Pure compounds, standard enthalpy

Compiled from Daubert, T. E., R. R Danner, H. M. Sibiil, and C. C. Stebbins, DIPPR Data Compilation of Pure Compound Properties, Project 801 Sponsor Release, July, 1993, Design Institute for Physical Property Data, AlChE, New York, NY and from Thermodynamics Research Center, Selected Values of Properties of Hydrocarbons and Related Compounds, Thermodynamics Research Center Hydrocarbon Project, Texas A M University, College Station, Texas (extant 1994). The compounds are considered to be formed from the elements in their standard states at 298.15 K and 101,325 P. These include C (graphite) and S (rhombic). Enthalpy of combustion is the net value for the compound in its standard state at 298.15K and 101,325 Pa. 

As a thermodynamicist working at the Lower Slobbovian Research Institute, you have been asked to determine the standard Gibbs free energy of formation and the standard enthalpy of formation of the compounds ds-butene-2 and trans-butene-2. Your boss has informed you that the standard enthalpy of formation of butene-1 is 1.172 kJ/mole while the standard Gibbs free energy of formation is 72.10 kJ/mole where the standard state is taken as the pure component at 25 °C and 101.3 kPa. 

Enthalpies of reaction can also be calculated from individual enthalpies of formation (or heats of formation), AHf, for the reactants and products. Because the temperature, pressure, and state of the substance will cause these enthalpies to vary, it is common to use a standard state convention. For gases, the standard state is 1 atm pressure. For a substance in an aqueous solution, the standard state is 1 molar concentration. And for a pure substance (compound or element), the standard state is the most stable form at 1 atm pressure and 25°C. A degree symbol to the right of the H indicates a standard state, AH°. The standard enthalpy of formation of a substance (AHf) is the change in enthalpy when 1 mol of the substance is formed from its elements when all substances are in their standard states. These values are then tabulated and can be used in determining A//°rxn. 

Having thus settled on Pedley s tables for the pure organic compounds, we have then decided to use NBS Tables to derive the solution enthalpies in figure 2.1. The values can be easily evaluated from the differences between the standard enthalpies of formation of the compounds in solution and the standard enthalpies of formation of pure substances, viz. 

The important point to be noted here is that the calculation uses NBS data only. Had we combined Pedley s standard enthalpies of formation for the pure compounds with the NBS values for the solutions, we would have obtained incorrect results. 

In summary, we selected one database (Pedley s) to quote the standard enthalpies of formation of the pure organic compounds and another database (NBS) to derive the solution enthalpies. Although these databases are not mutually consistent, that did not affect our final result because the experimental enthalpies of solution were calculated with NBS data only. The exercise illustrates the sort of caution one should keep in mind whenever two or more nonconsistent databases are used. 

References (20, 22, 23, 24, 29, and 74) comprise the series of Technical Notes 270 from the Chemical Thermodynamics Data Center at the National Bureau of Standards. These give selected values of enthalpies and Gibbs energies of formation and of entropies and heat capacities of pure compounds and of aqueous species in their standard states at 25 °C. They include all inorganic compounds of one and two carbon atoms per molecule. 

We observe that the standard change of enthalpy for the formation of a solute in an infinitely dilute solution is the sum of the standard change of enthalpy for the formation of the pure compound and the difference between its partial molar enthalpy in the infinitely dilute solution and its molar enthalpy when pure. 

The concept of R. Pretorius et al 261,262 can briefly be explained as follows. Consider the formation of the Ni2Al3 compound between the pure nickel and aluminium phases as an example. For the purpose of illustration, let us arbitrarily assume that at the interface between those phases the effective concentration of nickel is 70 at.% and that of aluminium 30 at.%. It is clear that in this case the limiting element is aluminium, whereas nickel is in excess. The standard enthalpy of formation of the Ni2Al3 compound is -57 kJ g-atom Hence, if the total number of the nickel and aluminium atoms is equal to the Avogadro number NA, then 57 0.30 0.60 = 28.5 kJ of heat is released in the system after consuming all the aluminium atoms in the reaction of formation of Ni2Al3. Thus, R. Pretorius et al 261,262 introduce the concept of the effective heat, AH, of formation of a compound through the equation... 

The prediction of the adsorption behavior of a transactinide compound starts with the calculation of the sublimation enthalpy (AH°subi) of the pure compound. Depending on the availability of data, the standard sublimation enthalpy can be calculated using the following methods ... 

With assignment of zero for the standard enthalpies of pure elements one may equate A with the standard enthalpy of the compound species i, Hfj.. This quantity at temperature T may then be taken relative to the molar enthalpy at 298 K or at 0 K,. , 298 difference is ordinarily tabulated as... 

The difference between the enthalpy of one mole of a pure compound and the total enthalpy of the elements from which it is composed is called the o/formation of the compound. It represents the change in energy due to rearrangement of the atoms, and to be made precise it must be specified under certain standard conditions and the state of aggregation of each species must be given. The standard pressure is 1 atm and the standard temperature... 

