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Question

Which of the following reaction(s) has a positive entropy change?
(I)Ag+(aq)+Cl(aq)AgCl(s)(II)NH4Cl(s)NH3(g)+HCl(g)(III)2NH3(g)N2(g)+3H2(g)
  1. I and II
  2. III
  3. II and III
  4. II


Solution

The correct option is C II and III
Entropy change will be positive in the cases where the number of gaseous moles of the product is more than the number of gaseous moles of the reactants.
Which means, ng>0
where, ng=npnR
nP= total number of gaseous moles of the products
nR= total number of gaseous moles of the reactants 

In the cases,

(II)  ng=2

(III)  ng=2
 
In both these cases, as
ng>0 S>0 

For a given substance at a given temperature, Sgas>Sliquid>Ssolid 
In case (I), entropy decreases as the conversion is taking place from liquid to solid state.
So for case (I), S<0 

So, S>0 in cases (II) and (III).

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