Reaction Kinetics:
Further to the application discussed above the TGA has also been used to elucidate the kinetics of decomposition reactions. This includes analyzing the shape of the TG curve. In common the rate of reaction at any measured temperature is proportional to the slope of the curve, other than a number of uncertainties sometimes make these analyses of questionable value. Even by lot of work has been reported on the subject. Here we are providing a brief discussion on the scope of kinetic analysis through TGA method.
The both isothermal and dynamic techniques are in use for kinetic studies but the isothermal measurement at elevated temperature and measure the time taken for a definite extent of mass loss to occur is easier.
The temperature dependence of chemical processes might be expressed through the Arrhenius equation.
K= A exp (- Ea/RT)
while K is the rate constant, R the gas constant and T the thermodynamic (Kelvin) temperature. A values of the Arrhenius parameter (Ea and A) gives measures of the magnitude of the energy barrier to reaction (the activation energy, Ea) and the frequency of the occurrence of a condition which might lead to reaction (the frequency factor A, s-1. The rate constant K is defined through the relationship among the rate of reaction (dα/ dt) and the extent of conversion or fraction reacted decomposed (α). The general relation to describe the relationship among (dα/dt) and (α) is
(dα/dt) = kα m (1-α) n
Thus, from Eq. (10.10) it is probable to derive several sub class of rate equation like as first order decay, nucleation, growth, etc. through changing the values of m and n. Isothermal experiments gives the means of determining the form of kinetic equation by discrimination among different models is not straight forward. An easier approach is to substitute the reciprocal of the isothermal life time for the rate constant in Eq. (10.9) and extrapolate the data to the region of interest.