 ad 2011) at the same mass fractis. Whe cmparig the values frm preset study with thse fr water medium (Višić Mekjavić, 1993), it ca e see that these values are higher i water. Oviusly, this is related t the degree f cmplexati. Aalgy was estalished with aqueus mixtures with 5 mass % 2-utae, 2-utal, ad t-utal (Tmaš et al., 2004, 2005, ad 2011). Furthermre, values f fr cadmium chlride at the same mass fracti (Z = 5 mass %) are similar; certai differeces are due t ifluece f relative permittivity ad ature f rgaic cmpet i the mixed slvet. Cclusis The preset ivestigati f thermdyamic prperties f cadmium chlride i aqueus mixture with 5 mass % 2-prpal (Z) ca e summarised as fllwig: Fr the preset system CdCl 2 i Z, was determied stadard mlal ptetial differece fr the cell reacti at differet temperatures: this value decreases with a icrease i the temperature. At cstat temperature, the mea mlal activity cefficiet f cadmium chlride decreases with a icrease i the mlality f sluti. At fixed mlality, the mea mlal activity cefficiet f cadmium chlride decreases with a icrease the temperature. The cell reacti is sptaeus, exthermic ad leads t reduced i etrpy. 48

9 Refereces Åkerlöf, G. (1932): Dielectric cstats f sme rgaic slvet water mixtures at varius temperatures, J. Am. Chem. Sc. 54 (11), Bald, A., Kiart, Z., Tmaš, R. (2013): Vlumetric studies f aqueus slutis f msdium salts f sme aliphatic dicarxylic acids at K. A ew methd f data aalysis, J. Ml. Liq. 178, Tmaš, R., Višić. M., Mekjavić, I. (2000): Thermdyamics f cadmium chlride i t-utal water mixtures (w t-buoh = 10%, 30%, ad 50 %) frm electrmtive frce measuremets, Crat. Chem. Acta 73 (2), Tmaš, R., Tmiić, I., Višić, M., Skl, V. (2004): Thermdyamics f Cadmium Chlride i 2- Butae + Water mixtures (5, 10, ad 15 Mass%) frm Electrmtive Frce Measuremets, J. Sluti Chem. 33 (11), Tmaš, R., Tmiić, I., Višić, M., Skl, V. (2005): Thermdyamic study f cadmium chlride i aqueus mixtures f 2-utal frm ptetial differece measuremets, J. Sluti Chem. 34 (8), Tmaš, R., Skl, V., Bškvić, P. (2011): Thermdyamic prperties f CdCl 2 i tert. utal (5 mass %) + water mixture. I Prceedigs: Iteratial Scietific ad Prfessial Cferece 13 th Ružička Days, Vukvar, Septemer 2010, Šuarić, D. (Ed.), Osijek, pp Višić, M., Mekjavić, I. (1989): Thermdyamics f the cell: Cd(s)+Hg CdCl 2 (aq, m)agclag, J. Chem. Thermdyamics 21 (2), Višić, M., Jadrić, A., Mekjavić, I. (1993): The staility cstats f cadmium chlride cmplexes i 2-prpal water mixtures (0, 10, 30 ad 50 mass per cet) frm electrmtive frce measuremets, Crat. Chem. Acta 66 (3-4), Višić, M., Mekjavić, I. (1993): Thermdyamics f the cell: Cd(Hg) satd. CdCl 2 (m)agclag i (10, 30 ad 50 Mass per Cet) 2-Prpal Water Mixtures, Crat. Chem. Acta 66 (3-4), Zhag, L., Lu, X., Wag, Y., Shi, J. (1993): Determiati f activity cefficiets usig a flw emf methd. 1. HCl i methal water mixtures at 25, 35 ad 45C, J. Sluti Chem. 22 (2),