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The enthalpy of formation of tetracyanomethane
J. Chem. Thermodynamics1973, 5, 481-483
The enthalpy of formation of tetracyanomethane D. S. BARNES and C. T. MORTIMER
Department of Chemistry, University of Keele, Keele, Staffordshire ST5 5BG, U.K. and E. MAYER
Institut fiir Anorganische und Analytische Chemie, Universitgit lnnsbruck, Innsbruck, Austria (Received 2 November 1972) Tetracyanomethane has been burned in a rotating-bomb combustion calorimeter. The standard enthalpy of combustion was AHg(CsN4, c)=--(616.43 :~ 0.4) kcalt~mo1-1, which yields a standard enthalpy of formation AH~(C~N4,c) =--(146.18 ~ 0.43) kcalth mol-L The mean bond dissociation energy
1. Introduction Cyanocarbon compounds have been the subject of several thermochemical investigations, ~1) but tetracyanomethane, which has been prepared only recently, (2) has not been studied thermochemically. The purpose o f this investigation was to establish the enthalpy of formation of this compound.
2. Experimental Tetracyanomethane, a white crystalline solid, was prepared by the method reported previously. (2) It was sublimed twice and was estimated to have a purity of not less than 99.9 moles per cent. (2) The compound showed no loss of mass in air and was burned without the precaution of enclosing the pelleted material in a Melinex bag. Combustions were made in a rotating-bomb calorimeter (261 cm 3 capacity) designed by Professor S. Sunner and constructed at the University of Lund, Sweden. Temperature was measured by noting the change AR in the resistance of a platinum resistance thermometer. The bomb, which was charged with 50 cm 3 of de-ionized distilled water, and oxygen at an initial pressure of 30 atm, was rotated after combustion to ensure that the final solution of nitric acid was homogeneous.t The combustion was vigorous and the large volume of water was used to prevent the neoprene ring, which sealed the bomb, from coming into contact with the hot gases. t Throughout this paper atria = 101.325 kPa; ealt~ = 4.t84 J.
482
D. S. BARNES, C. T. MORTIMER, AND E. MAYER
A f t e r each c o m b u s t i o n the contents o f the b o m b were e x a m i n e d for traces o f soot which were weighed. A t h e r m o c h e m i c a l c o r r e c t i o n was m a d e for this residue. A l l c o m b u s t i o n s were i n i t i a t e d electrically at 298.15 K b y fusing a thin p l a t i n u m wire, which set fire to a c o t t o n t h r e a d ( f o r m u l a : CHLTOo.s) a t t a c h e d to the pellets o f t e t r a c y a n o m e t h a n e . N i t r i c acid in the final b o m b s o l u t i o n was estimated b y t i t r a t i o n with p o t a s s i u m hydroxide. The s t a n d a r d e n t h a l p y o f c o m b u s t i o n o f t e t r a c y a n o m e t h a n e was calculated b y use o f the m e t h o d given in detail b y H u b b a r d , Scott, a n d W a d d i n g t o n J 3~ The e n t h a l p y o f s u b l i m a t i o n o f the c o m p o u n d was m e a s u r e d by use o f a PerkinE l m e r differential scanning calorimeter ( D S C 1), a c c o r d i n g to the m e t h o d o f Beech a n d L i n t o n b o n . ~4~
