Thermal degradation is a spontaneous process. The peaks in thermodynamic curves show release of heat of reactions during degradation of your sample not energy barriers.

Mohammad Reza Toosi,

Thank you very much for the guidance,

all obtained values of delta G and H are positive, which implies that it was non-spontaneous. The state functions shown in fig are only thermodynamic parameters. My question was also about kinetic parameters like activation energy, frequency factor, and mechanism of degradation. As a whole package, What exactly they express?

You mentioned that the diagram shows thermal degradation of mango wood. It was happened so it is a spontaneous process. In addition, thermal degradation releases a large amount of gas so delta S must be positive. I think you have to check details.

What's the range of T during TGA analysis?

What's the meaning of fractional conversion?

Mohammad Reza Toosi

Sir, The heating rate was 20 C/minute and the T range was room temp to 850 C.

The fractional conversion means %age change in mass. e,g 0.2 means 20% drop of mass.

My main question was what do Thermodynamic parameters tell us in general? Is that only about spontaneousness of a reaction and vice versa?

Delta G is positive

Delta H is positive

Delta S got negative after the half run.

What does it mean?

I have checked the data analysis, the following equations were used,

Delta H= Ea+RT

Delta G= Ea+ RTm*Ln(Kb*Tm/h.A)

Deta S= (Deta H-Deta G)/Tm

where Ea= energy of activation

Tm= Peak temperature

Kb= Boltzmann constant

h= Planks constant

A= Frequency factor

How did you calculate delta H by TGA analysis? Relation between delta H and Ea you noted is only for unimolecular reactions. I suggest observe delta H by calorimetric method for each fractional conversion and different temperatures. However, I think it's difficult because you can not stop the decomposition reaction in high temperatures.

Mohammad Reza Toosi

Sir delta H was calculated as follow

Delta H=Ea+RT

where Ea=Energy of activation of on given fractional conversion. it was calculated by isoconversoinal model free kinetic method (FWO).

T= temp in Kelvin at given fractional conversion

R= universal gas constant.

But my question is about the expression of all parameters.

Del G id positive

Del H is positive

Del S is positive till half run then got negative.

What should i conclude from all this?

I studied your model for Ea. It's based on determination of the variation of reaction rate by the temperature. I think your main problem is the relation between delta H and Ea that you used:

1- This equation is usually applied for the reactions in gas phase with small molecularity. Your reaction doesn't occur in gas phase.

2- In this equation, it's assumed that delta H is temperature independent. This assumption is true only in low range of T variation not in very high range of your case (25-850). So I think you need some advanced relations.

3- By activation energy we usually find delta H, delta G, and delta S as thermodynamic parameters of activation complex not the reaction.

Mohammad Reza Toosi

Sir,

More than 1K research papers used the same equation to determine delta H from Ea of biomasses. ( A paper is attached)

My question was about to conclude something from the above analysis ( graphs)