Tables of average standard bond enthalpies make the assumption that the standard enthalpy of a bond is independent of the molecule in which it exists. This is only roushly true. Since standard bond enthalpies vary from one compound to another, the use of avaage standara bond enthalpies gives only approximate values for standard enthalpies of reaction calculated from them. Experimental methods are used to obtain standard enthalpies of reaction whenever possible. Calculations bwd on average standard bond enthalpies are used only for reaaiona which cannot ce studied experimentally —for example, the reactions of a substance which has not been isolated in a pure state.

AHf is a standard enthalpy change, and therefore applies to a reaction in which both the elements and compound involved are present in their pure forms at an atmospheric pressure of 1 atm. 

In thermodynamic data tables (standard enthalpies or Gibbs energies of formation and standard molar entropies) which relate to compounds other than ions in a solution, the common convention that is applied involves setting the values of standard enthalpy and standard Gibbs energy of formation (or chemical potential) equal to OJmor for all simple pure elements in their stable physical state at the temperature in question. The data therefore refer to the formation of substances from simple elements. 

The problem this creates is that we do not want to have to tabulate an enthalpy change for every process or chemical reaction which might become of interest to us - there are too many. We would like to be able to associate an enthalpy with every substance - solids, liquids, gases, and solutes - for some standard conditions, so that having tabulated these, we could then easily calculate an enthalpy change between any such substances under those standard conditions. After that, we could deal with the changes introduced by impurities and other nonstandard conditions. The method developed to allow this is to determine, for every pure compound, the difference between the enthalpy of the compound and the sum of the enthalpies of the elements, each in its most stable state, which make up the compound. This quantity is called the standard molar enthalpy of formation from the elements. For aqueous ions, the quantity determined is a little more complicated (Chapter 15), but the principle is the same. It is this enthalpy quantity which is invariably tabulated in compilations of data. 

The main application of such a calorimeter, the burning of substances inside a sealed, pressure-tight reaction vessel, was developed by Berthelot (1827-1907) into a standard procedure. Berthelot was the first to fill the reaction vessel with pure oxygen to excess pressure in order to obtain a quick, thorough combustion into definite reaction products. Calorimeters of this type were soon given the name bomb calorimeter because of the bomb-like appearance of the reaction vessel. Berthelot s numerous thermochemical measurements owe their success to this experimental procedure. Even today, this instrument remains a valuable aid for the determination of the standard enthalpies of formation of chemical compounds, of the combustion heats of foodstuffs, and of the gross heating values of fuels (Rossini, 1956 Skinner, 1962). 

For a Pure Compound The change in enthalpy when one mole of the compound forms from its constituent elements in their standard states. 

What is the standard enthalpy of formation for a compound For a pure element in its standard state ... 

Even if the enthalpy of 210 J/g is not the corresponding value for the crystallization enthalpy of MEl and ME2 it has been chosen in order to compare the variation of the crystallized fraction for all studied blends. The ME2 and DF have similar low percentages of crystals at —15 °C. However, ME 1 shows a high percentage of crystals at -15 " C. The cold flow parameters of pure compounds are listed in Table 13.3. The CFPP and PP were determined by DSC at 0.45 and 1 % of crystals, respectively, using the same AHcryst as mentioned above. Claudy et al. [9] realized a study on 40 different diesel fuels and established correlations between CFPP and PP determined by ASTM standards and CFPP and PP obtained by DSC measurements. The best correlations were obtained at 0.45 and 1 % of crystals, respectively. In our study, the same values have been retained in order to compare the samples. 

In Eq. (2), Ts is the sample temperature, T0 is the melting point of the pure major component, X, is the mole fraction of the impurity, F is the fraction of solid melted, and AHf is the enthalpy of fusion of the pure component. A plot of Ts against 1 IF should yield a straight line whose slope is proportional to X,. This method can therefore be used to evaluate the absolute purity of a given compound without reference to a standard, with purities being obtained in terms of mole... 

Table 5.12 reports a compilation of thermochemical data for the various olivine components (compound Zn2Si04 is fictitious, because it is never observed in nature in the condition of pure component in the olivine form). Besides standard state enthalpy of formation from the elements (2) = 298.15 K = 1 bar pure component), the table also lists the values of bulk lattice energy and its constituents (coulombic, repulsive, dispersive). Note that enthalpy of formation from elements at standard state may be derived directly from bulk lattice energy, through the Bom-Haber-Fayans thermochemical cycle (see section 1.13). 

Several comments need to be made concerning the state of aggregation of the substances. For gases, the standard state is the ideal gas at a pressure of 1 bar this definition is consistent with the standard state developed in Chapter 7. When a substance may exist in two allotropic solid states, one state must be chosen as the standard state for example, graphite is usually chosen as the standard form of carbon, rather than diamond. If the chemical reaction takes place in a solution, there is no added complication when the standard state of the components of the solution can be taken as the pure components, because the change of enthalpy on the formation of a compound in its standard state is identical whether we are concerned with the pure... 

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