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Chapter IV Thermal Studies of …..
The standard free energy ∆ # G o , is the change in the Gibbs free
energy, which accompanies the conversion of reactants in their standard
states into products in their standard states. It can be calculated from the
standard enthalpy and standard entropy using ∆ # G o = ∆ # H o – T∆ # S o or from
tabulated value for the standard free energy of formation ∆ # G o .
For spontaneous process, ∆ # S o is positive and ∆ # G o is negative. The
relationship ∆ # G o = ∆ # H o – T∆ # S o allows predictions of the conditions under
which a reaction is spontaneous. Temperature has a major effect on
spontaneity of reactions. For the reactions where ∆ # H o < 0 and ∆ # S o < 0,
|T∆ # S o | will be less than |∆ # H o | provided that T is small, and such a
reaction will be spontaneous at lower temperatures. Conversely, when
∆ # H o >0 and ∆ # S o >0, |T∆ # S o | will be greater than |∆ # H o | provided that T is
large, and such a reaction will become spontaneous at higher
temperatures. In both cases, the temperature at which the reaction
becomes spontaneous (when ∆ # G o = 0) is simply given by T = ∆ # H o /∆ # S o
[33]. The prediction for the processes is given in table (4.3).
∆ # H o
Table (4.3): The conditions for the prediction of processes
∆ # S o
Spontaneous or not Spontaneity favored by
-ive +ive Under all conditions All conditions
-ive -ive If | T∆ # S o | < | ∆ # H o | Low temperature
+ive +ive If | T∆ # S o | > | ∆ # H o | High temperature
+ive -ive Never No condition
The thermodynamic parameters for dehydration of iron tartrate have
been estimated by Joseph and Joshi [34]. Dabhi and Joshi [30, 31] have also
reported the thermodynamic parameters for dehydration of various gel grown
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