State property enthalpy

The values of the thermodynamic properties of the pure substances given in these tables are, for the substances in their standard states, defined as follows For a pure solid or liquid, the standard state is the substance in the condensed phase under a pressure of 1 atm (101 325 Pa). For a gas, the standard state is the hypothetical ideal gas at unit fugacity, in which state the enthalpy is that of the real gas at the same temperature and at zero pressure. 

The enthalpy of a substance, like its volume, is a state property. A sample of one gram of liquid water at 25.00°C and 1 atm has a fixed enthalpy, H. In practice, no attempt is made to determine absolute values of enthalpy. Instead, scientists deal with changes in enthalpy, which are readily determined. For the process... 

This relationship is referred to as Hess s law, after Germain Hess (1802-1850), professor of chemistry at the University of St. Petersburg, who deduced it in 1840. Hess s law is a direct consequence of the fact that enthalpy is a state property, dependent only on initial and final states. This means that, in Figure 8.6, AH must equal the sum of AH, and AH2, because the final and initial states are the same for the two processes. 

The relation between AH° and enthalpies of formation is perhaps used more often than any other in thermochemistry. Its validity depends on the fact that enthalpy is a state property. For any reaction, AH° can be obtained by imagining that the reaction takes place in two steps. First, the reactants (compounds or ions) are converted to the elements ... 

Entropy, like enthalpy (Chapter 8), is a state property. That is, tine entropy depends only on the state of a system, not on its history. The entropy change is determined by the entropies of the final and initial states, not on the path followed from one state to another. 

FIGURE 6.25 Because enthalpy is a state property, the enthalpy of sublimation can be expressed as the sum of the enthalpies of fusion and vaporization measured at the same temperature. 

With most properties (enthalpies, volumes, heat capacities, etc.) the standard state is infinite dilution. It is sometimes possible to obtain directly the function near infinite dilution. For example, enthalpies of solution can be measured in solution where the final concentration is of the order of 10-3 molar. With properties such as volumes and heat capacities this is more difficult, and, to get standard values, it is usually necessary to measure apparent molal quantities 0y at various concentrations and extrapolate to infinite dilution (y° = Y°). Fortunately, it turns out that, at least with volumes and heat capacities, the transfer functions AYe (W — W + N) do not vary significantly with the electrolyte concentration as long as this concentration is relatively low (3). With most of the systems investigated, the transfer functions were calculated from apparent molal quantities at 0.1m and assumed to be equivalent to the standard values. 

FIGURE 6.7 (a) The altitude of a location on a mountain is like a state property it does not matter what route you take between two points, the net change in altitude is the same, (b) Enthalpy is a state property if a system changes from state A to state B (as depicted highly diagrammatically here), the net change in enthalpy is the same whatever the route—the sequence of chemical or physical changes— between the two states. 

The enthalpy of a system, a state property, is a measure of the energy of a system that is available as heat at constant pressure. For an endothermic process, AH > 0 for an exothermic process, AH < 0. 

Various parameterizations of NDDO have been proposed. Among these are modified neglect of diatomic overlap (MNDO),152 Austin Model 1 (AMI),153 and parametric method number 3 (PM3),154 all of which often perform better than those based on INDO. The parameterizations in these methods are based on atomic and molecular data. All three methods include only valence s and p functions, which are taken as Slater-type orbitals. The difference in the methods is in how the core-core repulsions are treated. These methods involve at least 12 parameters per atom, of which some are obtained from experimental data and others by fitting to experimental data. The AMI, MNDO, and PM3 methods have been focused on ground state properties such as enthalpies of formation and geometries. One of the limitations of these methods is that they can be used only with molecules that have s and p valence electrons, although MNDO has been extended to d electrons, as mentioned below. 

The system of our choice will usually prevail in a certain macroscopic state, which is not under the influence of external forces. In equilibrium, the state can be characterized by state properties such as pressure (P) and temperature (T), which are called "intensive properties." Equally, the state can be characterized by extensive properties such as volume (V), internal energy (U), enthalpy (H), entropy (S), Gibbs energy (G), and Helmholtz energy (A). 

