Decomposition Kinetics Using TG

Previously shown was how the activation energy of crystallization may be determined using DTA/DSC (section 3.6). A technique for determining the activation energy of a decomposition reaction using TG will now be developed. 

Decomposition (e.g. decomposition of CaC03 to CaO and CO2) differs from nucleation and growth in that the transformation of one site is not dependent on whether the neighboring sites have transformed. This can be illustrated by visualizing popcorn kernels in hot oil. From experience, we know that once the popcorn kernels transform, they do so fervently. With time, the rate of popping dies down since there axe less and less kernels left to pop. Thus, it is expected that the rate of this reaction is proportional to how many kernels are left unpopped. The same argument would hold true for a first order reaction  

The weight fraction product is defined as / = w/woo, which shortens the previous expression to  

Assuming that decomposition is an activated process, the rate constant is taken to follow an Arrhenius temperature dependence  

All that remains is to manipulate this equation into the form of a line. Taking logarithms  

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