Enthalpy , 527 calculate changes

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. 

If the surface of the metal can be described as a two-dimensional electron gas, where the calculated adsorption enthalpy will change linearly with 0. 

The reaction favours the formation of ozone with a significant equilibrium constant. Appendix C also lists the enthalpies of formation and the standard enthalpy of the reaction ArH° can be calculated. The answer for the enthalpy calculation is ArH° = —106.47 kJ mol, showing this to be an exothermic reaction, liberating heat. The entropy change at 298 K can also be calculated because ArG° = ArH° — T ArS°, so ArS° = 25.4 Jmol-1 K-1, indicating an increase in the entropy of the reaction as it proceeds by creating one molecule from two. 

The partial molal entropies are calculated from the partial molal free energies and enthalpies. The change in the partial molal entropy of water increases monotonically to a value near zero with increasing amount adsorbed. The partial molal entropy of barium sulfate decreases with increasing amount adsorbed. 

Attempts to derive theoretical relations describing various dissociation degrees are based on the contribution of electrostatic forces to the Gibbs energy, enthalpy, and change in dissociation entropy. These contributions can be calculated [68c]... 

Electromotive force measurements of the cell Pt, H2 HBr(m), X% alcohol, Y% water AgBr-Ag were made at 25°, 35°, and 45°C in the following solvent systems (1) water, (2) water-ethanol (30%, 60%, 90%, 99% ethanol), (3) anhydrous ethanol, (4) water-tert-butanol (30%, 60%, 91% and 99% tert-butanol), and (5) anhydrous tert-butanol. Calculations of standard cell potential were made using the Debye-Huckel theory as extended by Gronwall, LaMer, and Sandved. Gibbs free energy, enthalpy, entropy changes, and mean ionic activity coefficients were calculated for each solvent mixture and temperature. Relationships of the stand-ard potentials and thermodynamic functons with respect to solvent compositions in the two mixed-solvent systems and the pure solvents were discussed. 

In the last example we used molecular species as references for specific enthalpy calculations. This time we will use elemental species [C(s), H2(g), OtCg)] at 25°C and 1 atm. (For a single reaction both choices require about the same computational effort.) The energy balance neglecting shaft work and kinetic and potential energy changes becomes... 

You will write thermochemical equations and use them to calculate changes in enthalpy. 

To calculate entropy changes for chemical reactions, we find it convenient to use the same standard state already selected for enthalpy calculations in Section 12.3. For this purpose, we define the standard molar entropy to be the absolute molar entropy S° at 298.15 K and 1 atm pressure (Fig. 13.8) ... 

If an incomplete reaction occurs, you should calculate the standard heat of reaction only for the products which are actually formed from the reactants that actually react. In other words, only the portion of the reactants that actually undergo some change and liberate or absorb some energy are to be considered in calculating the overall standard heat of the reaction. If some material passes through the reactor unchanged, it can contribute nothing to the standard heat of reaction calculations (however, when the reactants or products are at conditions other than 25°C and 1 atm, whether they react or not, you must include them in the enthalpy calculations as explained in Sec. 4.7-5). If several reactions occur simultaneously, your material balance must reflect what enters the reactor and is produced via the independent reactions. 

The use of constant physical properties is acceptable in this case since the composition and temperature changes over the length of the column are not substantial. The system is at a pressure slightly above atmospheric but that has been ignored in making the enthalpy calculations. 

Use heat capacities to calculate changes in internal energy and enthalpy. 

The practical utility of the heat capacities is twofold. First, they allow us to calculate heat in constant-volume and constant-pressure processes. This is useful in energy balances. Second, they allow us to calculate changes in internal energy and enthalpy. This allows us to calculate these properties using equations rather than tables, or to obtain their values in states that are not found in tables. There is a limitation, however. Equation (. 17) maybe used only between two states of the same volume, and eg. f. iQ ) only between two states of the same pressure. The general calculation of properties between any two states will be discussed in Chanter r. 

Equations 8 and 9 provide recipes for calculating changes with temperature in the enthalpy and entropy of a substance. Complete evaluations still require knowing these... 

TABLE 3. Calculated changes of energies (AE 298,), enthalpies (AH 298), Gibbs free energies (AG m) and entropies (AS°29s) for the reaction of Mo complexes including alumina. T=298.15 K. 

Therefore, from an experimentally determined ACp and a measured enthalpy change at 25 °C, the corresponding enthalpy change at 60°C is calculated by AH6o=AH25 +ACp (60-25). If we admitted Eqs. 30 and 31 to be valid for binding processes, we would have a two equation system with two unknowns from which we could calculate changes in accessible surface... 

This reaction is highly endothermic. By varying the amount of oxygen and water, the two reactions can be done together in a manner so that the heat flow is controlled. The enthalpy calculations are not as simple as shown here because the reactions are not done at standard conditions and enthalpy changes with pressure and temperature. High pressures and temperatures of about 1200 °C are common. For precision, it needs to be calculated or experimentally determined for the conditions of interest. 

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