Temperature versus endothermic

These gas-phase fuel processing reactions have significantly different operating temperatures and thermal requirements (exothermic versus endothermic), so that thermal matching of the various gas streams through heat exchangers is an engineering requirement. 

Temperature versus enthalpy profiles similar to those shown in Figure 4-15 are obtained when starches are heated either in excess water, that is, when water stareh ratios are equal to two or greater, or under intermediate water content. The start of the endothermic peak at To corresponds to loss of birefringence, in the form of the typical Maltese cross, when the starch granule is viewed under polarized light. A single endotherm, such as that obtained under excess water conditions, is referred to as the... 

Figure 8.28 shows the DSC curves of the libers analyzed at elevated temperatures. The endothermic effects characterize the dehydration process. It starts at lower temperatures and values are lower for air-dry samples than those for swollen samples, suggesting that it is more difficult to dehydrate the swollen fibers than air-dry samples. Second, the unusual plateau in the endotherm was reproducibly observed for swollen cotton at 390-418 K (Figure 8.28a), which corresponds to a practically constant dehydration rate in this temperature region. This may be associated with a compensating effect of decreasing amount of adsorbed water versus temperature rise. 

This technique has been used to characterise polymers at temperatures up to 150 °C. Under a reactive gas (oxygen) or an inert gas (nitrogen) plots of the applied temperature versus the temperature of specimens (or calories per second) detect positive (i.e., exothermic) or negative (i.e., endothermic) temperature changes (AH) in the reactions or phase changes occurring upon heating the polymer. The theory of this technique has been discussed by Earnest [3]. 

For cases where AH0 is essentially independent of temperature, plots of in Ka versus 1/T are linear with slope —(AH°/R). For cases where the heat capacity term in equation 2.2.7 is appreciable, this equation must be substituted in either equation 2.5.2 or equation 2.5.3 in order to determine the temperature dependence of the equilibrium constant. For exothermic reactions (AH0 negative) the equilibrium constant decreases with increasing temperature, while for endothermic reactions the equilibrium constant increases with increasing temperature. 

Differential thermoanalysis involves recording the temperature difference between an inert compound and the sample during heating. Such differences occur if reactions take place which either release (exothermic effect) or consume (endothermic effect) energy. These effects are recorded as peaks on a plot of the temperature difference versus the temperature. Such thermal effects are associated with the loss of adsorbed H2O and structural OH as in TGA and also with phase transformations. 

Equation 16-7 not only shows the simple way that K, depends on temperature, it also shows a simple way to determine the enthalpy change for a reaction. By determining the value of e at several different temperatures, and then plotting log Ke versus 1 IT, we should get a straight line whose slope is -AE/2.3.R. If the reaction is exothermic LH is negative), the slope will be positive if the reaction is endothermic (A/f is positive), the slope will be negative (Figure 16-1). Equation 16-7 applies to all chemical equilibria and is independent of the concentration units used either Kp or < can be use(j equally... 

Core temperatures upon recovery on the catwalk were variable. Small areas of low temperatures (6-8°C versus other parts of the core at 11-13°C) were interpreted as indicating areas where endothermic hydrate decomposition decreased the core temperature. Cores evolved large amounts of gas, which was considered responsible for low core recovery—from a norm of > 80% to 20-60% in the hydrate region. 

Figure 10.12 Chain scission in polyethylene oxide) matrix, (a) Carbonyl region of photoaged PEO samples irradiated at lOOmWcnr2 and 75°C for various times (0, 1,2 and 4 minutes). The main band at 1725cm-1 is attributed to formate functions, whereas the shoulder at 1 750cm"1 is assigned to esters, (b) Evolution of the storage modulus (G ), the loss modulus (G"), and the tangent of the phase angle tan(8) versus time (temperature 90°C). The increase in tan(8) is evidence of chain scission, (c) Endotherms of the fusion of PEOs samples recorded at...

When the sample undergoes a transformation, it will either absorb (endothermic) or release (exothermic) heat. For example, the melting of a solid material will absorb heat, where that thermal energy is used to promote the phase transformation. The instrument will detect that the sample is cooler than the reference, and will indicate the transformation as an endotherm on a plot of differential temperature (AT) versus time.2 Figure 3.2 shows a typical DTA trace of the decomposition of dolomite. If the sample and reference are exposed to a constant heating rate, the x-axis is often denoted as tem-... 

Figure 3-7 Fraction transformed via partial area integration of a DTA/DSC peak. A partial area, such as the shaded region, divided by the area under the entire endotherm contributes one datum to the fraction transformed versus temperature plot. Successive area calculations with increasing temperature will permit generation of the entire curve.

The reference material is chosen to be one that exhibits no thermally induced transitions within the temperature range of interest. Thus, transitions that occur in the sample during the applied temperature programme appear as peaks or troughs, depending on whether they are exothermic or endothermic, on the plot of differential heat flow versus temperature (the thermogram) that is the output of the instrument (Figure 22.1). Commonly used reference materials are listed by Hatakeyama and Quinn (1994). 

As already stated, the usual output of a DSC instrument is a thermogram. This is a plot of differential heat flow rate (differential power) versus temperature for a temperature scan, or a plot of differential heat flow rate versus time for an isothermal scan. The thermogram is most logically plotted (by the instrument s software) such that a peak represents an exothermic event in the sample while a trough represents an endothermic event. However, the thermogram from a PC instrument is sometimes plotted in the... 

Figure 4.4 Relative heat production and heat withdrawal rates versus dimensionless pellet temperature 0p A) exothermic reactions, 0p, 0t and An > 0 B) endothermic reactions, 0p, 0t and An < 0 ( ) stable operating point (o) unstable operating point.

A typical DSC plot of the rate of heat absorption versus temperature Is shown for poly(llmonene oxide) In Figure 6. An endothermic transition corresponding to an enthalpy change of 0.3 cal g was observed between 65.2 and 84.5 C, the peak being at 74.2 C. A similar plot was obtained for poly(a-plnene oxide), the peak occurring at 81.3 C with a AH of 0.13 cal g measured between 78.3 and 84.3°C. Ruckel et al. (14) have reported very similar softening ranges of 67-80 and 65-80 C for the catalytlcally-prepared poly(o-plnene oxide) and poly(B plnene oxide). 

Differential scanning calorimetry monitors the energy required to maintain the sample and a reference at the same temperature as they are heated. A plot of heat flow (W/g or J/g) versus temperature is obtained. A thermal transition which absorbs heat (melting, volatilization) is called endothermic. If heat is released during a thermal transition (crystallization, degradation), it is called exothermic. The area under a DSC peak is directly proportional to the heat absorbed or released and integration of the peak results in the heat of transition. 

Equation 13 l-22b suggests that the logarithm of the equilibrium constant should be a linear function of the reciprocal of the absolute temperature if the heat of reaction is independent of temperature and, presumably, an almost linear function of l/T even if Arxnff° is a function of temperature. (Compare this behavior with that of the vapor pressure of a pure substance in Sec. 7.7, especially Eq. 7.7-6.) Consequently, it is common practice to plot the logarithm of the equilibrium constant versus the reciprocal of temperature. Figure 13.-1-2 gives the equilibrium constants for a number of reactions as a function of temperature plotted in this way. (Can you identify those reactions that are endothermic and those that are exothermic )... 

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