Bond breaking mass loss

Figure 5 shows a typical mass loss in a decomposition experiment. The obvious definition would seem to be where the mass loss is steepest, which corresponds to the peak temperature T in the DTG plot. However, this is merely the point where reaction is fastest and does not represent the start of reaction, e.g. where bonds in the compound begin to break. The position of T will depend upon the sample size, packing, and heat flow properties. The point Tjis the initial temperature or onset temperature, but is not easy to identify and depends on the sensitivity of the balance and the amount of drift or noise seen. There may be traces of impurities, which decompose or promote some decomposition ahead of the main reaction. A better definition of start of reaction is the extrapolated onset temperature T. This requires drawing of tangents to the curve at the horizontal baseline and the steepest part of the curve and marking their intersection. For a reaction that starts very slowly and only speeds up later, T and Tj will be very different and a more satisfactory point would be shown as temperature where the fraction reacted a is equal to 0.05, i.e. Tq.05- Another definition of reaction temperature, important in kinetic studies, is when the reaction is half over, that is, when the fraction reacted... 

Thermogravimetric analysis (TGA) is used to determine the decomposition temperature of a material. A material is placed in a small pan and then heated under a gaseous environment. When it evaporates, the pan becomes lighter due to the evaporation of the material as it decomposes. The temperature at which this occurs helps determine the quality of a material because weaker bonds break apart and evaporate sooner than more strongly bonded materials. It can be used to determine the quality of crystal materials, and it can be used to determine the mass loss under specific experimental conditions. 

As another example, aliphatic hydrocarbons contain a large number of bonds, all with nearly equal energies. The likelihood that at least one of the bonds will break is high, so molecular ion peaks from aliphatic hydrocarbons will be of low intensity if observed at all. Cyclic hydrocarbons, on the other hand, must cleave two carbon-carbon bonds before any loss of mass occurs (neglecting the relatively unimportant loss of a hydrogen atom), with the result of greatly increased molecular ion intensities. 

The reaction proces.ses can be described by a combination of results from leach tests and SIMS. The nominal composition and leach rates (g in - d" ) of a glass formulation, PNL 76-68 (Pacific North-West Laboratories), used as the. standard high-level nuclear waste glass matrix [4], are shown in Table 4. The definition of leach rate u.sed normalizes the loss (in g) to the proportion of that element in the nominal bulk composition. It will be seen that the leach rates for Cs, Na, Mo, Si and B exceeded the overall rate of mass loss from the gla.ss surface, whereas the elements Fe, Zn and Ti did not show significant loss to solution under these conditions. There were also several elements, e.g.. Ca. Ba, Cd and Sr, that were neither rapidly leached nor apparently retained in the surface layers. Studies described in the above-mentioned reviews, using XPS, FTIR, SIMS, SEM and dissolution rates, have established clearly that the primary reaction occurring in solution is the bond-breaking attack by OH at Si (or Al and... 

Hiroshima exploded with energy equivalent to about 20,000 tons of TNT.18 But where does all of this energy come from Unlike ordinary chemical reactions, nuclear fission does not involve breaking and forming chemical bonds. Instead, the energy comes from the loss of mass that accompanies the fission reaction. Most, if not all, of the students will be familiar with Einstein s famous equation, E = me2, but few are likely to understand what it means.19 In 1939, Lise Meitner and her nephew Robert Frisch reported their discovery of nuclear fission.20 They realized that the energy that accompanied the fission of uranium nuclei could be accounted for by using Einstein s equation. 

The mass spectrum of a simple molecule, CO2, is shown in Figure 31-11. Note that several fragment ions are present. Breaking a C—O bond in the molecular ion leads to CO" " miz = 28) and miz = 16). Loss of both oxygen atoms leads to miz = 12). Only positive ions are present in this example. Negative ions can also be produced and detected. 

---