3. Results and discussion Weights used were c a l i b r a t e d against N P L standards. T h e a t o m i c weights used are those r e c o m m e n d e d b y the I U P A C C o m m i s s i o n J 5) U n c e r t a i n t y intervals are given as twice the s t a n d a r d d e v i a t i o n o f the mean. The results o f five c o m b u s t i o n experiments are s h o w n in table 1. The energy equivalent e~f o f the s t a n d a r d c a l o r i m e t e r system, c o n t a i n i n g neither water, c a r b o n dioxide, oxygen, n o r crucible, was d e t e r m i n e d b y c o m b u s t i o n o f a sample o f benzoic acid ( B D H T h e r m o c h e m i c a l S t a n d a r d , b a t c h N o . 503341) having AuB - ( 6 3 1 8 . 1 4 + 0.7) calth g - l , u n c e r t a i n t y interval s(Aua) = 0.011 p e r cent u n d e r s t a n d a r d conTABLE 1. Combustion of tetracyanomethane, M(CsN4) = 116.0818 g mol-l: m(C~N4) and m are the masses burned of tetracyanomethane and of cotton fuse; AR is the corrected change during combustion of the resistance of the platinum resistance thermometer; AE~.bp. is the energy change for the isothermal bomb process (item 68 in reference 3); AE(HNO3) is the energy correction for the formation of nitric acid (item 92); AE(corr) is the energy correction to standard states (items 81 to 85, 87 to 90, 93, and 94); AE(carbon) is the energy correction for the carbon residue, which was taken as the product of its mass and its specific energy of combustion (7.84 caleb rag-l); AE(fuse) is the energy correction for combustion of the cotton fuse (3881 calt~ g-1); esr is the energy equivalent of the standard calorimeter system [(68648 ± 8) calth f~-1; see text]; Ae ° is the standard specific energy of combustion of tetracyanomethane (calth = 4.184 J) Experiment
1
m(CsN4)/g a 0.98859 mJg 0.00587 AR/f~ 0.07654 AELb.p./calt~ --5295.24 AE(HNO3)/calth 10.01 AE(corr)/calth 27.89 AE(carbon)]calth -30.02 AE(fuse)]calth 22.78 Ae~*(CsN4)/cal~hg-1 --5325.34
2~
3
4
5
0.991525 1.011805 0.98836 0.987935 0.00576 0.006055 0.00574~ 0.00608 0.07669 0.07845 0.07655 0.07671 --5305.61 --5427.37 --5295.92 --5306.59 10.41 10.52 10.63 10.28 27.97 28.57 27.88 27.87 --29.40 --14.97 --21.64 -13.01 22.35 23.50 22.30 23.60 --5319.36 --5316.98 --5318.66 --5322.06
Mean values: AU~(CsN4, c) =--(617.61 ± 0.4) kcalth mol-1; AnRT = +1.18 kealth mol-1; AH~(CsN4, c) = --(616.43 ± 0.4) kcalth mo1-1 p(298.15 K) = 1.57 gcm -~.
THE ENTHALPY OF FORMATION OF TETRACYANOMETHANE
483
ditions. The "recommended" conditions were used, in that the bomb was charged with 0.78 cm a of water and oxygen at an initial pressure of 30 arm. The mass of sample burned (1.1 g) was greater than the recommended quantity and under these conditions had AuB = - ( 6 3 1 8 . 4 7 _+ 0.7) calthg -1. The bomb was rotated after combustion. The combustion of tetracyanomethane is represented by the equation: C(CN),(c) + 502(g ) = 5CO2(g) +2N2(g), for which we obtain the value AH°(CsN4, c ) = - ( 6 1 6 . 4 3 ___ 0.4) kcalthmO1-1. Using the value ~6) AH~(CO2, g ) = - ( 9 4 . 0 5 1 __+0.031) kcalth mol -~, we derive the standard enthalpy of formation of crystalline tetracyanomethane: AH~(CsN4, c) = +(146.18 __+0.43) kcalth tool -1. The enthalpy of sublimation of tetracyanomethane has been found, from the mean value of seven experiments, to be (14,6 __+2.1) kealth tool -1, so that we obtain AH~(CsN~, g) = +(160.8 _+ 2.2) kcalth mo1-1. The mean bond dissociation energy ( D ) ( C - - C N ) which is one quarter of the enthalpy of the gas-phase reaction C(CN).~ = C +4CN, may be calculated from the relation: ( D ) ( C - - C N ) = ¼AH~(C, g)+ AH~(CN, g)-kAH~(CsN4, g). Using the AH~(g) values: ~7) C, 171.3, and CN, 109 kcalth tool -~, we obtain the value ( D ) ( C - - C N ) = 112 kcalth mol -a. REFERENCES 1. Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds. Academic Press, London, 1970. 2. Mayer, E. Monatsh. Chem. 1969, 100, 462. 3. Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental Thermochemistry, Vol. 1. Rossini, F. D.; editor. Interscience: New York. 1956, chap. 5. 4. Beech, G.; Lintonbon, R. M. Thermochem. Aeta 1971, 2, 86. 5. Greenwood, N. N. Chem. Brit. 1970, 6, 119. 6. CODATA, J. Chem. Thermodynamics 1972, 4, 331. 7. Nat. Bur. Stand. (U.S.) Tech. Note 270-3. 1968.
