Thermodynamic feasibility of a reaction is very important as it would let us know in the preliminary stage if further evaluation of the project should be carried out or not.
The feasibility and condition for the chemical equilibrium is derived on the basis of energy, enthalpy and entropy. The Gibbs Free Energy predicts the feasibility and equilibrium condition at constant temperature. Hence the Gibbs Free Energy of formation is very important in the analysis of chemical reactions. Values of individual compounds (reactants and products) are required to determine the change in the Gibbs Free Energy of the reaction. The change is significant for the because of the associated chemical equilibrium for the reaction.
If the change in the Gibbs Free Energy of the reaction is Negative, the thermodynamics for the reaction is favourable.
If the change in the Gibbs Free Energy of the reaction is Highly Positive, the thermodynamics for the reaction is not favourable.
The chemical equilibrium for the reaction is associated with the change in Gibbs Free Energy for the reaction.
S G reaction = S (n DGf) Products - S (n DGf) Reactants
The change in Gibbs FREE Energy for a reaction may be used in preliminary work to determine if a reaction is thermodynamically favourable at a given temperature.
For a thermodynamic equillibrium the following rough criteria is useful for quick screening of chemical reactions.
DGreaction < 0 (Reaction is favourable.)
DGreaction > 0 (Reaction possibly favourable.)
DGreaction > 50 (Reaction is not favourable.)
DGreaction = - RT ln (Kp)
Where Kp is equilibrium constant based on pressure.
Thermodynamic data is calculated by G values of various components of reaction at various temperatures and then DGr is calculated at various temperatures and then we would have a behaviour of Kp with temperature.
First we should check the DGr and Kp at normal temperature (298oK).
And if the reaction is possible at 298oK then we should check it at the broad spectrum of temperature and then we would analyse the Kp at the whole (possible) spectrum of temperature. If the value of Kp is low at the whole (possible) spectrum of temperature then the yield of product would be very low.
Kp vs Temp.
In Gas phase reactions: We can also vary pressure and then check its effects on DGreaction and equilibrium constant. (Focus on this in the light of Le-Chatelier's principle also.)
In liquid phase reactions : the reactions are generally carried out at a pressure at which the reaction media would be in the liquid phase. But we can check the effect of pressure variation on G and then equilibrium constant in the liquid phase reactions also.
Most of the times the equillibrium constant shows fluctuations with temperature and pressure.
By Thermodynamic data and Kinetic data we actually come to know the feasibility, yeild and the rate of reaction as well as it defines various process parameters (temperature and pressure) to choose from, and select the best possible set of temperature and pressure for a given process.
Here we have selected 100oC reaction temperature because the Toluene would be in liquid phase at 100oC and then k (rate constant) would be at the maximum possible.
We have k higher at 40oC than 100oC but the main reason of carrying out reaction at 100oC rather than at the 40oC even though the k value at 40oC is higher than that of the k value at 100oC, is the wt% of the nuclear chloroproducts formed, these are nearly 10 times at the 40oC than that of the nuclear chloroproducts formed at 100oC, therefore the 100oC is selected because at 40oC ring chlorination of toluene is accelarated.
So the bottom line would be at what temperature and pressure we would get maximum possible yeild and that too at a desired rate.
The rate constant is a direct function of temperature but not that much affected with the pressure, where as in gas phase reactions the Kpwould be much more affected by pressure and these both k and Kp would play an important rold in selecting process parameters.
Miscellaneous factors such as undesired reactions in the above case (chloronuclear products restricts the reaction to be carried out at 100oC rather than at 40oC). Always a smart compromise between these factors would decide the economic feasibility of the process.
