Enthalpy change, definition

For polymers in the rubbery range the volume change dV is small and if the pressure is only the atmospheric pressure the pdV term is so small as to be negligible. The negative sign for the pV term in the enthalpy definition here is due to the fact that the work of atmospheric pressure is in opposition to the positive work of a tensile force on an uniaxial specimen. 

The definitions of enthalpy, H, Helmholtz free energy. A, and Gibbs free energy, G, also give equivalent forms of the fundamental relation (3) which apply to changes between equiUbrium states in any homogeneous fluid system ... 

Solution. Figure 12-8 shows the path on a psychrometric chart. The leaving dry-bulb temperature is obtained directly from Fig. 12-2 as 72.2 F. Since the spray water enters at the wet-bulb temperature of 70 F and there is no heat added to or removed from it, this is by definition an adiabatic process and there will be no change in wet-bulb temperature. The only change in enthalpy is that from the heat content of the makeup water. This can be demonstrated as follows ... 

Technical calculations dealing with humid air are reasonable to solve with dry air mass flow rates, because these remain constant in spite of changes in the amount of water vapor in the air. For that reason a definition for enthalpy,... 

Consider a process at constant pressure for which the change in internal energy is AU and the change in volume is A V. It then follows from the definition of enthalpy in Eq. 9 that the change in enthalpy is... 

It follows from the definition just given that the standard enthalpy of formation of an element in its most stable form is zero. For instance, the standard enthalpy of formation of C(gr) is zero because C(gr) — C(gr) is a null reaction (that is, nothing changes). We write, for instance, AHf°(C, gr) = 0. However, the enthalpy of formation of an element in a form other than its most stable one is nonzero. For example, the conversion of carbon from graphite (its most stable form) into diamond is endothermic ... 

The temperature profile of a planetary atmosphere depends both on the composition and some simple thermodynamics. The temperature decreases with altitude at a rate called the lapse rate. As a parcel of air rises, the pressure falls as we have seen, which means that the volume will increase as a result of an adiabatic expansion. The change in enthalpy H coupled with the definition of the specific heat capacity... 

From the definition of the heat of reaction, Qp will depend on the temperature T at which the reaction and product enthalpies are evaluated. The heat of reaction at one temperature T0 can be related to that at another temperature 7. Consider the reaction configuration shown in Fig. 1.1. According to the First Faw of Thermodynamics, the heat changes that proceed from reactants at temperature T() to products at temperature 7), by either path A or path B must be the same. Path A raises the reactants from temperature T0 to 7, and reacts at 7). Path B reacts at T0 and raises the products from T0 to 7). This energy equality, which relates the heats of reaction at the two different temperatures, is written as... 

It is also evident from the definition [Equation (4.3)] that absolute values of H are unknown because absolute values of U cannot be obtained from classic thermodynamics alone. Therefore, from an operational point of view, it is possible only to consider changes in enthalpy AH. Such changes can be defined readily by the expression... 

However, to estimate the standard enthalpy of formation, it is necessary to add two reactions to Equation (4.46), because, by definition, the standard enthalpy of formation refers to the formation of the compound in its standard state from the elements in their standard states. Therefore we introduce the following enthalpy changes to convert the elements from their standard states to the gaseous atoms at 298 K ... 

Often, it is difficult to distinguish definitely between inner sphere and outer sphere complexes in the same system. Based on the preceding discussion of the thermodynamic parameters, AH and AS values can be used, with cation, to obtain insight into the outer vs. inner sphere nature of metal complexes. For inner sphere complexation, the hydration sphere is disrupted more extensively and the net entropy and enthalpy changes are usually positive. In outer sphere complexes, the dehydration sphere is less disrupted. The net enthalpy and entropy changes are negative due to the complexation with its decrease in randomness without a compensatory disruption of the hydration spheres. 

B) The formation of hydrogen sulfide from hydrogen gas and sulfur conforms to the definition of both standard enthalpy of reaction and standard enthalpy of formation since the heat change that is measured is for the formation of one mole of compound from the elements in their standard state. In Choice (D), diamond is not the stable aUotrope of carbon—it is graphite. 

A closely related quantity to the internal energy is the enthalpy, H. It, too, has SI units of joules and is defined as the internal energy plus the pressure-volume product, PV. As in most cases, we are concerned with changes in internal energy and enthalpy from one state to another, so that the definition of enthalpy for infinitesimal changes in state is... 

The conditions for eqnilibrinm have not changed, and application of the phase rnle is conducted as in the previous section. The difference now is that composition can be counted as an intensive variable. Composition is accounted for through direct introduction into the thermodynamic quantities of enthalpy and entropy. The free energy of a mixtnre of two pure elements, A and B, is still given by the definition... 

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