The enthalpy of formation of tetracyanomethane D. S. BARNES and C. T. MORTIMER
Department of Chemistry, University of Keele, Keele, Staffordshire ST5 5BG, U.K. and E. MAYER
Institut fiir Anorganische und Analytische Chemie, Universitgit lnnsbruck, Innsbruck, Austria (Received 2 November 1972) Tetracyanomethane has been burned in a rotating-bomb combustion calorimeter. The standard enthalpy of combustion was AHg(CsN4, c)=--(616.43 :~ 0.4) kcalt~mo1-1, which yields a standard enthalpy of formation AH~(C~N4,c) =--(146.18 ~ 0.43) kcalth mol-L The mean bond dissociation energy
1. Introduction Cyanocarbon compounds have been the subject of several thermochemical investigations, ~1) but tetracyanomethane, which has been prepared only recently, (2) has not been studied thermochemically. The purpose o f this investigation was to establish the enthalpy of formation of this compound.
2. Experimental Tetracyanomethane, a white crystalline solid, was prepared by the method reported previously. (2) It was sublimed twice and was estimated to have a purity of not less than 99.9 moles per cent. (2) The compound showed no loss of mass in air and was burned without the precaution of enclosing the pelleted material in a Melinex bag. Combustions were made in a rotating-bomb calorimeter (261 cm 3 capacity) designed by Professor S. Sunner and constructed at the University of Lund, Sweden. Temperature was measured by noting the change AR in the resistance of a platinum resistance thermometer. The bomb, which was charged with 50 cm 3 of de-ionized distilled water, and oxygen at an initial pressure of 30 atm, was rotated after combustion to ensure that the final solution of nitric acid was homogeneous.t The combustion was vigorous and the large volume of water was used to prevent the neoprene ring, which sealed the bomb, from coming into contact with the hot gases. t Throughout this paper atria = 101.325 kPa; ealt~ = 4.t84 J.
482
D. S. BARNES, C. T. MORTIMER, AND E. MAYER
A f t e r each c o m b u s t i o n the contents o f the b o m b were e x a m i n e d for traces o f soot which were weighed. A t h e r m o c h e m i c a l c o r r e c t i o n was m a d e for this residue. A l l c o m b u s t i o n s were i n i t i a t e d electrically at 298.15 K b y fusing a thin p l a t i n u m wire, which set fire to a c o t t o n t h r e a d ( f o r m u l a : CHLTOo.s) a t t a c h e d to the pellets o f t e t r a c y a n o m e t h a n e . N i t r i c acid in the final b o m b s o l u t i o n was estimated b y t i t r a t i o n with p o t a s s i u m hydroxide. The s t a n d a r d e n t h a l p y o f c o m b u s t i o n o f t e t r a c y a n o m e t h a n e was calculated b y use o f the m e t h o d given in detail b y H u b b a r d , Scott, a n d W a d d i n g t o n J 3~ The e n t h a l p y o f s u b l i m a t i o n o f the c o m p o u n d was m e a s u r e d by use o f a PerkinE l m e r differential scanning calorimeter ( D S C 1), a c c o r d i n g to the m e t h o d o f Beech a n d L i n t o n b o n . ~4~
3. Results and discussion Weights used were c a l i b r a t e d against N P L standards. T h e a t o m i c weights used are those r e c o m m e n d e d b y the I U P A C C o m m i s s i o n J 5) U n c e r t a i n t y intervals are given as twice the s t a n d a r d d e v i a t i o n o f the mean. The results o f five c o m b u s t i o n experiments are s h o w n in table 1. The energy equivalent e~f o f the s t a n d a r d c a l o r i m e t e r system, c o n t a i n i n g neither water, c a r b o n dioxide, oxygen, n o r crucible, was d e t e r m i n e d b y c o m b u s t i o n o f a sample o f benzoic acid ( B D H T h e r m o c h e m i c a l S t a n d a r d , b a t c h N o . 