sign of ∆G and ∆H, ∆S, and T

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

 0=∆H - T∆S

   ∆H = T∆S

  T =

T is the temp at which change over between spontaneous and nonspontaneous occur.