Formation, enthalpy of

Values of A H° were also obtained from the appearance potentials of N2H arising from nitrogen-hydrogen compounds (see p. 16). The recommended values are 1036.0 3.8 [6] and 1035.5 kJ/mol [2]. Other values given in [7 to 10] infer an incorrect value of Ap(N2). 

The entropy of N2H- at 298 K, S° = 200 2 [3] and 202 J-mol K [14], was calculated using statistical mechanics. Ab initio values of vibrational frequencies were used in the latter calculation [14]. Vibrational contributions to the entropy were neglected in the former calculation [3]. 

To calculate the heat of fomation of I2O5 by application of Hess law. 

The following reactions (1)—(9) are all such that AH can be measured directly (Long, Quart. Rev. Chem. Soc. 1953, 7, 140). The values of —AH according to Rossini et al. ( Selected values of diemical thermodjmamic properties , U.S. Nat. Bureau Stand. 1952) are quoted — 

To calcTilate the enthalpy change in the hj othetical reaction (Long, Quart. Rev. Chem. Soc. 1953, 7, 142) 

The calorie used in the tables is the thermochemical calorie defined by 1 cal = 4.1840 J. 

Enthalpy of formation of the product in a chemical reaction is the difference in enthalpy of the product and sum of enthalpy of all reactants. In order to demonstrate this, let us consider the combustion process under an SSSF process. The first law of thermod5mamics for a reaction process in a control volume and assuming SSSF process is given as 

Considering the enthalpy of the reactants as zero for a reference state of 25°C, 0.1 MPa, the enthalpy of the product at the reference state is then given by the net heat transfer, and this is termed as the enthalpy of formation of product 

The enthalpy of formation or the heat transfer quantity can be determined by experiment but are typically determined by statistical thermodynamics for different compounds. Enthalpy of formation for some of the common elements and compounds are given in Table C.6. 

Enthalpy of formation of the components and compounds at any other states relative to the reference base states is estimated by adding the change in enthalpy between the given state and the reference state of 25°C, 0.1 MPa, as 

Aft298 c, 0.1 MPa. Tj = Change of enthalpy between a state and the reference state 

Only one investigation of the absorption spectrum of NH2 in the electronic ground state X Bi, using a tunable difference-frequency laser spectrometer in the 3 pm region, was reported. Fifty-three rovibronic transitions in the V3 band were measured in the limits from 3510.677 cm (assigned to the transition 11q n(J = 10)- -10o o( = 9)) and 3186.295 cm (9o,9(J = 8) 10o,io(J = 9)) [11- 

Oscillator strengths and vertical energy differences were calculated for transitions between electronic states of the same multiplicity using ab initio (SCF and multi reference double Cl) methods [2]. 

Reactions of NHj and ND2 with a number of molecules were investigated using various methods. The results (partly reviewed in [1]) are listed in Table 18. 

Total rate constant 10 ° k in cm -molecule s at room temperature if not otherwise stated. ArH (product ion) is the enthalpy of reaction for NH2 + reactantproduct ion + neutral. 

Significant discrepancies are apparent [2] between ICR [7 to 11] and SIFT [2] data especially concerning the product ion distribution. 

2 Enthalpy of Formation AHf (in kcal/mol unless stated otherwise) 

AHf 0 = 26.1 2.3 was derived [1] from the dissociation energy Do(OF) = 51.4 kcal/mol [2]. The error limits obviously correspond to an error of 0.1 eV [2] in the measured appearance potentials. This enthalpy of formation was cited by Baulch etal. [3,4]. AHJ 3qo = 26 1 was given in a data compilation by Benson [5]. An older estimate Dq(OF) = 51.4 10 kcal/mol led to AH = 26 10 [6]. The OFenthalpy of formation was also discussed in [7] AH =106 kJ/mol was derived from a lower limit D(OF) 2.2 eV from ab initio calculations [8]. 

The smaller value AH 298 = 22( 10) corresponds to an older estimate D298(OF) = 56 10 kcal/mol [9]. Higher values are AH o 30.2 4.1, derived [10] from D(FO-F), which was determined from the kinetics of thermal decomposition of OF2, and from AH (OF2) [11] and 

The standard enthalpy of formation was calculated in this review from the careful measurements of [76VAS/LYT]. A value of-(607.4 1.6) kJ-moP was reported by these authors. However, it was necessary to correct their published experimental data for the formation of polynuclear complexes at high Zr concentrations (see Appendix A). 

Therefore, the corrected value selected in this review is  

In their investigation, Ackermaim and Ranh [1972ACK/RAU] measured the vapour pressure of hquid thorium containing less than 3 at.% dissolved tungsten from 2010 to 2460 K by combined mass-effusion and mass-spectrometric techniques and derived the vapom pressure equation equivalent to  

This is identical to the value selected by the CODATA Key Values group [1989COX/WAG]. 