503341) having AuB - ( 6 3 1 8 . 1 4 + 0.7) calth g - l , u n c e r t a i n t y interval s(Aua) = 0.011 p e r cent u n d e r s t a n d a r d conTABLE 1. Combustion of tetracyanomethane, M(CsN4) = 116.0818 g mol-l: m(C~N4) and m are the masses burned of tetracyanomethane and of cotton fuse; AR is the corrected change during combustion of the resistance of the platinum resistance thermometer; AE~.bp. is the energy change for the isothermal bomb process (item 68 in reference 3); AE(HNO3) is the energy correction for the formation of nitric acid (item 92); AE(corr) is the energy correction to standard states (items 81 to 85, 87 to 90, 93, and 94); AE(carbon) is the energy correction for the carbon residue, which was taken as the product of its mass and its specific energy of combustion (7.84 caleb rag-l); AE(fuse) is the energy correction for combustion of the cotton fuse (3881 calt~ g-1); esr is the energy equivalent of the standard calorimeter system [(68648 ± 8) calth f~-1; see text]; Ae ° is the standard specific energy of combustion of tetracyanomethane (calth = 4.184 J) Experiment
1
m(CsN4)/g a 0.98859 mJg 0.00587 AR/f~ 0.07654 AELb.p./calt~ --5295.24 AE(HNO3)/calth 10.01 AE(corr)/calth 27.89 AE(carbon)]calth -30.02 AE(fuse)]calth 22.78 Ae~*(CsN4)/cal~hg-1 --5325.34
2~
3
4
5
0.991525 1.011805 0.98836 0.987935 0.00576 0.006055 0.00574~ 0.00608 0.07669 0.07845 0.07655 0.07671 --5305.61 --5427.37 --5295.92 --5306.59 10.41 10.52 10.63 10.28 27.97 28.57 27.88 27.87 --29.40 --14.97 --21.64 -13.01 22.35 23.50 22.30 23.60 --5319.36 --5316.98 --5318.66 --5322.06
Mean values: AU~(CsN4, c) =--(617.61 ± 0.4) kcalth mol-1; AnRT = +1.18 kealth mol-1; AH~(CsN4, c) = --(616.43 ± 0.4) kcalth mo1-1 p(298.15 K) = 1.57 gcm -~.
THE ENTHALPY OF FORMATION OF TETRACYANOMETHANE
483
ditions. The "recommended" conditions were used, in that the bomb was charged with 0.78 cm a of water and oxygen at an initial pressure of 30 arm. The mass of sample burned (1.1 g) was greater than the recommended quantity and under these conditions had AuB = - ( 6 3 1 8 . 4 7 _+ 0.7) calthg -1. The bomb was rotated after combustion. The combustion of tetracyanomethane is represented by the equation: C(CN),(c) + 502(g ) = 5CO2(g) +2N2(g), for which we obtain the value AH°(CsN4, c ) = - ( 6 1 6 . 4 3 ___ 0.4) kcalthmO1-1. Using the value ~6) AH~(CO2, g ) = - ( 9 4 . 0 5 1 __+0.031) kcalth mol -~, we derive the standard enthalpy of formation of crystalline tetracyanomethane: AH~(CsN4, c) = +(146.18 __+0.43) kcalth tool -1. The enthalpy of sublimation of tetracyanomethane has been found, from the mean value of seven experiments, to be (14,6 __+2.1) kealth tool -1, so that we obtain AH~(CsN~, g) = +(160.8 _+ 2.2) kcalth mo1-1. The mean bond dissociation energy ( D ) ( C - - C N ) which is one quarter of the enthalpy of the gas-phase reaction C(CN).~ = C +4CN, may be calculated from the relation: ( D ) ( C - - C N ) = ¼AH~(C, g)+ AH~(CN, g)-kAH~(CsN4, g). Using the AH~(g) values: ~7) C, 171.3, and CN, 109 kcalth tool -~, we obtain the value ( D ) ( C - - C N ) = 112 kcalth mol -a. REFERENCES 1. Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds. Academic Press, London, 1970. 2. Mayer, E. Monatsh. Chem. 1969, 100, 462. 3. Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental Thermochemistry, Vol. 1. Rossini, F. D.; editor. Interscience: New York. 1956, chap. 5. 4. Beech, G.; Lintonbon, R. M. Thermochem. Aeta 1971, 2, 86. 5. Greenwood, N. N. Chem. Brit. 1970, 6, 119. 6. CODATA, J. Chem. Thermodynamics 1972, 4, 331. 7. Nat. Bur. Stand. (U.S.) Tech. Note 270-3. 1968.