Data at higher temperatures in .NET Make Code 39 Full ASCII in .NET Data at higher temperatures UPC-A Supplement 2

5.6 Data at higher temperatures generate, create none none in none projectsmake upc-a exact mechanisms are c none for none omplex. The study of such reactions forms a large part of the science of biochemistry..

Java programming language 5.6 Data at higher temperatures Everything we have dis none for none cussed so far is about determining data for standard conditions, which usually means pure phases at 25 C, 1 bar, although we will see later that it can mean something else. But as geologists we often deal with reactions at metamorphic and igneous temperatures of many hundreds of degrees. Specialized calorimeters can be used up to a few hundred degrees, but the experimental difficulties become great.

Obviously other methods are needed. As usual, there are several, but we will mention just two..

5.6.1 Drop calorimetry The amount of heat req none for none uired to raise the temperature of a mole of substance from Tr to T at constant pressure is simply HT HTr (or HT HTr for a standard reference substance); again, a difference between two unknown quantities. This quantity is conveniently determined by cooling the substance from T to Tr and measuring the amount of heat given up by the substance during this process.6 To do this, a calorimeter is placed directly under a furnace and the sample is dropped from the furnace where it has temperature T1 , into the calorimeter, where it gives up its heat and achieves temperature T2 (Figure 5.

7). The amount of heat given up by the sample is determined by using this heat to melt a working substance in the calorimeter (either H2 O or diphenyl ether C6 H5 2 O), and measuring the volume change of this substance by the displacement of mercury. The relationship between the volume change and the H of the solid liquid phase transition (T2 in the calorimeter is 273.

15 K for H2 O; 303.03 K for diphenyl ether) is accurately known, so this amount of heat equals HT1 HT2 . Small corrections are then applied using heat capacities to adjust this H to HT HTr , where Tr is invariably 298.

15 K. More details of the method are given by Robie (1987). Experimental results for muscovite are shown in Figure 5.

8. Values of HT HTr can be combined to give f H for substances at high temperatures. Thus for any substance.

f HT f H Tr HT HTr (5.16). As a matter of fact, d rop calorimetry has been largely superseded by differential scanning calorimetry (DSC) ( 5.6.2), but I include a description here because it illustrates the acquisition of high temperature enthalpies and heat capacities more intuitively than does DSC, and because much of presently used data were obtained by this method.

. Getting data Figure 5.7 A drop calo rimeter. (Simpli ed from Douglas and King (1968).

). Furnace Sample holder Mercury measuring system Ice calorimeter 10 cm where f refers to the none none reaction in which the substance is formed from its elements. For example,. HT HTr SiO2 = HT HTr SiO2 HT HTr Si HT HTr HTr = HT SiO2 HT Si HT O2 HTr = f H T SiO2 SiO2 Si HTr O2 f H Tr SiO2 (5.17). and therefore f H T SiO2 f H Tr SiO2 HT HTr SiO2 (5.18). To get heat capacities none none from these measurements, the experimental values of HT HTr for the substance and its elements are first fitted to a function, which is commonly. HT HTr = A + BT + CT 2 + DT 1 (5.19). 5.6 Data at higher temperatures 60 Muscovite H T H298 kcal mol 1 50 Slope = CP Figure 5.8 Values of H none none T H298 for muscovite as measured in a drop calorimeter. The slope of the curve at any point equals the heat capacity at that temperature.

Data from Pankratz (1964).. 0 300. 600 700 Temperature, K Once the best fit va lues of A, B, C, and D are calculated, HT HTr may be computed for any desired temperature. For example, the equation for the muscovite data in Figure 5.8 is.

HT H298 = 38 793 + 97 65 T + 13 19 10 3 T 2 + 25 44 105 T 1 The heat capacity A kn owledge of how the quantity HT H298 varies with T is useful because the first derivative, or the slope of the curve, is the heat capacity, CP . As we have said, HTr is an unknown quantity, but it is certainly a constant, so that.
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