COM MUNICAT1ONS
Themochemical Properties of Peroxyacetyl (PAN) and Peroxybenzoyl Nitrate ( PBN) Eugene S . Domalski National Bureau of Standards, Institute for Materials Research, Washington, D.C. 20234
The thermochemical properties of peroxyacetyl nitrate (PAN) and peroxybenzoyl nitrate (PBN) were estimated by means of bond and group additivity schemes. Values are given at 298.15 K for the following properties: AHfo(g), Cpo(g), So(g), Cpo(liq), AHvap, and AHsublim. ; these values were used to calculate ASfo(g), AGf"(g), ACpo[(liq) - (g)], AHfO(lis), and AHf"(s), which are also cited.
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eroxyacetyl nitrate (PAN), CHZ(CO)OONOZ, and peroxybenzoyl nitrate (PBN), CsH5(CO)OON02, are formed when sunlight acts upon air which is polluted with trace amounts of partially oxidized hydrocarbons and nitrogen oxides. Thermodynamic information is necessary in systems involving these compounds to determine whether certain reactions are likely to proceed, to estimate equilibrium concentrations, and to calculate the energetics of specific reactions which may be taking place. Experimental data from which the thermochemical properties of PAN and PBN may be derived are virtually nonexistent. An experimental program designed to provide reliable thermochemical data on these compounds will undoubtedly be expensive and time consuming. We believe that certain estimation methods in thermochemistry have developed sufficiently to supply values which will be useful in the absence of measured data. The estimates presented here are to be considered as forerunners, and should ultimately be superceded by data derived from experimental measurements. [NOTE : The names peroxyacetyl nitrate and peroxybenzoyl nitrate are misnomers for the structures they represent. The peroxyacetyl group, CH3(CO)O0, when combined with the nitrate group, O N 0 2 ,would give CH3(CO)OOON02. Similar logic can be applied to peroxybenzoyl nitrate. Acetyl peroxynitrate and benzoyl peroxynitrate are names which are more accurate descriptions of the intended structures.] The estimation methods of Benson (1968), Benson and Buss (1958), Benson et al. (1969), Bondi (1963, 1968), and Shaw (1969) were used to obtain the estimated thermochemical data on PAN and PBN. Although other estimation schemes are available, the above empirical methods are easy to apply, straightforward, and are based upon existing data extensive enough to provide values usually approaching, or within, experimental error. The standard thermochemical properties at 298.15OK we have estimated by the fore-mentioned procedures * are : enthalpy of formation, AHfo(g); heat capacity at constant pressure, Cpo(g) and Cpo(liq); entropy, So(g); enthalpy of vaporization, AHvapO ; and enthalpy of sublimation, AHsublimo. Other thermochemical properties calculated from
these estimates are: entropy of formation, ASfo(g); Gibbs energy of formation, AGfo(g); heat capacity change accompanying the liquid-vapor transition, ACpo[(liq) - (g)]; the enthalpies of formation of the liquid, AHfO(1iq); and solid states, AHf"(s). The bond additivity schemes of Benson and Buss (1958) and Benson (1968) rather than their group additivity method were used to estimate AHfO, Cpo, and So in the gaseous states because data were lacking for the contribution of the group in which an oxygen atom is bonded to a nitro group and another oxygen atom, 0-(N02)(0). The recent publication of Benson et al. (1969), which uses the group additivity method exclusively, was very useful in examining the thermochemical estimates for compounds with a related structure. Although we chose the bond additivity procedure, the group additivity method could have been applied after having found an appropriate estimate for the 0-(N02)(0) grouping. In calculating Cpo(g), we assigned a value of 10 cal deg-1 mol-' to the contribution from the 0-NOz bond, and obtained it from data on Cpo(g) for alkyl nitrates (Stull et al., 1969). The group increments provided by Bondi (1963, 1968) were used to obtain estimates of the enthalpy of vaporization and sublimation. Group increments for > C = 0, -0-, and -ONOz were added to obtain the contribution for -(CO)-O-ONO2. We acknowledge the dangers in adding increments of polar atoms and groups to obtain values for larger groups as above. Resonance interactions usually tend to lower the values of larger polar groups calculated by summing smaller ones. We have applied an arbitrary correction of -2 kcal mol-' to the vaporization data, and -3 kcal mol-' to the sublimation data for these interactions. The group contribution method of Shaw (1969) was used to estimate Cpo in the liquid state at 298.15"K. The difference between the contribution for the CO-(C)z and CO-(C)(O) groupings, 5.6 cal deg-l mol-', was used as an estimate of the change which occurs when carbon is replaced by oxygen in polar groups. Using this substitution, we assigned values of 3 and 23 cal deg-l mol-' to the respective groups, 0-(CO)(O) and 0-(NOz)(0), which were not part of Shaw's (1969) group listing. The usual magnitude of the uncertainties developed from the estimation methods of Benson (1968) and Benson and Buss (1958) are =t2 kcal mol-' for AHfO, and = t l cal deg-l mol-' for Cp" and So. However, for highly branched or polar molecules, the uncertainty can easily double. Shaw (1969) reports an uncertainty within i1.5 cal deg-l mol-' for the heat capacities of the liquids he has calculated. The estimation schemes of Bondi (1963, 1968) for AHvapO and AHsublimO are uncertain to about + l kcal mol-'. Along with the thermochemical data on PAN and PBN provided in Table I, we have given uncerVolume 5, Number 5, May 1971 443
