Endothermicity, definition

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 ... 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Exothermic Reaction. A reaction in which heat is liberated. It usually proceeds rapidly sometimes explosively. In an endothermic reaction a chemical change proceeds slowly with absorption of a definite number of calories Ref Hackh s (1944), 328-R (Exothermic) 305 R (Endothermic)... 

When the enthalpies of reaction between branched ketones and the corresponding 1,1-disubstituted alkenes are calculated using the multiple enthalpies of formation available for the latter, the following ranges are obtained Me/i-Pr, 196.6 to 200.5 Et/i-Pr, 201.2 to 206.6 and Me/t-Bu, 200.5 to 205.1 kJmol-1. Perhaps it is reasonable to conclude that the reaction enthalpies for the branched compounds either will be approximately constant, as for the unbranched ketone/alkene conversions, or will be more endothermic with branching, as in the branched aldehyde/alkene conversions. In either case, the least endothermic reaction enthalpy for the Me/i-Pr conversion above seems inconsistent and therefore the enthalpies of formation for 2,3-dimethyl-l-butene from References 16 or 26, which are essentially identical, should be selected. These enthalpies were also selected in a previous section. However, there is too much inconstancy, as well as too much uncertainty, in the replacement reactions of carbonyls and olefins to be more definitive in our conclusions. 

A particularly critical case is the exothermal decomposition immediately following the endothermal melting peak. In such cases, the decomposition is faster in the liquid than in the solid state. In an industrial environment, this could mean that a hot spot may melt a small part of the solid, which begins to decompose and the decomposition may propagate through the entire volume of the product. In such cases, the definition of a safe operation temperature becomes critical. 

Tg of either polymer components. In addition to the two distinguishable Tg signals, the melting endotherm and cold-crystallization exotherm of CA are both detectable for every blend at almost the same temperature positions as those for the CA (DS = 2.95) alone, with a proportional reduction of the respective peak areas. In contrast to the result, the thermograms compiled in Fig. 9a for the pair of CA (DS = 2.70) and P(VP-co-VAc) (VP/VAc = 0.51/0.49) (combination A) indicate a definitely single Tg that shifts to the higher temperature side with increasing CA content. As summarized in Fig. 8, CA/PVAc blends are immiscible irrespective of the DS of the CA component, while PVP forms a miscible monophase with CAs unless the acetyl DS exceeds a value of... 

Quantitative estimation of the sulphate content by the fusion method was found to be difficult because of the low percentage of the impurity. The anatase, thus prepared, was amorphous. The surface area of this anatase sample (B.E.T.) was 54 m2/g and the differential thermal analysis curve of the anatase sample is shown in fig. 2a. Although no exothermic peak due to crystallization was observed, the endothermic peak shows a definite splitting around... 

Figure 5 shows the thermally induced (10-60°C) changes in the CHj stretching frequency of intact human SC plotted as a function of hydration and reheating. Corresponding data for the extracted lipid samples are depicted in Fig. 6. For SC, the frequency increased with temperature and showed a small, but definite, inflection point between 35 and 45°C. Under dry conditions, the midpoint was estimated to be 45°C, decreasing to a constant value of 35°C as the hydration level was increased. This behavior closely mimicked the calorimetric results described, suggesting that the 35°C inflection observed by IR corresponded to the same endothermic process measured by DSC. However, no evidence of a thermal transition at 35°C was observed by IR in the...

The complexity of the SC membrane hinders such definitive interpretation, but, nevertheless, alterations in endotherms can be used to screen molecules suspected of altering membrane function. Conversely, one should note that the absence of additive-induced alterations in the phase transition profile does not rale out their perturbing effect, but rather indicates that the additive does not modify the gel phase. As described earlier, a DSC thermogram of hydrated but untreated human SC yields four endotherms, the first three of which can be identified as noncooperative lipid-associated phase transitions, while the high-temperature endotheim is attributed to keratin denaturation [33,37]. 

Fig. 5.10 DSC measurements on individual samples of polymorphs R, Y, OP and ON of 5-XII (see caption for Fig. 5.9 for definition of terms), recorded at 10 °C min , each exhibiting homogeneous melting. Inset DSC trace of a mixture of the four modifications, recorded at 0.5 °C min, showing better separated melting endotherms and exotherms resulting from crystallization from supercooled melts, (from Yu et al. 2000, with permission.)...

The normalized temperature t, given by equation (40), is a convenient independent variable only if it varies monotonically with distance through the flame. Although it is conceivable that nonmonotonic behavior of t could occur—for example, for flames in which endothermic reactions become dominant at the highest temperatures—no practical examples of adiabatic flames with temperature peaks or valleys are known the use of t seems appropriate from the viewpoint of monotonicity. The main limitation to the formulation given is assumption 7 to the effect that Cp i = Cp = constant. This assumption can be removed, with t = (T — Tq)I(T — Tq), merely at the expense of complicating the definition of / in equation (82), namely, /=Z7= 1 oo)/Cp(T — To), where each hj is a known function... 

Heat accompanying a chemical change. Thermochemistry, exothermic reaction, endothermic reaction. Definition of heat of reaction. Heat content. Heat of formation. Heat of combustion. Heat values of foods. Heat of neutralization. Heats of formation and relative electronegativity of atoms. The production of high temperatures and low temperatures. 

All decomposition reactions are endothermal except that of FeU04, presumably because this is the only reaction which involves oxidation of the double oxide. No significant diflFerence was noted in the DTA or TGA curves of the two NiU04 phases. It is interesting to note the alternating pattern in the decomposition reactions of the uranates. The iron, nickel, and zinc double oxides tend to decompose directly into their constituent oxides, while the manganese, cobalt, and copper compounds decompose to other double oxides. The pattern is not carried over into the decomposition temperatures. In this instance, the thermal stability of the double oxides appears to vary directly with the characteristic transition element oxidation states Gr(III) > Mn, Go (III, II) > Ni, Zn(II) > Gu(II, I). The iron compounds constitute a definite exception to this pattern. 

P) is positive the reaction is endothermic at T and/ . These definitions of exothermic and endothermic are equivalent to the ones given earlier in terms of chemical bond strengths. (Convince yourself)... 

There is a gradual reduction in Tg with some broadening and loss of definition of the high temperature endotherms near 200 °C as the amount of prestretch increases (Figure 21). The orientation produced by low deformations appears to produce an internal stress in the amorphous regions which acts as an external load to lower the softening temperature (72). 

From these definitions, if a compound C is exothermic in the given conditions, its heat of formation in these conditions is positive it is negative if the compound C is endothermic. 

Carbon dioxide alone is left and this is reduced to carbon monoxide by passage through red-hot coke. In order to prevent the coke being cooled by the endothermic nature of the reaction to a temperature at which the combination no longer takes place or occurs only incompletely and very slowly, the carbon dioxide is mixed with a definite quantity of oxygen. 

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