- Email: [email protected]
The thermochemistry of the chloro-benzoyl chlorides
J. Chem. Thermodynamics 1975,I, 911-982
The thermochemistry chlorides GALINA
M. MOSELHYa
of the chloro-benzoyl and HUW 0. PRITCHARD
ChemistryDepartment, York University, Downsview,Ontario M3J lP3, Canada (Received7 April I975) The enthalpies of hydrolysis of the three isomers of chloro-benzoyl chloride have been measured in a diphenyl-ether calorimeter at 300.05 K. The enthalpy change for the reactions: (0, m, orp)Cl *CBH&OCl(l) + HaO(l) = (o, m,or p)Cl *CBH&OaH(s) -t HCl(g), is AZ&(300.05 K)/kcalth mol-1 AHZ(298.15 K)/kcalSh mol-l (observed) (corrected) -(9.502 f 0.060) ortho: -(9.561 & 0.045) meta : -(9.885 A 0.062) -(9.826 i 0.080) -(10.453 I-t 0.100) para: -(lo.512 rt 0.087) where the error limits refer to twice the standard deviation of the mean. A small error detected in our earlier work on the hydrolysis of benzoyl chloride leads to revised values for the analogous reaction of benzoyl chloride itself of -(8.189 i 0.050) and -(8.200 & 0.058) kcal,, mol-” for the two separate determinations at 300.05 K; thus AIG’(CsH,COCl, L298.15 K) = -(37.60 f 0.09) kcal,, mol-l.
1. Introduction A recent paper from this laboratory described an accurate determination of the enthalpy of formation of benzoyl chloride via hydrolysis to benzoic acid.(l) The value obtained was in poor agreement with that determined by Hu and Sinke from combustion calorimetry,@’ suggesting that some minor imperfections may still remain in their experimental techniques. In view of the central importance of the chlorobenzoic acids in modern combustion calorimetry(354) and in view of small errors that have occurred in recent combustion determinations for two chlorine-containing molecules,(13 5, we felt that it was a worthwhile exercise to obtain very accurate differences in enthalpies of formation for the three chloro-benzoic acids and their respective acid chlorides; these molecules are all stable and well-characterized, and can be used if necessary for further tests in the refinement of combustion techniques for organochlorine compounds. 2. Experimental The construction of the calorimeter and its operation have been described alreadyc6) and the techniques used in the present work follow very closely those used before in the hydrolysis of benzoyl chloride. (‘) The energy release was measured (a) for the hydrolysis of the acid chloride in aqueous acetone and (b) for the solution of gaseous a Formerly r. M. nETPAYKOBA.
978
G. M. MOSELHY
AND H. 0. PRITCHARD
HCI and the corresponding solid acid, to form a final solution of the same composition. The only difference in procedure arose from the necessity in one of these experiments to use acetone (80 per cent by mass) + water in which to carry out the reaction. Gaseous HCl did not dissolve instantly in this mixture and so the stirring device (figure 1 of reference 1) was modified slightly to accommodate a second inverted bell which trapped all escaping HCl bubbles and held them until they had time to dissolve.(‘) Ortho-, meta- and para-chloro-benzoyl chlorides were prepared by reaction of thionyl chloride with the parent carboxylic acid. @) To ensure isomeric purity of the samples, the starting acids were recrystallized before use, and following the usual vacuum distillation, each acid chloride was further purified by some 4 to 6 fractional recrystallizations : the melting temperatures of ortho-, meta-, and para-chloro-benzoyl chlorides were 269.4 to 269.6 K, 268.9 to 269.2 K, and 286.9 to 287.0 K, respectively. The hydrolysis of these acid chlorides takes place quite quickly in aqueous acetone but the rate of hydrolysis decreases with increasing fraction of acetone in the reaction mixture. However, the dissolution of the chloro-benzoic acids in the reaction mixture is slow, but is accelerated by increasing the fraction of acetone. Thus, it was possible in two cases to choose acetone + water mixtures in which both the hydrolysis experiment and the dissolution experiment were complete in less than 2 h, but in the case of the para-isomer, the best compromise we could achieve was about 3 h each for both the hydrolysis and the dissolution experiments using a less than ideal mass of reactant. The aqueous acetone solvent mixtures used were: ortho, 50 mass per cent of acetone; meta, 65 mass per cent of acetone; para, 80 mass per cent of acetone. The absence of side reactions in these hydrolyses was shown by the recovery of the acid obtained by allowing 3 g of each acid chloride to undergo complete hydrolysis in the appropriate acetone + water mixture, followed by evaporation of the whole reaction mixture to dryness. In each case the recovered acid was intermediate in both colour and melting properties between the best available commercial product and our own recrystallized pure acid; this test therefore established not only the cleanliness of the reaction, but also the isomeric purity of each of the acid chlorides. Other details concerned with the establishment of purity, analysis of materials, and the performance of the calorimetric experiments have been reported extensively elsewhere*(l> 9 6, ‘1 it need only be noted here that all mass measurements have been corrected for buoyancy where appropriate. 3. Results Table 1 shows the results of determination of the enthalpy of hydrolysis AH1 of each of the three acid chlorides in aqueous acetone, according to the reaction: Cl. C,H,COCl(l)
+ (x(CH3)&0 + yH,O) = {C1C6H,C02H+HCl+x(CH3)2CO+(y-1)H,0), and table 2 shows the complementary determinations of AH, for the reactions: Cl. CsH,C02H(s)
+ HCl(g) + {x(CH,),CO + (y - l)H,O) = (C1CGH4COzH+HCl+x(CH3)2CO+(y-1)HzO).