Table I. Thermochemical Properties of PAN and PBN PAN PBN Literature Estimated property at 298.15 K [Benson (1968) AHf" (g), kcal -57 ( 1 2 ) molL1 Buss (1958) Benson (1968) 26 ( 1 3 ) 35 (=t3) { Benson and Buss (1958) IBenson (1968) So (g), cal deg-1 98 ( 1 4 ) 116 ( 1 4 ) Benson and mol-' Buss (1958) AHvap", kcal 13 ( 1 2 ) 18 (+2) Bondi(1968) mol-' AHsublim", 17 ( 1 2 ) 23 ( 1 2 ) Bondi (1963) kcal mol-' (Bondi (1968) Cp" (liq), cal 42 (is) 60 ( 1 5 ) Shaw (1969) deg-I mol-' Calculated property at 298.1 5°K ASfo (g), cal -97 ( 1 4 ) deg-1 mol-' AGf" (g), kcal -28 ( 1 3 ) mol-' AHf " (liq), kcal - 70 (+3) mol-' AHf" (s), kcal -74 ( 1 3 ) mol-' 16 ( 1 6 ) ACp" [(liq) ( d l , deg-l mol-'
-117 ( 1 4 ) calcd +5 ( 1 3 ) calcd
-48 ( 1 3 ) calcd
- 53 ( 1 3 ) calcd 25 ( 1 6 ) calcd
tainties, in parentheses, for the properties we have estimated. A certain amount of arbitrariness has been exercised in the uncertainty assignments. To calculate ASf", we used values for the entropies of the elements in their standard states at 298.15'K, as tabulated by Wagman et al. (1968). The recent thermodynamic and thermochemical texts by Stull et al. (1969) and Cox and Pilcher (1970) were very helpful in providing data for use as auxiliary checks. It is interesting to note that, as a result of applying the estimation methods, we find that under equivalent conditions, PAN would form spontaneously from its elements (negative AGf"), and PBN would decompose to its elements (positive AGf "). Literature Cited
Benson, S. W., "Thermochemical Kinetics. Methods for the Estimation of Thermochemical Data and Rate Parameters," J. Wiley and Sons, Inc., New York, 1968. Benson, S. W., Buss, J. H., J . Chem. Plzys. 29, 546-72 (1958). Benson, S. W., Cruickshank, F. R., Golden, D. M., Haugen, G . R., O'Neal, H. E., Rodgers, A. S., Shaw, R., Walsh, R., Chem. Rev. 69,279-324 (1969). Bondi, A,, J. Chem. Eng. Data 8, 371-81 (1963). Bondi, A,, "Physical Properties of Molecular Crystals, Liquids, and Glasses," J. Wiley and Sons, Inc., New York, 1968. Cox, J. D., Pilcher, G., "Thermochemistry of Organic and Organometallic Compounds," Academic Press, Inc., New York and London, 1970. Shaw, R., J. Cliem. Eng. Data 14, 461-5 (1969). Stull, D. R., Westrum, E. F., Jr., Sinke, G. C., "The Chemical Thermodynamics of Organic Compounds," J. Wiley and Sons, Inc., New York, 1969. Wagman, D. D., Evans, W. H., Parker, V. B., Halow, I., Bailey, S. M., Schumm, R. H., NBS Technical Note 270-3, January 1968. Received for review October 12, 1970. Accepted December 26, 1970.
Calculation of Excess Air for Combustion Processes Using 0,-to-N, Ratio R. L. Miller and J. D. Winefordner Department of Chemistry, University of Florida, Gainesville, Fla. 32601
For most incinerators, it is possible to estimate the percentage of air in excess of the air needed for complete combustion of the fuel by simply measuring the ratio of O2to N z . This method is simpler and more reliable than the standard method of estimating excess air by measuring the percentage of coz.
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n the analysis of incinerators and stack gases, it is desirable to have a convenient means for measuring the amount of air in excess of the air needed to burn the fuel completely-Le., stoichiometric combustion. In the Orsat analysis (Stern, 1968), information is provided that can be used to estimate the percentage excess air. From a brief ex444
Environmental Science & Technology
posure to stack sampling, it seemed apparent that for many incinerators, the ratio of O2to Nawould provide a better means for estimating the percentage of excess air where the composition of the fuel is variable and the incinerator temperature is not excessively hot, causing the Na in the air to oxidize. A theoretical comparison of the percent COa and 0 2 to NZ method is given below.
Comparison of Methods Percent COZMethod. The typical determination of excess air from the percentage of COz using the Orsat instrument has the following limitations : (a) the percentage excess air represented by a certain percentage COz varies with the fuel type; and (6) other acidic gases-eg., SOn, HC1, NO,, etc.typically present in stack gases interfere with the determination of C 0 2by NaOH absorption.
