Exothermic process enthalpy change

For simplicity, we can imagine the solution process taking place in the three distinct steps shown in Figure 13.3. Step 1 is the separation of solute molecules from one another, and step 2 is the separation of solvent molecules from one another. Both of these steps require an input of energy to overcome intermolecuiar attractions, so they are endothermic. In step 3 the solvent and solute molecules mix. This process is usually exothermic. The enthalpy change for the overall process, sdn. is given by... 

The scale-up of exothermic processes is greatly enhanced through the use of the coefficient of thermal stability. Kafarov [2] defined this as the ratio of the slope (tan ttj) of the line representing the heat removal (due to the heat transfer medium and changes in enthalpy) to the slope (tan ttj) of the line representing heat generation (by the reaction) at the intersection of the two lines when plotted on the T versus Q coordinates. This is expressed as... 

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 question is Find the molar heat of reaction. This means we need the enthalpy change (AH) in kJ/mol (or J/mol) of glucose. The increase in temperature means that this is an exothermic process (negative enthalpy change). 

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. 

The temperature dependence of the equilibrium concentration of a product in a thermodynamically controlled process is determined by the heat (enthalpy change) of the catalyzed reaction. For an exothermic process an increase in temperature... 

The thermodynamic data of Tables 2 and 3 are remarkably selfconsistent, and correlate well with data for the various phenyltrimethyl ammonium perchlorates in 1,2-dichloroethane originally studied by Denison and Ramsey (44). Clearly dissociation is nominally exothermic though the actual enthalpy changes are quite small. Solvation of the free ions, therefore, is also small (in these solvents) but exceeds that of the ion pair species. The overall entropy changes confirm some ordering of solvent molecules on dissociation, the loss in entropy for this process being more than sufficient to balance the entropy increase associated with... 

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 thermal effect of any reactions is mild and does not induce a phase change or a significant change in the temperature of the process units. Hence, the specific enthalpies of the feed and effluent streams, h and h2, of the process units are of comparable magnitude 2 = 2,s/ i,s = 0(1). (Note that this assumption is by no means restrictive rather, it reflects current industrial practice. For example, the use of adiabatic units in highly exothermic processes is avoided for safety reasons, and external cooling systems are preferred. This issue is addressed in detail in Chapter 7.)... 

Sample The complete combustion of butane gas, C4H10, in oxygen gas, Oz, produces C02 and H20. It is a highly exothermic process releasing 2845 kj of heat per mole of butane. Write the balanced thermochemical equation, using all whole-number coefficients. Also, determine the enthalpy change in burning 50.0 g of butane gas. 

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. 

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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