Discuss the factor ∆H, ∆S and ∆G for the spontaneous and non spontaneous
processes
OR
classify the reaction with respect to sign of
∆G and ∆H, ∆S, and T
The
process is spontaneous and non spontaneous is can be explained by Gibb’s
equation.
∆G = ∆H - T∆S
Case
1- if ∆H and ∆S are both are negative than ∆G will be
negative only when T ∆S < ∆H or temp is low (then this reaction is carried
out at low temp) hence this reaction is spontaneous.
Case
2- if ∆H and ∆S both are positive, then ∆G will be
negative if T ∆S > ∆H such reaction must be carried out at high temperature
hence this reaction is spontaneous
Case3-
if ∆H is negative (∆H< 0) and ∆S is positive (∆S>0) then for all temp ∆G
will be negative and reaction is spontaneous.
Case
4- if ∆H is positive (∆H>0) and ∆S is negative
(∆S<0) , ∆G is always positive ∆G >0 hence the reaction will non
spontaneous at all temp.
This
can be summarized in the following table
Nature
of reaction
|
∆H
|
∆S
|
∆G
|
Spontaneity
of reaction
|
Exothermic
|
-ve
|
-ve
|
-ve
at low temp
|
spontaneous
at low temp
|
Endothermic
|
+ve
|
+ve
|
-ve
at high temp
|
spontaneous
only at high temp
|
Exothermic
|
-ve
|
+ve
|
-ve
at all temp
|
spontaneous
at all temp
|
Endothermic
|
+ve
|
-ve
|
+ve
at all temp
|
non
spontaneous at all temp
|
Temperature
of equilibrium (Cross over temp)
We know that
∆G = ∆H - T∆S
At equilibrium, ∆G=0
T is the temp at
which change over between spontaneous and nonspontaneous occur.