Enthalpy endothermic processes

Let us consider the formation of sodium chloride from its elements. An energy (enthalpy) diagram (called a Born-Haber cycle) for the reaction of sodium and chlorine is given in Figure 3.7. (As in the energy diagram for the formation of hydrogen chloride, an upward arrow represents an endothermic process and a downward arrow an exothermic process.)... 

We can therefore report that AH = —208 kj because the enthalpy of the reaction mixture decreases by 208 kj in this reaction (Fig. 6.18). An endothermic process absorbs heat, and so when ammonium nitrate dissolves in water the enthalpy of the system increases (Fig. 6.19). Note that AH < 0 for exothermic reactions, whereas AH > 0 for endothermic reactions. 

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. 

At a given ambient water vapor pressure (usually the level found in the open atmosphere), the temperature of the material is raised so that the equilibrium water vapor pressure over the hydrated material is higher than the ambient water vapour pressure. Generally, heating up to 400 °C is sufficient to remove all the water of crystallization from materials. This removal of water yields a material which may contain some more strongly bound water. To remove this water, the material requires to be heated to a higher temperature (400-600 °C) so that the equilibrium water vapour pressure exceeds the ambient water vapour pressure. For near-complete removal of the last traces of water, temperatures as high as 1000 °C may be required. In addition to the heat required to raise the temperature of the material, heat is also required for the evaporation of water, which is an endothermic process. The enthalpy of evaporation increases as the water content, and hence the equilibrium water vapor pressure, decreases. 

Because average bond enthalpies refer to the endothermic process of bond breaking, they always have a positive sign. 

Figure 3.8 Born-Haber cycle constructed to obtain the lattice enthalpy A//(E, lce) of sodium chloride. All arrows pointing up represent endothermic processes and arrows pointing down represent exothermic processes (the figure is not drawn to scale)...

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. 

Electrochemistry, 20 standard potentials for, 22 Electromagnetic spectrum, 45 Electronegativity, 13, 30 Electrons, transfer of, 18 Endothermic process, 23 Energy of activation, 28 Enthalpy, 21 Entropy, 21, 23, 31 Exothermic process, 23... 

First, we have seen from the previous calculation that raising the temperature introduces more defects. We would have expected this to happen because defect formation is an endothermic process and Le Chatelier s principle tells us that increasing the temperature of an endothermic reaction will favour the products—in this case defects. Second, if it were possible to decrease the enthalpy of formation of a defect, A//s or A//p, this would also increase the proportion of defects present. A simple calculation as we did... 

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 difference to the copper process is, that the reduction of nickel oxide with methane is an endothermic process, thus a heat engine could be employed. Figures 9 and 10 show the enthalpy, entropy diagrams of the reactions outlined in equations (8) and (9). (A metallurgist will not favor the reoxidation of nickel since it is very difficult, but equilibrium thernodynamic considerations do allow it.)... 

The enthalpy of reaction, AH, is the other important thermodynamic parameter to consider. On its own, whether a reaction is exothermic or endothermic will not determine if a reaction is industrially feasible or not. Both exothermic and endothermic processes are known in industry, methanol carbonylation to acetic acid (Equation 3 AH —123 kJ/mol at 200°C), being an example of the former and the steam reforming of methane to synthesis gas, (Equation 4 AH + 227 kJ/mol at 800°C), being an example of the latter. 

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 very low initial enthalpy of adsorphon observed by calorimetry on sulfated titanias suggests the occurrence of an endothermic process (the dissociation of NH3) counterbalancing the exothermic process of adsorption. A plateau of heats around 150kjmoh is then observed, followed by a regular decrease of the heats. 

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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