Endothermic process enthalpy change

The change in enthalpy of a system is equal to the heat supplied to the system at constant pressure. For an endothermic process, AH > 0 for an exothermic process, AH < 0. 

Both these everyday processes are spontaneous, but whereas one process is endothermic, the other is exothermic. The energy and enthalpy of the system increase in one process, but these quantities decrease in the other process. This simple example demonstrates that analyzing energy changes and enthalpy changes is not enough to predict whether a process will occur spontaneously. We need a property other than energy and enthalpy if we hope to use thermodynamics to determine when a process will be spontaneous. 

The enthalpy changes associated with exchange of chloride ions in a Zn2.2 A1 LDH by a variety of dicarboxylate anions (oxalate, succinate, adipate and tartrate) have been measured by microcalorimetry [222]. With the exception of tartrate, the exchange processes are endothermic and the enthalpies... 

When a reaction evolves heat, the sign of the enthalpy change AH is negative and the reaction is said to be exothermic. An endothermic process, on the other hand, is one in which heat is absorbed by the system and AH is positive. 

To understand the values in Table 8.6, we can think of dissolving as a two-step process (Fig. 8.23). In the first hypothetical step, we imagine the ions separating from the solid to form a gas of ions. The change in enthalpy accompanying this highly endothermic step is the lattice enthalpy, AHL, of the solid, which was introduced in Section 6.20 (see Table 6.3 for values). The lattice enthalpy of sodium chloride (787 kj-mol-1), for instance, is the molar enthalpy change for the process... 

G decreases for a spontaneous process, like the energy of a mechanical system. Since AG incorporates both driving forces for spontaneity—enthalpy (energy) decrease and entropy (disorder) increase—an endothermic process may be spontaneous if the increase in disorder is big enough to counteract the unfavorable enthalpy change, and a process that leads to increased order (negative AS) may be spontaneous if the process is sufficiently exothermic (negative AH). 

The activation energies, calculated from the change in enthalpy in going from the reactant to the transition state, are -1-20.3 and -1-15.6 kcal mol" for O-alkylation and C-alkylation, respectively. The overall enthalpy change of the reactions, obtained from the differences in the heat of formation between the reactant and the product, is -1-5.7 and —13.0 kcal mol for O-alkylation and C-alkylation, respectively. These results predict that the product obtained for C-alkylation is the preferred product because (i) the activation barrier is smaller (lower in energy) than that in O-alkylation and (ii) the reaction is exothermic while O-alkylation is an endothermic process. 

The equilibrium state of any process involving an enthalpy change must be affected by temperature. The conformational transitions in proteins discussed in Section Pf,E are a case in point. The transition from the native to the denatured form in a nonaqueous solvent is usually an endothermic process, and a decrease in temperature will favor the native form. In such cases, it is possible that the disruption of the native conformation in a given protein-solvent system, which observed at room temperatures, may be reversed at sufficiently low temperatures. [On the other hand, particularly in mixed solvents, the transition from an ordered to a disordered state may be an exothemic process (Doty and Yang, 1956 Foss and Schellman, 1959), and the reverse effect of temperature may be ob-... 

The lattice enthalpy for crystal formation is large enough to overcome all the endothermic processes (and the negative entropy change) and to make formation of LiF from the elements a very favorable reaction. 

Differential thermal analysis (DTA) measures the amount of heat released or absorbed by a sample as it is heated at a known rate." When the enthalpy change is determined, the method is called differential scanning calorimetry (DSC). The presence of exothermic or endothermic processes at certain temperatnres provides information about the nature of phase changes and chemical reactions occurring in the material as it is heated. DTA can often be used as a sensitive method for establishing the presence or absence of secondary phases in samples if these phases undergo phase transformations at known temperatures. ... 

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