THE THERMOCHEMISTRY
OF THE CHLORO-BENZOYL
CHLORIDES
979
TABLE 1. Determination of AHI for the hydrolysis of amount of substance n of o-, m-, and p-CICBH&OCl in a mixture consisting of amounts of substance nx of (CH&CO and ny of HZ0 (calth = 4.184 J) n mm01
-AH c&
-AHI kcal, mol-’
ortho: x = 59.489 y = 191.791 13.800 325.00 23.551 13.976 328.99 23.539 13.347 314.44 23.559 13.899 327.57 23.567 13.666 321.85 23.551 mean: 23.553 standard deviation: &0.005
- ?I mm01
-AH, -AH __ calzh kcal, mol - 1
meta: x = 80.128 y = 139.100 13.032 336.11 25.791 13.171 339.87 25.805 12.959 333.32 25.722 12.810 330.32 25.786 mean: 25.716 standard deviation: iO.018
-- n mm01
-AH __ c&
-AH, kcal,, mol - 1
para: x = 163.462 y = 132.749 7.535 211.84 28.115 7.554 212.62 28.148 7.892 221.00 28.003 7.772 218.40 28.100 28.058 7.604 213.37 28.098 7.528 211.52 mean: 28.087 standard deviation: kO.021
In table 3 are listed the small correction terms AH3 for the reaction: (x(CH,),CO
+(y - 1)&O)
+HzO
= {x(CH,),CO
+ yHzO).
The procedure used here, since the term is very small, was to dissolve sufficient water in the acetone + water mixture to bring about a 1 per cent change (by mass) in its composition, i.e. to start with a mixture 0.5 per cent too dilute in water, ending up with a mixture 0.5 per cent too concentrated in water. It was assumed that over this composition range, the enthalpy change was proportional to the amount of substance of H,O added, thus giving AH3 directly. Mixing enthalpies are not known for acetone and water at 300 K, but from the known results for 288 K,(‘) calculated values of AH3 lie in the range -0.2 to -0.3 kcal,, mole1 in reasonable qualitative agreement with the figures given in table 3.7 In re-examining our procedures for estimating AH,, we have found that the values given in our previous paper on benzoyl chloride(l) are incorrect, and so table 3 includes the correct results for those experiments also. The principal source of error in these determinations of AH3 arises from the stirring correction, which becomes very much more variable the greater the acetone content of the solution. We have found that whilst stirring corrections are quite reproducible in pure solvents, i.e. water, acetone, dioxan, and also in mixtures containing less than 70 mass per cent of acetone, they become rather variable for the 80 per cent of acetone + 20 per cent of water mixture, and in fact the stirring corrections appear to depend upon how long the mixture has been standing since it was last stirred. We believe that much of this variability arises because there is an unavoidable temperature gradient in any diphenyl-ether calorimeter: our reaction vessel is between 15 and 20 cm tall, of which about the top one-third is air space. Since the pressure coefficient of the melting temperature of diphenyl ether is(“) 0.0284 K atm-l and its density is 1.0692 g cmm3, we have a natural temperature gradient of about 5 x 10S4 K between the top and the bottom of our vessel, which gives rise to rather t Throughoutthis papercalth= 4.184J; atm = 101.325kPa.