Because enthalpy is a state property, the enthalpy change depends on the initial and final states only, not on the path the process follows. As die sum of all the reactions in path 2 results in the same reaction as the one in path 1, the enthalpy change should be the same for both paths. 

The above is in fact a different expression of the state property of enthalpy. 

Explain why the reference state used to generate a table of specific internal energies or enthalpies is irrelevant if one is only interested in calculating Af/ or AW for a process, (The term state property should appear in your explanation.)... 

Tables of enthalpies and other state properties of many substances may be found in tables B5-B9 of this text and Perry s Chemical Engineers Handbook on pp. 2-206 through 2-316.

The next step in the calculation would be to determine the values of A/ " for Steps 1. 3. 5. and 6 using methods to be given in Section 8.2 read the values of AH2 and A//4 from Table B. 1 and then use the fact that enthalpy is a state property to calculate the desired (A// for the upper dashed line in the figure) as... 

Since condensation is the reverse of vaporization and enthalpy is a state property, the heat of condensation must be the negative of the heat of vaporization. Thus, the heat of condensation of water at 100°C and 1 atm must be -40.6 kJ/moi. Similarly, the heat of solidification is the negative of the heat of fusion at the same temperature and pressure. 

The fact that internal energy and enthalpy are state properties means that any conv enient process path from a reference state to a process state may be chosen, even if the actual process proceeds by a different path. As a rule, you would choose a path that allows ou to make use of heat capacities, phase transition temperatures, and latent heats tabulated in an avtulable reference (like this text). 

Property values in the standard state are denoted by the degree symbol. For example, Cp is the standard-state heat capacity. Since the standard state for gases is the ideal-gas state, Cp for gases is identical with Cp , and the data of Table C.l apply to the standard state for gases. All conditions for a standard state are fixed except temperature, which is always the temperature of the system. Standard-state properties are therefore functions of temperature only. The standard state chosenfor gases is a hypothetical one, for at 1 bar actual gases are not ideal. However, they seldom deviate much from ideality, and in most instances enthalpies for the real-gas state at 1 bar and the ideal-gas state are little different. 

Among other things, it became established that the nature of the structure adopted by a given compound on crystallization would then exert a profound effect on the solid-state properties of that system. For a given material, the heat capacity, conductivity, volume, density, viscosity, surface tension, diffusivity, crystal hardness, crystal shape and color, refractive index, electrolytic conductivity, melting or sublimation properties, latent heat of fusion, heat of solution, solubility, dissolution rate, enthalpy of transitions, phase diagrams, stability, hygroscopicity, and rates of reactions were all affected by the nature of the crystal structure. 

Since U, P, and V are state functions, enthalpy is also a state function. Like internal energy, for an ideal gas enthalpy depends only on temperature. Enthalpy is an extensive property. 

Properties of A and A5 -> 5 is an extensive, property of state, like enthalpy. 

It may be added here, that, as extensions to these computational predictors of solubility, initial information has been published recently on the search for models for the prediction of drug solubility and permeability, bioavailability and for the determination of the influence of some solid-state properties (melting point, enthalpy of melting and entropy of melting) on the intrinsic solubility of drugs. °... 

Since enthalpy is a state property, — Hp can be evaluated by any conveniently chosen path. Rigorously, for the path described, AH applies at the product temperature and the specific heat is for a mixture of the composition of the feed. Except for simple gaseous systems, the thermodynamic data available are insufiicient to take into account variations in AH with temperature and c with composition. Often these variations are small. 

The three approaches mentioned in the previous section may also be used to describe caloric properties (enthalpy, entropy, heat capacity) of mixtures. The same considerations mentioned earlier are also true for the application of mixture equations of state and corresponding-states methods for the prediction of caloric properties. 

---