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Chapter IV Thermal Studies of …..
∆ # S o = 2.303 × R × log10 [Ah/kTm ] (4.3)
Where, k = Boltzmann constant, h = Planck’s constant,
Tm = Temperature, A = Frequency factor.
The standard enthalpy of activation ∆ # H o can be calculated by using
the following relation,
∆ # H o = E – 2RT (4.4)
The standard Gibbs energy of activation ∆ # G o is possible to estimate
from the equation.
∆ # G o = ∆ # H o – T∆ # S o (4.5)
Enthalpy is a state function whose absolute value cannot be known.
∆ # H o can be ascertained, either by direct method or indirectly. An increase
in the enthalpy of a system, for which ∆ # H o is positive, is referred to as an
endothermic process. Conversely, loss of heat from a system, for which
∆ # H o has a negative value, is referred to as an exothermic process.
Entropy is a thermodynamic property of a system. It is a state
function and it is defined in terms of change rather than its absolute value.
A spontaneous process has a natural tendency to occur, without the need
for input work into the system. In contrast to this, the non-spontaneous
process does not have a natural tendency to occur.
However, ∆ # G o
is negative for a spontaneous process. An
exothermic reaction (∆ # H o > 0) with positive (∆ # S o > 0) is always
spontaneous. A reaction for which ∆ # H o < 0 and ∆ # S o < 0 is spontaneous
only at low temperatures, whilst a reaction for which ∆ # H o > 0 and ∆ # S o > 0
is spontaneous only at high temperatures. The temperature at which the
reaction becomes spontaneous in each case is given by T = ∆ # H o /∆ # S o .
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