G. M. MOSELHY
980
AND H. 0. PRITCHARD
TABLE 2. Determination of AH2 for the dissolution of amounts of substance n of o-, m-, and p-Cl-C$H+COCl and n of HCl(g) in a mixture consisting of amounts of substance PIX of (CH&CO and n(r - 1) of HzO. There is always a slight mismatch between the amounts of substance of chloro-benzoic acid and hydrogen chloride, expressed as an excess &r(HCl) which results in a small correction SAH to the observed a this is estimated on the assumption that the enthalpy of solution of HCl in the mixture is the same as the enthalpy of solution in water(Q (C&I, = 4.184 J) n(C1 . CsH&OaH) lllDlO1
n(HC1)
-AH c&h
IlllTlOI
ortho: 13.737 13.737 13.732 13.738, 13.742
13.825 13.704 13.822 13.750 13.732
13.179 13.158 13.160 13.183
13.174 13.263 13.115 13.145
1.584 7.584 7.585 7.585
7.420 7.567 7.584 7.537
meta:
para:
An(HC1) pm01
-8AH kc&
caltn
x = 59.489, 196.36 194.98 395.75 195.54 194.73
(y - 1) = 190.791 87.8 -1.56 -32.8 -to.58 90.0 -1.59 11.9 -0.21 -9.1 +0.16
x = 80.128, 212.60 214.46 211.68 212.00
(v - 1) = 138.100 -4.4 +0.08 104.5 -1.85 -44.S i-O.80 -37.2 $0.66
x = 163.462, 132.25 135.25 135.01 134.47
(y - 1) = 131.749 -163.5 +2.90 -7.8 $0.14 -0.2 +o.oo -47.8 +0.85
14.187 14.238 14.155 14.195 14.176 mean : 14.190 standard deviation: f0.014
16.138 16.158 16.146 16.132 mean : 16.144 standard deviation: ~tO.006
mean : standard deviation:
TABLE
-A.& mol-l
17.821 17.853 17.802 17.842 17.830 rto.011
3. Correction terms Al& for the differential enthalpy of dissolution of water in the reaction mixture. The required reaction is {x*solvent + (y - l)HzO} + Hz0 = {x*solvent $ yH~0) (cahh = 4.184 J)
Experiment benzoyl chloride benzoyl chloride ortho-chloro-benzoyl meta-chloro-benzoyl para-chloro-benzoyl
chloride chloride chloride
Solvent
-&
Y iii&l
dioxan acetone acetone acetone acetone
44.126 50.264 59.489 80.128 163.462
143.865 198.053 191.791 139.100 132.749
-AH3 srnol-l 0.201 0.172 0.198 0.253 0.255
rt 0.004 f 0.002 f 0.002 2~ 0.003 f 0.007
THE THERMOCHEMISTRY
OF THE CHLORO-BENZOYL
981
CHLORIDES
TABLE 4. Determination of A& at 300.05K (caIth = 4.184 I) benzoyl chloride set 1 set 2 AiTJkcalth mol-l A&/kc&, mol-” AHs/kcalth mol-l AH,/kcah
mol-l
a
cbloro-benzoyl chlorides orthometapara-
-22.455
-24.138
-23.553
-25.776
- 14.438 -0.172
-16.139
-14.190
-16.144
-0.201 -8.200
-0.198 -9.561
-0.253 -9.885
-8.189
-28.087 -17.830 -0.255 -10.512
“AHq=AHI-AHa$AH3
complicated evaporational effects, significant in size for the 80 mass per cent acetone solution in comparison with the magnitudes of the stirring corrections. The raw experimental results are then assembled in table 4 to give the enthalpy change AH4 for the overall reaction:
Cl* C,H,COCl(l)
+ H20(l)
= Cl- C,H,CO,H(s)
+ HCl(g),
and table 5 presents a summary of the errors involved in each determination, how the individual terms contribute to the final overall error.
