Corresponding states enthalpy change from

The enthalpy change, AH, can be calculated for a steady-state process, using H°f, which is the enthalpy of formation of the various output and input components. Under the assumption that the inputs and outputs are at ambient conditions, the enthalpy of the components corresponds to the standard enthalpy of formation of each component. The kinetic and potential energy terms are neglected from the energy balance. It is also assumed that water enters the process as a liquid and hydrocarbon products leave the process as a liquid. All other components are in the gas phase. 

The experiments are usually carried out at atmospheric pressure and the initial goal is the determination of the enthalpy change associated with the calorimetric process under isothermal conditions, AT/icp, usually at the reference temperature of 298.15 K. This involves (1) the determination of the corresponding adiabatic temperature change, ATad, from the temperature-time curve just mentioned, by using one of the methods discussed in section 7.1 (2) the determination of the energy equivalent of the calorimeter in a separate experiment. The obtained AT/icp value in conjunction with tabulated data or auxiliary calorimetric results is then used to calculate the enthalpy of an hypothetical reaction with all reactants and products in their standard states, Ar77°, at the chosen reference temperature. This is the equivalent of the Washburn corrections in combustion calorimetry... 

The negative of the enthalpy change for this particular process corresponds to the net heat of adsorption, since the process involves the transfer of the adsorbate from its liquid state to the surface of the adsorbent. (The net heat of adsorption of liquid adsorbate is in turn the difference between the heat of adsorption of the adsorbate vapor and the heat of condensation of the vapor.)... 

Don t confuse ArH° and ArH.li is important to remember that the enthalpy term given by the slope of log K versus 1/T (e.g.. Figure 13.2) is a standard state enthalpy of reaction (ArH°), the meaning of which is determined entirely by the standard states of the reaction constituents, and may or may not correspond to an enthalpy that is directly measurable (ArH). In Figure 13.2 the slope is exactly equal at all temperatures to the AH° calculable from the tables of Robie et al. (1978) (Table 13.1) because we did not change their standard states in calculating K. Anderson (1970) considers the difference between these terms in more detail. 

Hence, the enthalpy change between T, and Tj. Pi niay be computed from the variation for an ideal gas plus the variation of the departure function, which accounts for non-ideality. The big advantage of the departure functions is that they can be evaluated with z PVT relationship, including the corresponding states principle. Moreover, the use of departure functions leads to a unified framework of computational methods, both for thermodynamic properties and phase equilibrium. 

The change in state of a system produced by a specified chemical reaction is definite. The corresponding enthalpy change is definite, since the enthalpy is a function of the state. Thus, if we transform a specified set of reactants to a specified set of products by more than one sequence of reactions, the total enthalpy change must be the same for every sequence. This rule, which is a consequence of the first law of thermodynamics, was originally known as Hess s law of constant heat summation. Suppose that we compare two different methods of synthesizing sodium chloride from sodium and chlorine. 

If 5i2 for an associating mixture is measured and jBi2(phys.) is calculated by a corresponding-states procedure, laCchem.) can be obtained by difference. The enthalpy change in the association reaction can also be derived from studies of the temperature dependence of J i2(chem.). 

Solve In (a) 1 mol NajO is formed from the elements sodium and oxygen in their proper states, solid Na and Oj gas, respectively. Therefore, the enthalpy change for reaction (a) corresponds to a standard enthalpy of formation. 

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