Question

Question 6.16 For an isolated system, ΔU = 0, what will be ΔS?

Class XI Thermodynamics Page 183

Answer 1

we know, for an isolated system, internal energy ( ΔU )= 0 and for spontaneous reaction, total entropy change must be the positive.for example, consider the diffusion of two gases A  and B into each other in a closed vessel.which is isolated from surroundings.gases are separated by the movable piston. when  piston is removed, the system becomes disordered. it shows that,
         ΔS > 0 and ΔU = 0
  ∴ΔS = Qsurr/T 
         =(ΔU + PΔV)/T
         = PΔV/T   (∵ ΔU = 0 )
because ΔV = + positive 
  TΔS = PΔV >0  e.g., TΔS > 0 ⇒ ΔS > 0