showing
4. Discussion
The main purpose of this work has been to try to establish pairs of differences in enthalpies of formation between acids and acid chlorides for compounds which are significant in the combustion calorimetry of organic chlorine-containing moleculesTABLE 5. Summary of accumulation of errors 6AH in the determination of quoted in the body of this table are standard deviations (calth = 4.184 .I) AH,
Source of error
Calculation of error
SAH/kca&,,
ortho-
individual measurements (observed) a z!zO.O07 individual measurements (observed) a xtO.016 analytical measurements 10.1 per cent of AH, ztO.014 stirringcorrection observed zto.002 electricalcalibration ~tO.007 per cent of AH4 &O.OOl Accumulatedstandarddeviation: 10.023 Total error (twicethe standarddeviation): 10.045
the values
AHa;
mol-1
meta50.019 1tO.018 ho.016 z!to.O03 &O.OOl
para10.025 kO.030 ho.018 +0.007 +0.001
ho.031
-&0.044
10.062
ho.087
* The errorsquotedin this tablefor the individualmeasurements of AHI andAH2 ace somewhat largerthan thoseshownin tables1 and2: thoseerrorsdo not includeallowancefor the tmcertainty in the magnitudeof the stirringcorrection,sincein eachseriesof experiments, a constantaverage stirring correctionwasused.As describedin the text, a differentstirring devicewasusedin the determinationof AHa and, asthis wasmuchmorecumbersome, it gaveriseto muchlargeruncertaintiesfrom thiscause.The samedevicewasusedto measure AHi andAH,. but the errorsarising from the stirring correctionare smallerfor AH3 than for AH1 becauseof the short stirring times requiredin the formerexperiments.
982
G. M. MOSELHY
AND
H. 0. PRITCHARD
and at least one if not more of these three acids is in fact being considered at the present time as a secondary thermochemical standard for organic chlorine-containing molecules.(3’ 4, These differences AH, have been established within very close limits, which we believe have been assessed realistically; the errors become larger, as expected, with increasing acetone content of the reaction mixtures since in general they reflect the increasing difficulty of performing accurate thermochemistry and accurate chemical analyses of increasingly volatile solutions. Nevertheless, as far as combustion calorimetry is concerned, all three vaIues of AH4 (and also the-now corrected-corresponding difference for benzoic acid and benzoyl chloride) may be taken as being virtually exact, and therefore to provide the basis for a thorough evaluation of rotating-bomb calorimeter techniques for organochlorine compounds. Using the auxiliary data quoted in our earlier paper on benzoyl chloride,@ together with the recent enthalpy of formation results for the mono-chloro-benzoic acids,(4) we can write down the following estimates for the standard enthalpies of formation for these four acid chlorides, as follows:
benzoyl chloride ortho-chloro-benzoyl meta-chloro-benzoyl para-chloro-benzoyl
chloride chloride chloride
AHi(298.15 K) kcal,, mol- 1 -(8.136&0.050) -(9.502+0.060) -(9.826,0.080) -(10.453~0.100)
AH;(R.COCI, 1,298.15 K) kcal,, mol-l -(37.60+0.09) -(41.00~0.19) -(45.40&0.38) -(45.63+0.21)
To do this, we have assumed that over the range 298 to 300 K, the molar
heat capacities for all four acids are the same,(” 4, and for all four acid chlorides are the same;(‘) we have also included a correction of 0.011 kcal,, mol-‘, being the difference between the enthalpy of formation of gaseous HCl at 1 atm for the real gas and for the standard state.@) We have expanded the error limits of AHF(298.15 K) somewhat arbitrarily to accommodate these estimates.
This work was supported by the National Research Council of Canada, and was greatly facilitated by the collaboration of Dr J. V. Davies in the early stages. REFERENCES 1. Davies, J. V.; Dunning, B. K.; Pritchard, H. 0. J. Chem. Thermodynamics 1972, 4, 731. 2. Hu, A. T.; Sinke, G. C. J. Chem. Thermodynamics 1969, 1, 507. 3. Hu, A. T.; Sinke, G. C.; Mbsson, M.; Ring&r, B. J. Chem. Thermodynamics 1972, 4, 283. 4. Johnson, W. H.; Prosen, E. J. J. Res. Nat. Bur. Stand. 1974, 78A, 683. 5. Hu, A. T. ; Sinke, G. C.; Mintz, M. J. J. Chem. Thermodynamics 1972, 4, 239. 6. Davies, J. V.; Pritchard, H. 0. J. Chem. Thermodynamics 1972, 4, 9. 7. Moselhy, G. M. M.Sc. Thesis, York University, Toronto, Canada. 1974. 8. Mever. H. Mona&h. 1901.22, 415. 9. I&r&ional Critical Tabies, i929, 5, 157. 10. Dainton, F. S.; Diaper, J.; Ivin, K. J.; Sheard, D. R. Trans. Faradav Sot. 1957, 53, 1269.