The enthalpy of formation of Th is derived from measurements of the enthalpy of solution in HCl solutions. Unless some fluorosilicate ion is present, this reaction leaves variable amounts of a black residue, probably an oxide hydride containing chloride and hydroxide ions, ThO(Cl,OH)H [1962KAT/KAP], [1973ACK/RAU3] (see Appendix A). 

The standard enthalpy of formation of Th , Af//°(Th , 298.15 K), is obtained from Af/7° (ThCLt, P, 298.15 K), and the extrapolation of the enthalpy of solution of ThCl4 in HCl solutions to infinite dilution, according to reaction  

The enthalpy of formation of thorium tetrachloride itself (see Section V111.2) involves the enthalpy of dissolution of thorium metal and of thorium tetrachloride in HCl solutions at various concentrations, the data for which are summarised in Table Vl-1. 

If the reaction in question represents the formation of one mole of the compound from its elements in their standard states, as in 

Hf° = /products - /reactants = 286 kJ - 0 = -268 kJ mol 1 which defines the standard enthalpy of formation of water at 298K. 

In general, the standard enthalpy change for any reaction is given by the expression 

The following examples illustrate some important aspects of the standard enthalpy of formation of substances. 

The constant C3 (bar) is calculated by inserting the reference pressure so that the pressure unit of measurement must be in bar abs. 

Equilibrium constants in reforming reactions at selected temperatures. Pressure unit is bar. 

CH4+H20= CO-h3H2= C0-hH20= C02+H2= CH4-hC02= C2H6-H2H20  

If any thermodynamic assessment involving coal is intended, it is necessary to define its heat of formation. In particular, temperature calculation during the modeling of gasification or pyrolysis is extremely sensitive to consistent enthalpy calculation. The problem is that normally only clearly defined substances such as carbon (graphite) can be consistently treated in chemical reaction systems. Therefore, coal must be theoretically decomposed to its elements and a correctly assigned standard enthalpy of formation is required. 

It is assumed that the standard heat of formation of coal A(H° can be obtained by subtracting the specific heats of combustion of the individual elements Ac//° (carbon, hydrogen, and suUur) from the HHV of the coal, both referring to a water-and-ash-free basis. 

If empirical correlations are used for the determination of the coal HHV, they are usually not consistent to the mentioned decline of the HHV with increasing carbon content, which would then require predicting the HHV of graphite while approaching 100% carbon. Also if char resulting from pyrolysis should be modeled, estimations of the standard formation enthalpy of artificial high-rank material are required. 

Sciazko [48] recentiy provided an approach to directly assess the heat of formation from the elemental composition. In the first step, the heat of combustion AcH° in MJ/kg is calculated from the weight fractions (wt%/100) of carbon C, hydrogen H, and the combustible Iraction of the suUur Sc, assuming oxidation to CO2, liquid H2O, and SO2. 

In a second step, the heat of formation is calculated by the following equations  

---

Table 5.1 gives a sample calculation of the NHVj for toluene, starting from the molar enthalpies of formation of the reactants and products and the enthalpies of changes in state as the case requires. 

Generally speaking, intermolecular forces act over a short range. Were this not the case, the specific energy of a portion of matter would depend on its size quantities such as molar enthalpies of formation would be extensive variables On the other hand, the cumulative effects of these forces between macroscopic bodies extend over a rather long range and the discussion of such situations constitutes the chief subject of this chapter. 

The enthalpy (strictly, the enthalpy change) for a reaction can readily be calculated from enthalpies of formation AHf which can often be obtained from tables of data. 

A/14 the enthalpy of reaction, which is in this case twice the enthalpy of formation of hydrogen chloride. Clearly A/14 is the difference between the total bond energies of the products and the total bond energies ol the reactants, lhat is... 

For the formation of the hydrogen halides by the direct combination of the elements, the enthalpies of formation are ... 

A/ij the lattice energy of sodium chloride this is the heat liberated when one mole of crystalline sodium chloride is formed from one mole of gaseous sodium ions and one mole of chloride ions, the enthalpy of formation of sodium chloride. 

The positive enthalpy of formation of NaClj is so large that the possibility of the reaction Na(s) + Cljlg) -< NaCljfs) occurring under any conditions is extremely remote. 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

Note the much larger enthalpy of formation of silieon dioxide as compared with carbon dioxide this arises in part because of greater strength in the Si—O bonds and also because the Si—Si bond in silieon is mueh weaker than the C—C bond (p. 162). 

As expected from the enthalpy of formation, water is thermally very stable but when steam is heated to above 1300 K slight dissociation to the elements does occur. Pure water is almost a nonconductor of electricity but slight ionic dissociation occurs ... 

These gases have lower thermal stabilities than hydrogen sulphide as expected from their enthalpies of formation Table 10.2) and they are consequently more powerful reducing agents than hydrogen sulphide. 

Data on proton affiri itics (gas ph asc) of m any differen t com poti u ds (see Table 2) deni on strate Lh e h igh level of accuracy possible in determ in in g energies of related species. In th is report by Dew-ar and Dieter , the enthalpy of formation of II is the experimental value (367.2 kcal/moll. The calculated value for H is unreliable. 