The thermochemistry chlorides GALINA
M. MOSELHYa
of the chloro-benzoyl and HUW 0. PRITCHARD
ChemistryDepartment, York University, Downsview,Ontario M3J lP3, Canada (Received7 April I975) The enthalpies of hydrolysis of the three isomers of chloro-benzoyl chloride have been measured in a diphenyl-ether calorimeter at 300.05 K. The enthalpy change for the reactions: (0, m, orp)Cl *CBH&OCl(l) + HaO(l) = (o, m,or p)Cl *CBH&OaH(s) -t HCl(g), is AZ&(300.05 K)/kcalth mol-1 AHZ(298.15 K)/kcalSh mol-l (observed) (corrected) -(9.502 f 0.060) ortho: -(9.561 & 0.045) meta : -(9.885 A 0.062) -(9.826 i 0.080) -(10.453 I-t 0.100) para: -(lo.512 rt 0.087) where the error limits refer to twice the standard deviation of the mean. A small error detected in our earlier work on the hydrolysis of benzoyl chloride leads to revised values for the analogous reaction of benzoyl chloride itself of -(8.189 i 0.050) and -(8.200 & 0.058) kcal,, mol-” for the two separate determinations at 300.05 K; thus AIG’(CsH,COCl, L298.15 K) = -(37.60 f 0.09) kcal,, mol-l.
1. Introduction A recent paper from this laboratory described an accurate determination of the enthalpy of formation of benzoyl chloride via hydrolysis to benzoic acid.(l) The value obtained was in poor agreement with that determined by Hu and Sinke from combustion calorimetry,@’ suggesting that some minor imperfections may still remain in their experimental techniques. In view of the central importance of the chlorobenzoic acids in modern combustion calorimetry(354) and in view of small errors that have occurred in recent combustion determinations for two chlorine-containing molecules,(13 5, we felt that it was a worthwhile exercise to obtain very accurate differences in enthalpies of formation for the three chloro-benzoic acids and their respective acid chlorides; these molecules are all stable and well-characterized, and can be used if necessary for further tests in the refinement of combustion techniques for organochlorine compounds. 2. Experimental The construction of the calorimeter and its operation have been described alreadyc6) and the techniques used in the present work follow very closely those used before in the hydrolysis of benzoyl chloride. (‘) The energy release was measured (a) for the hydrolysis of the acid chloride in aqueous acetone and (b) for the solution of gaseous a Formerly r. M. nETPAYKOBA.
978
G. M. MOSELHY
AND H. 0. PRITCHARD
HCI and the corresponding solid acid, to form a final solution of the same composition. The only difference in procedure arose from the necessity in one of these experiments to use acetone (80 per cent by mass) + water in which to carry out the reaction. Gaseous HCl did not dissolve instantly in this mixture and so the stirring device (figure 1 of reference 1) was modified slightly to accommodate a second inverted bell which trapped all escaping HCl bubbles and held them until they had time to dissolve.(‘) Ortho-, meta- and para-chloro-benzoyl chlorides were prepared by reaction of thionyl chloride with the parent carboxylic acid. @) To ensure isomeric purity of the samples, the starting acids were recrystallized before use, and following the usual vacuum distillation, each acid chloride was further purified by some 4 to 6 fractional recrystallizations : the melting temperatures of ortho-, meta-, and para-chloro-benzoyl chlorides were 269.4 to 269.6 K, 268.9 to 269.2 K, and 286.9 to 287.0 K, respectively. The hydrolysis of these acid chlorides takes place quite quickly in aqueous acetone but the rate of hydrolysis decreases with increasing fraction of acetone in the reaction mixture. However, the dissolution of the chloro-benzoic acids in the reaction mixture is slow, but is accelerated by increasing the fraction of acetone. Thus, it was possible in two cases to choose acetone + water mixtures in which both the hydrolysis experiment and the dissolution experiment were complete in less than 2 h, but in the case of the para-isomer, the best compromise we could achieve was about 3 h each for both the hydrolysis and the dissolution experiments using a less than ideal mass of reactant. The aqueous acetone solvent mixtures used were: ortho, 50 mass per cent of acetone; meta, 65 mass per cent of acetone; para, 80 mass per cent of acetone. The absence of side reactions in these hydrolyses was shown by the recovery of the acid obtained by allowing 3 g of each acid chloride to undergo complete hydrolysis in the appropriate acetone + water mixture, followed by evaporation of the whole reaction mixture to dryness. In each case the recovered acid was intermediate in both colour and melting properties between the best available commercial product and our own recrystallized pure acid; this test therefore established not only the cleanliness of the reaction, but also the isomeric purity of each of the acid chlorides. Other details concerned with the establishment of purity, analysis of materials, and the performance of the calorimetric experiments have been reported extensively elsewhere*(l> 9 6, ‘1 it need only be noted here that all mass measurements have been corrected for buoyancy where appropriate. 3. Results Table 1 shows the results of determination of the enthalpy of hydrolysis AH1 of each of the three acid chlorides in aqueous acetone, according to the reaction: Cl. C,H,COCl(l)
+ (x(CH3)&0 + yH,O) = {C1C6H,C02H+HCl+x(CH3)2CO+(y-1)H,0), and table 2 shows the complementary determinations of AH, for the reactions: Cl. CsH,C02H(s)
+ HCl(g) + {x(CH,),CO + (y - l)H,O) = (C1CGH4COzH+HCl+x(CH3)2CO+(y-1)HzO).
THE THERMOCHEMISTRY
OF THE CHLORO-BENZOYL
CHLORIDES
979
TABLE 1. Determination of AHI for the hydrolysis of amount of substance n of o-, m-, and p-CICBH&OCl in a mixture consisting of amounts of substance nx of (CH&CO and ny of HZ0 (calth = 4.184 J) n mm01
-AH c&
-AHI kcal, mol-’
ortho: x = 59.489 y = 191.791 13.800 325.00 23.551 13.976 328.99 23.539 13.347 314.44 23.559 13.899 327.57 23.567 13.666 321.85 23.551 mean: 23.553 standard deviation: &0.005
- ?I mm01
-AH, -AH __ calzh kcal, mol - 1
meta: x = 80.128 y = 139.100 13.032 336.11 25.791 13.171 339.87 25.805 12.959 333.32 25.722 12.810 330.32 25.786 mean: 25.716 standard deviation: iO.018
-- n mm01
-AH __ c&
-AH, kcal,, mol - 1
para: x = 163.462 y = 132.749 7.535 211.84 28.115 7.554 212.62 28.148 7.892 221.00 28.003 7.772 218.40 28.100 28.058 7.604 213.37 28.098 7.528 211.52 mean: 28.087 standard deviation: kO.021
In table 3 are listed the small correction terms AH3 for the reaction: (x(CH,),CO
+(y - 1)&O)
+HzO
= {x(CH,),CO
+ yHzO).
The procedure used here, since the term is very small, was to dissolve sufficient water in the acetone + water mixture to bring about a 1 per cent change (by mass) in its composition, i.e. to start with a mixture 0.5 per cent too dilute in water, ending up with a mixture 0.5 per cent too concentrated in water. It was assumed that over this composition range, the enthalpy change was proportional to the amount of substance of H,O added, thus giving AH3 directly. Mixing enthalpies are not known for acetone and water at 300 K, but from the known results for 288 K,(‘) calculated values of AH3 lie in the range -0.2 to -0.3 kcal,, mole1 in reasonable qualitative agreement with the figures given in table 3.7 In re-examining our procedures for estimating AH,, we have found that the values given in our previous paper on benzoyl chloride(l) are incorrect, and so table 3 includes the correct results for those experiments also. The principal source of error in these determinations of AH3 arises from the stirring correction, which becomes very much more variable the greater the acetone content of the solution. We have found that whilst stirring corrections are quite reproducible in pure solvents, i.e. water, acetone, dioxan, and also in mixtures containing less than 70 mass per cent of acetone, they become rather variable for the 80 per cent of acetone + 20 per cent of water mixture, and in fact the stirring corrections appear to depend upon how long the mixture has been standing since it was last stirred. We believe that much of this variability arises because there is an unavoidable temperature gradient in any diphenyl-ether calorimeter: our reaction vessel is between 15 and 20 cm tall, of which about the top one-third is air space. Since the pressure coefficient of the melting temperature of diphenyl ether is(“) 0.0284 K atm-l and its density is 1.0692 g cmm3, we have a natural temperature gradient of about 5 x 10S4 K between the top and the bottom of our vessel, which gives rise to rather t Throughoutthis papercalth= 4.184J; atm = 101.325kPa.