The bond matrix expresses 2 C—C bonds plus 8 C—H bonds for propane and 3 C—C bonds plus 10 C—H bonds for n-butane. Eaeh enthalpy of atomization is obtained by subtraeting the enthalpy of formation of the alkane from the sum of atomie atomization enthalpies (C 716 H 218 kJ mol ) for that moleeule. For example, the moleeular atomization enthalpy of propane is 3(716) +8(218) — (—104) = 3996 kJ mol . Enthalpies of formation are available from Pedley et al. (1986) or on-line at www.webbook.nist.gov. 

Solve the same problem for propane and isobutane (2-methylpropane). The bond matrix is the same as it is for n-butane, but the enthalpy of formation is somewhat different (n-butane) = —127.1 kJ mol vs. (isobutane) = —134.2... 

A variant on this procedure produces a first approximation to the molecular mechanics (MM) heat paiameters (Chapters 4 and 5) for C—C and C—H. Instead of atomization energies, the enthalpies of formation of propane and butane (—25.02 and —30.02 kcal mol ) are put directly into the b vector. The results (2.51 kcal mol and —3.76 kcal mol ) are not very good approximations to the heat parameters actually used (2.45 kcal mol and —4.59 kcal mol ) because of other factors to be taken up later, but the calculation illustrates the method and there is rough agreement. 

Our results are in very good agreement with Benson s simpler bond additivity values (2.5 kcal mol and —3.75 kcal mol Benson and Cohen, 1998), as indeed they must be because they were obtained from the same set of experimental enthalpies of formation. Note that many applications in themiochemishy use energy units of kilocalories per mole, where 1.000 kcal mol =4.184 kJ mol . ... 

Determine the molecular mechanics heat parameters for C—C and C—H using the enthalpies of formation of n-butane and n-pentane, which are —30.02 and —35.11 kcal mol respectively. 

The difference between an MM calculation of the enthalpy of formation and a bond energy scheme comes in the steric energy, which was shown in Eile 4-3. The sum of compression, bending, etc. energies is the steric energy, E = 2.60 kcal mol in Eile 4-3. This is added to BE, as is the partition function energy contribution (see below), PCE = 2.40 kcal moP, to yield... 

The goal of this project is to determine the enthalpies of formation of cis- and trans-2-butene and to calculate the enthalpy of isomerization between them. 

If you are running an updated version (V 8.0) of PC Model, click on force fields mm3. Omit this step for older versions. Click on Analyze (or compute depending on the version of PCMODEL) to obtain a menu of options. Select minimize. The geometry changes can be seen on the screen and a sequence of numbers appears in the right panel of the CRT screen, ending in Hf, the enthalpy of formation. This is the PCMODEL-MM3 calculated value of for cis-2-... 

Suppose that the computed ground-state enthalpy of formation of a molecule is =... 

Add the necessary control lines to obtain the full MM3 minimal input file and run the file under the MM3 force field to obtain the enthalpy of formation of the aromatic molecule benzene. 

Very early force fields were used in an attempt to calculate structures, enthalpies of formation, and vibrational spectra, but it was soon found that accuracy suffered severely in either the structure-energy calculations or the vibrational spectra. Force constants were, on the whole, not transferable from one field to another. The result was that early force fields evolved so as to calculate either structure and energy or spectra, but not both. 

A force field that can produce vibrational spectra has a second advantage in that the Ay// calculations can be put on a much more satisfactory theoretical base by calculating an enthalpy of formation at 0 K as in ab initio procedures and then adding various thermal energies by more r igorous means than simply lumping them in with empirical bond enthalpy contributions to Ay//-. The stronger the theoretical base, the less likely is an unwelcome surprise in the output. 

Write and run an MM3 input file for methane from scratch, that is, open an empty file and put in all the necessary infomiation to do the MM3 calculation on CH4. What is the enthalpy of formation of CH4 What are the C—H bond lengths and angles ... 

What is the MM3 enthalpy of formation at 298.15 K of styrene Use the option Mark all pi atoms to take into account the conjugated double bonds in styrene. Is the minimum-energy structure planar, or does the ethylene group move out of the plane of the benzene ring ... 

Find the MM3 enthalpy of formation of 1- and 2-methyladamantane. Use the Rings tool and the adamant option to obtain the base structure of adamantane itself. Use the Build tool to add the methyl group. 1-Adamantane is the more symmetrical structure of the two isomers. 

W hich leads to an isomeri/ation enthalpy of —1.7 kcal mol. ("Heat of fonnation should he taken to mean enthalpy of formation in this conte.st.) Entropy effects being... 

By systematically applying a series of corrections to approximate solutions of the Schroedinger equation the Pople group has anived at a family of computational protocols that include an early method Gl, more recent methods, G2 and G3, and their variants by which one can anive at themiochemical energies and enthalpies of formation, Af and that rival exper imental accuracy. The important thing... 

---