G. M. MOSELHY
980
AND H. 0. PRITCHARD
TABLE 2. Determination of AH2 for the dissolution of amounts of substance n of o-, m-, and p-Cl-C$H+COCl and n of HCl(g) in a mixture consisting of amounts of substance PIX of (CH&CO and n(r - 1) of HzO. There is always a slight mismatch between the amounts of substance of chloro-benzoic acid and hydrogen chloride, expressed as an excess &r(HCl) which results in a small correction SAH to the observed a this is estimated on the assumption that the enthalpy of solution of HCl in the mixture is the same as the enthalpy of solution in water(Q (C&I, = 4.184 J) n(C1 . CsH&OaH) lllDlO1
n(HC1)
-AH c&h
IlllTlOI
ortho: 13.737 13.737 13.732 13.738, 13.742
13.825 13.704 13.822 13.750 13.732
13.179 13.158 13.160 13.183
13.174 13.263 13.115 13.145
1.584 7.584 7.585 7.585
7.420 7.567 7.584 7.537
meta:
para:
An(HC1) pm01
-8AH kc&
caltn
x = 59.489, 196.36 194.98 395.75 195.54 194.73
(y - 1) = 190.791 87.8 -1.56 -32.8 -to.58 90.0 -1.59 11.9 -0.21 -9.1 +0.16
x = 80.128, 212.60 214.46 211.68 212.00
(v - 1) = 138.100 -4.4 +0.08 104.5 -1.85 -44.S i-O.80 -37.2 $0.66
x = 163.462, 132.25 135.25 135.01 134.47
(y - 1) = 131.749 -163.5 +2.90 -7.8 $0.14 -0.2 +o.oo -47.8 +0.85
14.187 14.238 14.155 14.195 14.176 mean : 14.190 standard deviation: f0.014
16.138 16.158 16.146 16.132 mean : 16.144 standard deviation: ~tO.006
mean : standard deviation:
TABLE
-A.& mol-l
17.821 17.853 17.802 17.842 17.830 rto.011
3. Correction terms Al& for the differential enthalpy of dissolution of water in the reaction mixture. The required reaction is {x*solvent + (y - l)HzO} + Hz0 = {x*solvent $ yH~0) (cahh = 4.184 J)
Experiment benzoyl chloride benzoyl chloride ortho-chloro-benzoyl meta-chloro-benzoyl para-chloro-benzoyl
chloride chloride chloride
Solvent
-&
Y iii&l
dioxan acetone acetone acetone acetone
44.126 50.264 59.489 80.128 163.462
143.865 198.053 191.791 139.100 132.749
-AH3 srnol-l 0.201 0.172 0.198 0.253 0.255
rt 0.004 f 0.002 f 0.002 2~ 0.003 f 0.007
THE THERMOCHEMISTRY
OF THE CHLORO-BENZOYL
981
CHLORIDES
TABLE 4. Determination of A& at 300.05K (caIth = 4.184 I) benzoyl chloride set 1 set 2 AiTJkcalth mol-l A&/kc&, mol-” AHs/kcalth mol-l AH,/kcah
mol-l
a
cbloro-benzoyl chlorides orthometapara-
-22.455
-24.138
-23.553
-25.776
- 14.438 -0.172
-16.139
-14.190
-16.144
-0.201 -8.200
-0.198 -9.561
-0.253 -9.885
-8.189
-28.087 -17.830 -0.255 -10.512
“AHq=AHI-AHa$AH3
complicated evaporational effects, significant in size for the 80 mass per cent acetone solution in comparison with the magnitudes of the stirring corrections. The raw experimental results are then assembled in table 4 to give the enthalpy change AH4 for the overall reaction:
Cl* C,H,COCl(l)
+ H20(l)
= Cl- C,H,CO,H(s)
+ HCl(g),
and table 5 presents a summary of the errors involved in each determination, how the individual terms contribute to the final overall error.
showing
4. Discussion
The main purpose of this work has been to try to establish pairs of differences in enthalpies of formation between acids and acid chlorides for compounds which are significant in the combustion calorimetry of organic chlorine-containing moleculesTABLE 5. Summary of accumulation of errors 6AH in the determination of quoted in the body of this table are standard deviations (calth = 4.184 .I) AH,
Source of error
Calculation of error
SAH/kca&,,
ortho-
individual measurements (observed) a z!zO.O07 individual measurements (observed) a xtO.016 analytical measurements 10.1 per cent of AH, ztO.014 stirringcorrection observed zto.002 electricalcalibration ~tO.007 per cent of AH4 &O.OOl Accumulatedstandarddeviation: 10.023 Total error (twicethe standarddeviation): 10.045
the values
AHa;
mol-1
meta50.019 1tO.018 ho.016 z!to.O03 &O.OOl
para10.025 kO.030 ho.018 +0.007 +0.001
ho.031
-&0.044
10.062
ho.087
* The errorsquotedin this tablefor the individualmeasurements of AHI andAH2 ace somewhat largerthan thoseshownin tables1 and2: thoseerrorsdo not includeallowancefor the tmcertainty in the magnitudeof the stirringcorrection,sincein eachseriesof experiments, a constantaverage stirring correctionwasused.As describedin the text, a differentstirring devicewasusedin the determinationof AHa and, asthis wasmuchmorecumbersome, it gaveriseto muchlargeruncertaintiesfrom thiscause.The samedevicewasusedto measure AHi andAH,. but the errorsarising from the stirring correctionare smallerfor AH3 than for AH1 becauseof the short stirring times requiredin the formerexperiments.
982
G. M. MOSELHY
AND
H. 0. PRITCHARD
and at least one if not more of these three acids is in fact being considered at the present time as a secondary thermochemical standard for organic chlorine-containing molecules.(3’ 4, These differences AH, have been established within very close limits, which we believe have been assessed realistically; the errors become larger, as expected, with increasing acetone content of the reaction mixtures since in general they reflect the increasing difficulty of performing accurate thermochemistry and accurate chemical analyses of increasingly volatile solutions. Nevertheless, as far as combustion calorimetry is concerned, all three vaIues of AH4 (and also the-now corrected-corresponding difference for benzoic acid and benzoyl chloride) may be taken as being virtually exact, and therefore to provide the basis for a thorough evaluation of rotating-bomb calorimeter techniques for organochlorine compounds. Using the auxiliary data quoted in our earlier paper on benzoyl chloride,@ together with the recent enthalpy of formation results for the mono-chloro-benzoic acids,(4) we can write down the following estimates for the standard enthalpies of formation for these four acid chlorides, as follows:
benzoyl chloride ortho-chloro-benzoyl meta-chloro-benzoyl para-chloro-benzoyl
chloride chloride chloride
AHi(298.15 K) kcal,, mol- 1 -(8.136&0.050) -(9.502+0.060) -(9.826,0.080) -(10.453~0.100)
AH;(R.COCI, 1,298.15 K) kcal,, mol-l -(37.60+0.09) -(41.00~0.19) -(45.40&0.38) -(45.63+0.21)
To do this, we have assumed that over the range 298 to 300 K, the molar
heat capacities for all four acids are the same,(” 4, and for all four acid chlorides are the same;(‘) we have also included a correction of 0.011 kcal,, mol-‘, being the difference between the enthalpy of formation of gaseous HCl at 1 atm for the real gas and for the standard state.@) We have expanded the error limits of AHF(298.15 K) somewhat arbitrarily to accommodate these estimates.
This work was supported by the National Research Council of Canada, and was greatly facilitated by the collaboration of Dr J. V. Davies in the early stages. REFERENCES 1. Davies, J. V.; Dunning, B. K.; Pritchard, H. 0. J. Chem. Thermodynamics 1972, 4, 731. 2. Hu, A. T.; Sinke, G. C. J. Chem. Thermodynamics 1969, 1, 507. 3. Hu, A. T.; Sinke, G. C.; Mbsson, M.; Ring&r, B. J. Chem. Thermodynamics 1972, 4, 283. 4. Johnson, W. H.; Prosen, E. J. J. Res. Nat. Bur. Stand. 1974, 78A, 683. 5. Hu, A. T. ; Sinke, G. C.; Mintz, M. J. J. Chem. Thermodynamics 1972, 4, 239. 6. Davies, J. V.; Pritchard, H. 0. J. Chem. Thermodynamics 1972, 4, 9. 7. Moselhy, G. M. M.Sc. Thesis, York University, Toronto, Canada. 1974. 8. Mever. H. Mona&h. 1901.22, 415. 9. I&r&ional Critical Tabies, i929, 5, 157. 10. Dainton, F. S.; Diaper, J.; Ivin, K. J.; Sheard, D. R. Trans. Faradav Sot. 1957, 53, 1269.